Re: refactor the i915 GVT support

Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.07.21 17:53:34 +0200, Christoph Hellwig wrote:
> Hi all,
> 
> the GVT code in the i915 is a bit of a mess right now due to strange
> abstractions and lots of indirect calls.  This series refactors various
> bits to clean that up.  The main user visible change is that almost all
> of the GVT code moves out of the main i915 driver and into the kvmgt
> module.
>

The reason we isolated hypervisor specific code from core vgpu
emulation is to make multiple hypervisor support possible. Yes, we do
have Xen support but never got way into upstream...And we also have
third party hypervisors which leverage gvt function through current
hypervisor interface.

Sorry I may not have more time to check in details for now, but some
of them look fine to me. I'll review more after vacation or let Zhi check details.

Thanks!

> Tested on my Thinkpad with a Kaby Lake CPU and integrated graphics.
> 
> Git tree:
> 
>     git://git.infradead.org/users/hch/misc.git i915-gvt
> 
> Gitweb:
> 
>     http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/i915-gvt
> 
> Diffstat:
>  b/drivers/gpu/drm/i915/Kconfig                         |   31 
>  b/drivers/gpu/drm/i915/Makefile                        |   30 
>  b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c |    4 
>  b/drivers/gpu/drm/i915/gvt/cfg_space.c                 |   89 --
>  b/drivers/gpu/drm/i915/gvt/cmd_parser.c                |    4 
>  b/drivers/gpu/drm/i915/gvt/dmabuf.c                    |   36 
>  b/drivers/gpu/drm/i915/gvt/execlist.c                  |   12 
>  b/drivers/gpu/drm/i915/gvt/gtt.c                       |   55 -
>  b/drivers/gpu/drm/i915/gvt/gvt.c                       |  100 --
>  b/drivers/gpu/drm/i915/gvt/gvt.h                       |  132 ++-
>  b/drivers/gpu/drm/i915/gvt/interrupt.c                 |   38 -
>  b/drivers/gpu/drm/i915/gvt/kvmgt.c                     |  634 ++++-------------
>  b/drivers/gpu/drm/i915/gvt/mmio.c                      |    4 
>  b/drivers/gpu/drm/i915/gvt/opregion.c                  |  148 ---
>  b/drivers/gpu/drm/i915/gvt/page_track.c                |    8 
>  b/drivers/gpu/drm/i915/gvt/scheduler.c                 |   37 
>  b/drivers/gpu/drm/i915/gvt/trace.h                     |    2 
>  b/drivers/gpu/drm/i915/gvt/vgpu.c                      |   22 
>  b/drivers/gpu/drm/i915/i915_drv.h                      |    7 
>  b/drivers/gpu/drm/i915/i915_params.c                   |    2 
>  b/drivers/gpu/drm/i915/intel_gvt.c                     |   64 +
>  b/drivers/gpu/drm/i915/intel_gvt.h                     |    4 
>  drivers/gpu/drm/i915/gvt/Makefile                      |    9 
>  drivers/gpu/drm/i915/gvt/hypercall.h                   |   82 --
>  drivers/gpu/drm/i915/gvt/mpt.h                         |  400 ----------
>  25 files changed, 541 insertions(+), 1413 deletions(-)

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RE: refactor the i915 GVT support

[Wang, Zhi A]

Hi Christoph:

Thanks so much for the patches and the testing.

The abstraction between i915 and KVMGT module is for our customers who can easily port GVT-g into their own hypervisors. As you can see, all the hypervisor related functions were put in "hypervisor" module. The GVT-g module talks with the "hypervisor" module through the MPT layer. The customers just need to focus on their "hypervisor" module, implement and attach their own "hypervisor" modules to the MPT layer without touching the GVT-g core logic, which reduce great efforts during the porting as the core logic of GVT-g in i915 is pretty vendor-specific and customers aren't motivated to touch it unless they have to.

The boundary between GVT-g core logic and "hypervisor" module also helps a lot on narrowing down the problems when supporting our customers. According to our experience during the support, the less a customer touches the core logic, the less problem will be introduced.

We get many customers who are using commercial hypervisors like QNX or their private hypervisors in many areas in the industry. An reference implementation of "Xen hypervisor" module to demonstrate our customer how to port GVT-g to a type-1 hypervisor instead of KVM can be found here:
https://github.com/intel/gvt-linux/blob/topic/gvt-xengt/drivers/gpu/drm/i915/gvt/xengt.c

But it's hard for some customers to contribute their own "hypervisor" module to the upstream Linux kernel. I am thinking what would be a better solution here? The MPT layer in the kernel helps a lot for customers, but only one open-source "hypervisor" module is there in the kernel. That can confuse people which don't know the story. One thing I was thinking is to put a document about the background and more description in the MPT headers. So it won't confuse more people. 

Feel free to drop more comments. 😊

Thanks,
Zhi.

-----Original Message-----
From: Christoph Hellwig <hch@lst.de> 
Sent: Wednesday, July 21, 2021 6:54 PM
To: Jani Nikula <jani.nikula@linux.intel.com>; Joonas Lahtinen <joonas.lahtinen@linux.intel.com>; Vivi, Rodrigo <rodrigo.vivi@intel.com>; Zhenyu Wang <zhenyuw@linux.intel.com>; Wang, Zhi A <zhi.a.wang@intel.com>
Cc: intel-gfx@lists.freedesktop.org; intel-gvt-dev@lists.freedesktop.org; dri-devel@lists.freedesktop.org; linux-kernel@vger.kernel.org
Subject: refactor the i915 GVT support

Hi all,

the GVT code in the i915 is a bit of a mess right now due to strange abstractions and lots of indirect calls.  This series refactors various bits to clean that up.  The main user visible change is that almost all of the GVT code moves out of the main i915 driver and into the kvmgt module.

Tested on my Thinkpad with a Kaby Lake CPU and integrated graphics.

Git tree:

    git://git.infradead.org/users/hch/misc.git i915-gvt

Gitweb:

    http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/i915-gvt

Diffstat:
 b/drivers/gpu/drm/i915/Kconfig                         |   31 
 b/drivers/gpu/drm/i915/Makefile                        |   30 
 b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c |    4 
 b/drivers/gpu/drm/i915/gvt/cfg_space.c                 |   89 --
 b/drivers/gpu/drm/i915/gvt/cmd_parser.c                |    4 
 b/drivers/gpu/drm/i915/gvt/dmabuf.c                    |   36 
 b/drivers/gpu/drm/i915/gvt/execlist.c                  |   12 
 b/drivers/gpu/drm/i915/gvt/gtt.c                       |   55 -
 b/drivers/gpu/drm/i915/gvt/gvt.c                       |  100 --
 b/drivers/gpu/drm/i915/gvt/gvt.h                       |  132 ++-
 b/drivers/gpu/drm/i915/gvt/interrupt.c                 |   38 -
 b/drivers/gpu/drm/i915/gvt/kvmgt.c                     |  634 ++++-------------
 b/drivers/gpu/drm/i915/gvt/mmio.c                      |    4 
 b/drivers/gpu/drm/i915/gvt/opregion.c                  |  148 ---
 b/drivers/gpu/drm/i915/gvt/page_track.c                |    8 
 b/drivers/gpu/drm/i915/gvt/scheduler.c                 |   37 
 b/drivers/gpu/drm/i915/gvt/trace.h                     |    2 
 b/drivers/gpu/drm/i915/gvt/vgpu.c                      |   22 
 b/drivers/gpu/drm/i915/i915_drv.h                      |    7 
 b/drivers/gpu/drm/i915/i915_params.c                   |    2 
 b/drivers/gpu/drm/i915/intel_gvt.c                     |   64 +
 b/drivers/gpu/drm/i915/intel_gvt.h                     |    4 
 drivers/gpu/drm/i915/gvt/Makefile                      |    9 
 drivers/gpu/drm/i915/gvt/hypercall.h                   |   82 --
 drivers/gpu/drm/i915/gvt/mpt.h                         |  400 ----------
 25 files changed, 541 insertions(+), 1413 deletions(-)

Re: refactor the i915 GVT support

[Gerd Hoffmann]

  Hi,

> https://github.com/intel/gvt-linux/blob/topic/gvt-xengt/drivers/gpu/drm/i915/gvt/xengt.c

> But it's hard for some customers to contribute their own "hypervisor"
> module to the upstream Linux kernel. I am thinking what would be a
> better solution here? The MPT layer in the kernel helps a lot for
> customers, but only one open-source "hypervisor" module is there in
> the kernel. That can confuse people which don't know the story.  One
> thing I was thinking is to put a document about the background and
> more description in the MPT headers. So it won't confuse more people. 

Getting the xengt module linked above merged into mainline
would also nicely explain why there are hypervisor modules.

take care,
  Gerd

Re: refactor the i915 GVT support

[Greg KH]

On Thu, Jul 22, 2021 at 10:49:47AM +0000, Wang, Zhi A wrote:
> But it's hard for some customers to contribute their own "hypervisor"
> module to the upstream Linux kernel.

What prevents them from doing this?  We will take any code that meets
our standards, what format is this external code in?

> I am thinking what would be a
> better solution here? The MPT layer in the kernel helps a lot for
> customers, but only one open-source "hypervisor" module is there in
> the kernel. That can confuse people which don't know the story. One
> thing I was thinking is to put a document about the background and
> more description in the MPT headers. So it won't confuse more people. 

If no one is using it in the kernel, it needs to be removed.  No
abstractions should be added that are not required by the in-tree code.

So this series should be accepted, _or_ the external code needs to be
submitted for inclusion.

thanks,

greg k-h

Re: refactor the i915 GVT support

[Jason Gunthorpe]

On Thu, Jul 22, 2021 at 01:26:36PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > https://github.com/intel/gvt-linux/blob/topic/gvt-xengt/drivers/gpu/drm/i915/gvt/xengt.c
> 
> > But it's hard for some customers to contribute their own "hypervisor"
> > module to the upstream Linux kernel. I am thinking what would be a
> > better solution here? The MPT layer in the kernel helps a lot for
> > customers, but only one open-source "hypervisor" module is there in
> > the kernel. That can confuse people which don't know the story.  One
> > thing I was thinking is to put a document about the background and
> > more description in the MPT headers. So it won't confuse more people. 
> 
> Getting the xengt module linked above merged into mainline
> would also nicely explain why there are hypervisor modules.

It would also be nice to explain why a GPU driver needs a hypervisor
specific shim like this in the first place.

        enum hypervisor_type type;
        int (*host_init)(struct device *dev, void *gvt, const void *ops);
        void (*host_exit)(struct device *dev, void *gvt);
        int (*attach_vgpu)(void *vgpu, unsigned long *handle);
        void (*detach_vgpu)(void *vgpu);

Doesn't vfio provide all this generically with notifiers?

        int (*inject_msi)(unsigned long handle, u32 addr, u16 data);

Isn't this one just an eventfd?

        unsigned long (*from_virt_to_mfn)(void *p);
        int (*read_gpa)(unsigned long handle, unsigned long gpa, void *buf,
                        unsigned long len);
        int (*write_gpa)(unsigned long handle, unsigned long gpa, void *buf,
                         unsigned long len);
        unsigned long (*gfn_to_mfn)(unsigned long handle, unsigned long gfn);

        int (*dma_map_guest_page)(unsigned long handle, unsigned long gfn,
                                  unsigned long size, dma_addr_t *dma_addr);
        void (*dma_unmap_guest_page)(unsigned long handle, dma_addr_t dma_addr);

        int (*dma_pin_guest_page)(unsigned long handle, dma_addr_t dma_addr);

        int (*map_gfn_to_mfn)(unsigned long handle, unsigned long gfn,
                              unsigned long mfn, unsigned int nr, bool map);
        bool (*is_valid_gfn)(unsigned long handle, unsigned long gfn);

Shouldn't the vfio page SW IOMMU do all of this generically?

        int (*enable_page_track)(unsigned long handle, u64 gfn);
        int (*disable_page_track)(unsigned long handle, u64 gfn);
        int (*set_trap_area)(unsigned long handle, u64 start, u64 end,
                             bool map);
        int (*set_opregion)(void *vgpu);
        int (*set_edid)(void *vgpu, int port_num);

edid depends on hypervisor??

        int (*get_vfio_device)(void *vgpu);
        void (*put_vfio_device)(void *vgpu);

Jason

RE: refactor the i915 GVT support

[Wang, Zhi A]

Hi Jason:

I guess those APIs you were talking about are KVM-only. For other hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not sure if you have already noticed that VFIO is KVM-only right now.

GVT-g is designed for many hypervisors not only KVM. In the design, we implemented an abstraction layer for different hypervisors. You can check the link in the previous email which has an example of how the MPT module "xengt" supports GVT-g running under Xen. 
For example, injecting a msi in VFIO/KVM is via playing with eventfd. But in Xen, we need to issue a hypercall from Dom0. So does others, like querying mappings between GFN and HFN. Some GPU related emulation logic might be implemented differently under different hypervisors because different hypervisors might provide not exact the APIs we want. That's the reason why they get a callback in the MPT (yet not perfect.)  

As you can see, to survive from this situation, we have to rely on an abstraction layer so that we can prevent introducing coding blocks like in the core logic:

If (in_hypervisor_xen)
	Issue hypercalls
else if (in_hypervisor_kvm)
	Play with eventfds.
Else if (in_hypervisor_other)
	xxxx

Thus some of the APIs have to be wrapped in the MPT module in GVT-g design.

Sadly, not all customers are motivated or allowed to get their hypervisor-specific modules into the kernel. We have a customer who runs GVT-g with their private hypervisor. In this case, they don't want to get their "xxxgt" MPT module into upstream since their hypervisor has been in the kernel yet. Also, we have customers who ported the GVT-g to QNX which is another widely used commercial hypervisor in the industry. They can't get the "qnxgt" MPT module into upstream since it's not allowed.

We do understand the situation and try to figure out a solution that can fulfill expectations from different people in the community and also customers. 

According to Greg KH's comments, we are collecting the requirements of MPT modules from other open-source hypervisors in the kernel, e.g. ACRN, to see if they can get another MPT module into the kernel, which will show an example that how the MPT abstraction can benefit. Also, we are evaluating the impact on our customers if we have to remove MPT abstraction in the kernel because there is only one MPT module. 

Thanks so much for the comments.

Thanks,
Zhi.

-----Original Message-----
From: Jason Gunthorpe <jgg@nvidia.com> 
Sent: Tuesday, July 27, 2021 3:12 PM
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Wang, Zhi A <zhi.a.wang@intel.com>; Christoph Hellwig <hch@lst.de>; Jani Nikula <jani.nikula@linux.intel.com>; Joonas Lahtinen <joonas.lahtinen@linux.intel.com>; Vivi, Rodrigo <rodrigo.vivi@intel.com>; Zhenyu Wang <zhenyuw@linux.intel.com>; intel-gfx@lists.freedesktop.org; intel-gvt-dev@lists.freedesktop.org; linux-kernel@vger.kernel.org; dri-devel@lists.freedesktop.org
Subject: Re: refactor the i915 GVT support

On Thu, Jul 22, 2021 at 01:26:36PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> > https://github.com/intel/gvt-linux/blob/topic/gvt-xengt/drivers/gpu/
> > drm/i915/gvt/xengt.c
> 
> > But it's hard for some customers to contribute their own "hypervisor"
> > module to the upstream Linux kernel. I am thinking what would be a 
> > better solution here? The MPT layer in the kernel helps a lot for 
> > customers, but only one open-source "hypervisor" module is there in 
> > the kernel. That can confuse people which don't know the story.  One 
> > thing I was thinking is to put a document about the background and 
> > more description in the MPT headers. So it won't confuse more people.
> 
> Getting the xengt module linked above merged into mainline would also 
> nicely explain why there are hypervisor modules.

It would also be nice to explain why a GPU driver needs a hypervisor specific shim like this in the first place.

        enum hypervisor_type type;
        int (*host_init)(struct device *dev, void *gvt, const void *ops);
        void (*host_exit)(struct device *dev, void *gvt);
        int (*attach_vgpu)(void *vgpu, unsigned long *handle);
        void (*detach_vgpu)(void *vgpu);

Doesn't vfio provide all this generically with notifiers?

        int (*inject_msi)(unsigned long handle, u32 addr, u16 data);

Isn't this one just an eventfd?

        unsigned long (*from_virt_to_mfn)(void *p);
        int (*read_gpa)(unsigned long handle, unsigned long gpa, void *buf,
                        unsigned long len);
        int (*write_gpa)(unsigned long handle, unsigned long gpa, void *buf,
                         unsigned long len);
        unsigned long (*gfn_to_mfn)(unsigned long handle, unsigned long gfn);

        int (*dma_map_guest_page)(unsigned long handle, unsigned long gfn,
                                  unsigned long size, dma_addr_t *dma_addr);
        void (*dma_unmap_guest_page)(unsigned long handle, dma_addr_t dma_addr);

        int (*dma_pin_guest_page)(unsigned long handle, dma_addr_t dma_addr);

        int (*map_gfn_to_mfn)(unsigned long handle, unsigned long gfn,
                              unsigned long mfn, unsigned int nr, bool map);
        bool (*is_valid_gfn)(unsigned long handle, unsigned long gfn);

Shouldn't the vfio page SW IOMMU do all of this generically?

        int (*enable_page_track)(unsigned long handle, u64 gfn);
        int (*disable_page_track)(unsigned long handle, u64 gfn);
        int (*set_trap_area)(unsigned long handle, u64 start, u64 end,
                             bool map);
        int (*set_opregion)(void *vgpu);
        int (*set_edid)(void *vgpu, int port_num);

edid depends on hypervisor??

        int (*get_vfio_device)(void *vgpu);
        void (*put_vfio_device)(void *vgpu);

Jason

Re: refactor the i915 GVT support

[Greg KH]

A: http://en.wikipedia.org/wiki/Top_post
Q: Were do I find info about this thing called top-posting?
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

A: No.
Q: Should I include quotations after my reply?

http://daringfireball.net/2007/07/on_top

On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:
> Hi Jason:
> 
> I guess those APIs you were talking about are KVM-only. For other hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not sure if you have already noticed that VFIO is KVM-only right now.

Please wrap your lines properly :(

> GVT-g is designed for many hypervisors not only KVM. In the design, we implemented an abstraction layer for different hypervisors. You can check the link in the previous email which has an example of how the MPT module "xengt" supports GVT-g running under Xen. 
> For example, injecting a msi in VFIO/KVM is via playing with eventfd. But in Xen, we need to issue a hypercall from Dom0. So does others, like querying mappings between GFN and HFN. Some GPU related emulation logic might be implemented differently under different hypervisors because different hypervisors might provide not exact the APIs we want. That's the reason why they get a callback in the MPT (yet not perfect.)  
> 
> As you can see, to survive from this situation, we have to rely on an abstraction layer so that we can prevent introducing coding blocks like in the core logic:
> 
> If (in_hypervisor_xen)
> 	Issue hypercalls
> else if (in_hypervisor_kvm)
> 	Play with eventfds.
> Else if (in_hypervisor_other)
> 	xxxx

That's horrid, and slow, please do this properly.

> Thus some of the APIs have to be wrapped in the MPT module in GVT-g design.
> 
> Sadly, not all customers are motivated or allowed to get their hypervisor-specific modules into the kernel. We have a customer who runs GVT-g with their private hypervisor. In this case, they don't want to get their "xxxgt" MPT module into upstream since their hypervisor has been in the kernel yet. Also, we have customers who ported the GVT-g to QNX which is another widely used commercial hypervisor in the industry. They can't get the "qnxgt" MPT module into upstream since it's not allowed.

Why is it not allowed?

> We do understand the situation and try to figure out a solution that can fulfill expectations from different people in the community and also customers. 
> 
> According to Greg KH's comments, we are collecting the requirements of MPT modules from other open-source hypervisors in the kernel, e.g. ACRN, to see if they can get another MPT module into the kernel, which will show an example that how the MPT abstraction can benefit. Also, we are evaluating the impact on our customers if we have to remove MPT abstraction in the kernel because there is only one MPT module. 

Until that happens, can we please just remove the unneeded layer here as
no one is using it?  Then, when you have a real user for this type of
middle-layer, you can add it back?  We have no need for it now.

thanks,

greg k-h

Re: refactor the i915 GVT support

[Jason Gunthorpe]

On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:

> I guess those APIs you were talking about are KVM-only. For other
> hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not
> sure if you have already noticed that VFIO is KVM-only right now.

There is very little hard connection between VFIO and KVM, so no, I
don't think that is completely true.

In an event, an in-tree version of other hypervisor support for GVT
needs to go through enabling VFIO support so that the existing API
multiplexers we have can be used properly, not adding a shim layer
trying to recreate VFIO inside a GPU driver.

> GVT-g is designed for many hypervisors not only KVM. In the design,
> we implemented an abstraction layer for different hypervisors. You
> can check the link in the previous email which has an example of how
> the MPT module "xengt" supports GVT-g running under Xen.  For
> example, injecting a msi in VFIO/KVM is via playing with
> eventfd. But in Xen, we need to issue a hypercall from Dom0. 

This is obviously bad design, Xen should plug into the standardized
eventfd scheme as well and trigger its hypercall this way. Then it can
integrate with the existing VFIO interrupt abstraction infrastructure.

> others, like querying mappings between GFN and HFN. 

This should be done through VFIO containers, there is nothing KVM
specific there.

> As you can see, to survive from this situation, we have to rely on
> an abstraction layer so that we can prevent introducing coding
> blocks like in the core logic:

No, you have to fix the abstractions we already have to support the
matrix of things you care about. If this can't be done then maybe we
can add new abstractions, but abstractions like this absoultely should
not be done inside drivers.

Jason

Re: [Intel-gfx] refactor the i915 GVT support

[Daniel Vetter]

On Thu, Jul 29, 2021 at 09:20:22AM +0200, Christoph Hellwig wrote:
> On Wed, Jul 28, 2021 at 02:59:25PM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:
> > 
> > > I guess those APIs you were talking about are KVM-only. For other
> > > hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not
> > > sure if you have already noticed that VFIO is KVM-only right now.
> > 
> > There is very little hard connection between VFIO and KVM, so no, I
> > don't think that is completely true.
> 
> The only connection is the SET_KVM notifier as far as I can tell.
> Which is used by a total of two drivers, including i915/gvt.  That
> being said gvt does not only use vfio, but also does quite a few
> direct cals to KVM.
> 
> > In an event, an in-tree version of other hypervisor support for GVT
> > needs to go through enabling VFIO support so that the existing API
> > multiplexers we have can be used properly, not adding a shim layer
> > trying to recreate VFIO inside a GPU driver.
> 
> Yes.  And if we go back to actually looking at the series a lot of
> it just removes entirely pointless indirect calls that go to generic
> code and not even the kvm code, or questionable data structure designs.
> If we were to support another upstream hypervisor we'd just need to
> union a few fields in struct intel_gpu and maybe introduce a few
> methods.  Preferably in a way that avoids expensive indirect calls
> in the fast path.

fwiw I concur with the direction of this series. gvt landed 5 years ago,
that should have been plenty of time to merge at least one of the other
backends that float around. If it didn't happen in 5 years it aint
suddenly happening in the next few, and the abstraction layer should be
sunset.

Also yes structuring it more as a helper layer with some
unions/subclassing than full blown backend abstractor layer would be a
good idea too I guess (it usually is the right thing to do).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: refactor the i915 GVT support

[Wang, Zhi A]

On 7/28/2021 8:59 PM, Jason Gunthorpe wrote:
> On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:
>
>> I guess those APIs you were talking about are KVM-only. For other
>> hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not
>> sure if you have already noticed that VFIO is KVM-only right now.
> There is very little hard connection between VFIO and KVM, so no, I
> don't think that is completely true.
>
> In an event, an in-tree version of other hypervisor support for GVT
> needs to go through enabling VFIO support so that the existing API
> multiplexers we have can be used properly, not adding a shim layer
> trying to recreate VFIO inside a GPU driver.

We were delivering the presentation of GVT-g in Xen summit 2018 and we 
were thinking and talking about supporting VFIO in Xen during the 
presentation (the video can be found from Youtube). But we didn't see 
any motiviation from the Xen community to adopt it.

If people take a look into the code in QEMU, in the PCI-passthrough 
part, Xen is actually not using VFIO even nowadays. We would be glad to 
see someone can influence on that part, especically making all the 
in-kernel hypervisor to use VFIO in PCI-passthrough and supporting mdev. 
That would be a huge benefit for all the users.

>> GVT-g is designed for many hypervisors not only KVM. In the design,
>> we implemented an abstraction layer for different hypervisors. You
>> can check the link in the previous email which has an example of how
>> the MPT module "xengt" supports GVT-g running under Xen.  For
>> example, injecting a msi in VFIO/KVM is via playing with
>> eventfd. But in Xen, we need to issue a hypercall from Dom0.
> This is obviously bad design, Xen should plug into the standardized
> eventfd scheme as well and trigger its hypercall this way. Then it can
> integrate with the existing VFIO interrupt abstraction infrastructure.
>
>> others, like querying mappings between GFN and HFN.
> This should be done through VFIO containers, there is nothing KVM
> specific there.
>
>> As you can see, to survive from this situation, we have to rely on
>> an abstraction layer so that we can prevent introducing coding
>> blocks like in the core logic:
> No, you have to fix the abstractions we already have to support the
> matrix of things you care about. If this can't be done then maybe we
> can add new abstractions, but abstractions like this absoultely should
> not be done inside drivers.
>
> Jason

That's a good point and we were actually thinking about this before and 
I believe that's the correct direction. But just like the situation 
mentioned above, it would be nice if people can really put a great 
influence on all in-kernel hypervisors to use VFIO which can really 
benefit all the users.

For now, we are just going to take christoph's patches.

Zhi

Re: [PATCH 01/21] drm/i915/gvt: integrate into the main Makefile

[Zhenyu Wang]

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On 2021.07.21 17:53:35 +0200, Christoph Hellwig wrote:
> Remove the separately included Makefile and just use the relative
> reference from the main i915 Makefile as for source files in other
> subdirectories.
>

We agreed to make gvt mostly self-contained in its own directory
before. Although I don't think we would change files much later, but
still need to get ack from i915 maintainers.

Thanks

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  drivers/gpu/drm/i915/Makefile     | 29 ++++++++++++++++++++++++-----
>  drivers/gpu/drm/i915/gvt/Makefile |  9 ---------
>  drivers/gpu/drm/i915/gvt/trace.h  |  2 +-
>  3 files changed, 25 insertions(+), 15 deletions(-)
>  delete mode 100644 drivers/gpu/drm/i915/gvt/Makefile
> 
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index 4f22cac1c49b..2153f67705b8 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -289,11 +289,30 @@ i915-$(CONFIG_DRM_I915_SELFTEST) += \
>  
>  # virtual gpu code
>  i915-y += i915_vgpu.o
> -
> -ifeq ($(CONFIG_DRM_I915_GVT),y)
> -i915-y += intel_gvt.o
> -include $(src)/gvt/Makefile
> -endif
> +i915-$(CONFIG_DRM_I915_GVT) += \
> +	intel_gvt.o \
> +	gvt/gvt.o \
> +	gvt/aperture_gm.o \
> +	gvt/handlers.o \
> +	gvt/vgpu.o \
> +	gvt/trace_points.o \
> +	gvt/firmware.o \
> +	gvt/interrupt.o \
> +	gvt/gtt.o \
> +	gvt/cfg_space.o \
> +	gvt/opregion.o \
> +	gvt/mmio.o \
> +	gvt/display.o \
> +	gvt/edid.o \
> +	gvt/execlist.o \
> +	gvt/scheduler.o \
> +	gvt/sched_policy.o \
> +	gvt/mmio_context.o \
> +	gvt/cmd_parser.o \
> +	gvt/debugfs.o \
> +	gvt/fb_decoder.o \
> +	gvt/dmabuf.o \
> +	gvt/page_track.o
>  
>  obj-$(CONFIG_DRM_I915) += i915.o
>  obj-$(CONFIG_DRM_I915_GVT_KVMGT) += gvt/kvmgt.o
> diff --git a/drivers/gpu/drm/i915/gvt/Makefile b/drivers/gpu/drm/i915/gvt/Makefile
> deleted file mode 100644
> index ea8324abc784..000000000000
> --- a/drivers/gpu/drm/i915/gvt/Makefile
> +++ /dev/null
> @@ -1,9 +0,0 @@
> -# SPDX-License-Identifier: GPL-2.0
> -GVT_DIR := gvt
> -GVT_SOURCE := gvt.o aperture_gm.o handlers.o vgpu.o trace_points.o firmware.o \
> -	interrupt.o gtt.o cfg_space.o opregion.o mmio.o display.o edid.o \
> -	execlist.o scheduler.o sched_policy.o mmio_context.o cmd_parser.o debugfs.o \
> -	fb_decoder.o dmabuf.o page_track.o
> -
> -ccflags-y				+= -I $(srctree)/$(src) -I $(srctree)/$(src)/$(GVT_DIR)/
> -i915-y					+= $(addprefix $(GVT_DIR)/, $(GVT_SOURCE))
> diff --git a/drivers/gpu/drm/i915/gvt/trace.h b/drivers/gpu/drm/i915/gvt/trace.h
> index 6d787750d279..348f57f8301d 100644
> --- a/drivers/gpu/drm/i915/gvt/trace.h
> +++ b/drivers/gpu/drm/i915/gvt/trace.h
> @@ -379,5 +379,5 @@ TRACE_EVENT(render_mmio,
>  #undef TRACE_INCLUDE_PATH
>  #define TRACE_INCLUDE_PATH .
>  #undef TRACE_INCLUDE_FILE
> -#define TRACE_INCLUDE_FILE trace
> +#define TRACE_INCLUDE_FILE gvt/trace
>  #include <trace/define_trace.h>
> -- 
> 2.30.2
> 
> _______________________________________________
> intel-gvt-dev mailing list
> intel-gvt-dev@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gvt-dev

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Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.07.29 09:20:22 +0200, Christoph Hellwig wrote:
> On Wed, Jul 28, 2021 at 02:59:25PM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 28, 2021 at 01:38:58PM +0000, Wang, Zhi A wrote:
> > 
> > > I guess those APIs you were talking about are KVM-only. For other
> > > hypervisors, e.g. Xen, ARCN cannot use the APIs you mentioned. Not
> > > sure if you have already noticed that VFIO is KVM-only right now.
> > 
> > There is very little hard connection between VFIO and KVM, so no, I
> > don't think that is completely true.
> 
> The only connection is the SET_KVM notifier as far as I can tell.
> Which is used by a total of two drivers, including i915/gvt.  That
> being said gvt does not only use vfio, but also does quite a few
> direct cals to KVM.

yeah, we mostly combined VFIO into hypervisor specific thing before,
e.g interface to set vgpu edid, etc. along with kvm specific calls.

> 
> > In an event, an in-tree version of other hypervisor support for GVT
> > needs to go through enabling VFIO support so that the existing API
> > multiplexers we have can be used properly, not adding a shim layer
> > trying to recreate VFIO inside a GPU driver.
> 
> Yes.  And if we go back to actually looking at the series a lot of
> it just removes entirely pointless indirect calls that go to generic
> code and not even the kvm code, or questionable data structure designs.
> If we were to support another upstream hypervisor we'd just need to
> union a few fields in struct intel_gpu and maybe introduce a few
> methods.  Preferably in a way that avoids expensive indirect calls
> in the fast path.

ok, agree on that.

Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>

Thanks a lot for this effort!

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Re: refactor the i915 GVT support

[Jason Gunthorpe]

On Tue, Aug 03, 2021 at 05:43:15PM +0800, Zhenyu Wang wrote:
> Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> 
> Thanks a lot for this effort!

Great, do we have a submission plan for this? how much does it clash
with my open_device/etc patch? ie does the whole thing have to go
through the vfio tree?

Thanks,
Jason

Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.03 11:30:58 -0300, Jason Gunthorpe wrote:
> On Tue, Aug 03, 2021 at 05:43:15PM +0800, Zhenyu Wang wrote:
> > Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> > 
> > Thanks a lot for this effort!
> 
> Great, do we have a submission plan for this? how much does it clash
> with my open_device/etc patch? ie does the whole thing have to go
> through the vfio tree?
> 

I think Alex would determine when to merge open_device series, gvt part
can be through vfio tree without problem. For this refactor, I would first
merge for gvt staging to do more regression testing before sending through
i915 tree.

Thanks

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Re: [PATCH 04/21] drm/i915/gvt: move the gvt code into kvmgt.ko

[Joonas Lahtinen]

Quoting Christoph Hellwig (2021-07-21 18:53:38)
> Instead of having an option to build the gvt code into the main i915
> module, just move it into the kvmgt.ko module.  This only requires
> a new struct with three entries that the main i915 module needs to
> request before enabling VGPU passthrough operations.
> 
> This also conveniently streamlines the GVT initialization and avoids
> the need for the global device pointer.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Hi,

Thanks for putting the work into this. This conversion has been
requested for a long time. For clarity, should we call the module
i915_kvmgt?

How far would we be from dynamically modprobing/rmmoding the kvmgt
module in order to eliminate the enable_gvt parameter?

<SNIP>

> +
> +/*
> + * Exported here so that the exports only get created when GVT support is
> + * actually enabled.
> + */
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_alloc, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_create_shmem, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_init, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_ggtt_pin_ww, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_pin_map, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_object_set_to_cpu_domain, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(__i915_gem_object_flush_map, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(__i915_gem_object_set_pages, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_gtt_insert, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_prime_export, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_ww_ctx_init, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_ww_ctx_backoff, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_gem_ww_ctx_fini, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_ppgtt_create, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_request_add, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_request_create, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_request_wait, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_reserve_fence, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_unreserve_fence, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_vm_release, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(i915_vma_move_to_active, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_context_create, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_context_unpin, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(__intel_context_do_pin, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_ring_begin, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_runtime_pm_get, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_runtime_pm_put_unchecked, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_uncore_forcewake_for_reg, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_uncore_forcewake_get, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(intel_uncore_forcewake_put, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(shmem_pin_map, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(shmem_unpin_map, I915_GVT);
> +EXPORT_SYMBOL_NS_GPL(__px_dma, I915_GVT);

This list is also a concern. At the least the double underscore
functions should be eliminated from being exported.

Zhi and Zhenyu, can we have some further patches to clean that up?

Regards, Joonas

Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.16 19:34:58 +0200, Christoph Hellwig wrote:
> On Wed, Aug 04, 2021 at 01:26:06PM +0800, Zhenyu Wang wrote:
> > On 2021.08.03 11:30:58 -0300, Jason Gunthorpe wrote:
> > > On Tue, Aug 03, 2021 at 05:43:15PM +0800, Zhenyu Wang wrote:
> > > > Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> > > > 
> > > > Thanks a lot for this effort!
> > > 
> > > Great, do we have a submission plan for this? how much does it clash
> > > with my open_device/etc patch? ie does the whole thing have to go
> > > through the vfio tree?
> > > 
> > 
> > I think Alex would determine when to merge open_device series, gvt part
> > can be through vfio tree without problem. For this refactor, I would first
> > merge for gvt staging to do more regression testing before sending through
> > i915 tree.
> 
> Any updates on this?  I'd really hate to miss this merge window.

I'm still waiting for our validation team's report on this. I'm afraid
it might be missing for next version as i915 merge window is mostly
till rc5...and for any change outside of gvt, it still needs to be
acked by i915 maintainers.

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Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.17 09:08:55 +0800, Zhenyu Wang wrote:
> On 2021.08.16 19:34:58 +0200, Christoph Hellwig wrote:
> > On Wed, Aug 04, 2021 at 01:26:06PM +0800, Zhenyu Wang wrote:
> > > On 2021.08.03 11:30:58 -0300, Jason Gunthorpe wrote:
> > > > On Tue, Aug 03, 2021 at 05:43:15PM +0800, Zhenyu Wang wrote:
> > > > > Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> > > > > 
> > > > > Thanks a lot for this effort!
> > > > 
> > > > Great, do we have a submission plan for this? how much does it clash
> > > > with my open_device/etc patch? ie does the whole thing have to go
> > > > through the vfio tree?
> > > > 
> > > 
> > > I think Alex would determine when to merge open_device series, gvt part
> > > can be through vfio tree without problem. For this refactor, I would first
> > > merge for gvt staging to do more regression testing before sending through
> > > i915 tree.
> > 
> > Any updates on this?  I'd really hate to miss this merge window.
> 
> I'm still waiting for our validation team's report on this. I'm afraid
> it might be missing for next version as i915 merge window is mostly
> till rc5...and for any change outside of gvt, it still needs to be
> acked by i915 maintainers.

Looks our validation team did have problem against recent i915 change.
If you like to try, we have a gvt-staging branch on
https://github.com/intel/gvt-linux which is generated against drm-tip
with gvt changes for testing, currently it's broken.

One issue is with i915 export that intel_context_unpin has been
changed into static inline function. Another is that intel_gvt.c
should be part of i915 for gvt interface instead of depending on KVMGT
config.

But the problem I see is that after moving gvt device model (gvt/*.c
except kvmgt.c) into kvmgt module, we'll have issue with initial mmio
state which current gvt relies on, that is in design supposed to get
initial HW state before i915 driver has taken any operation.  Before
we can ensure that, I think we may only remove MPT part first but
still keep gvt device model as part of i915 with config. I'll try to
split that out.

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Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.17 13:22:03 +0800, Zhenyu Wang wrote:
> > On 2021.08.16 19:34:58 +0200, Christoph Hellwig wrote:
> > > Any updates on this?  I'd really hate to miss this merge window.
> > 
> > I'm still waiting for our validation team's report on this. I'm afraid
> > it might be missing for next version as i915 merge window is mostly
> > till rc5...and for any change outside of gvt, it still needs to be
> > acked by i915 maintainers.
> 
> Looks our validation team did have problem against recent i915 change.
> If you like to try, we have a gvt-staging branch on
> https://github.com/intel/gvt-linux which is generated against drm-tip
> with gvt changes for testing, currently it's broken.
> 
> One issue is with i915 export that intel_context_unpin has been
> changed into static inline function. Another is that intel_gvt.c
> should be part of i915 for gvt interface instead of depending on KVMGT
> config.

I'm working on below patch to resolve this. But I met a weird issue in
case when building i915 as module and also kvmgt module, it caused
busy wait on request_module("kvmgt") when boot, it doesn't happen if
building i915 into kernel. I'm not sure what could be the reason?

> But the problem I see is that after moving gvt device model (gvt/*.c
> except kvmgt.c) into kvmgt module, we'll have issue with initial mmio
> state which current gvt relies on, that is in design supposed to get
> initial HW state before i915 driver has taken any operation.  Before
> we can ensure that, I think we may only remove MPT part first but
> still keep gvt device model as part of i915 with config. I'll try to
> split that out.

Sorry I misread the code that as we always request kvmgt module when
gvt init, so it should still apply original method that this isn't a
problem. Our current validation result has shown no regression as well.

---8<---
From 58ff84572f1a0f9d79ca1d7ec0cff5ecbe78d280 Mon Sep 17 00:00:00 2001
From: Zhenyu Wang <zhenyuw@linux.intel.com>
Date: Thu, 19 Aug 2021 16:36:33 +0800
Subject: [PATCH] TESTONLY:drm/i915/gvt: potential fix for refactor against
 current tip

---
 drivers/gpu/drm/i915/Makefile           | 4 +++-
 drivers/gpu/drm/i915/gt/intel_context.c | 5 +++++
 drivers/gpu/drm/i915/gt/intel_context.h | 3 ++-
 drivers/gpu/drm/i915/i915_trace.h       | 1 +
 4 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index c4f953837f72..2248574428a1 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -296,7 +296,9 @@ i915-$(CONFIG_DRM_I915_SELFTEST) += \
 
 # virtual gpu code
 i915-y += i915_vgpu.o
-i915-$(CONFIG_DRM_I915_GVT_KVMGT) += intel_gvt.o
+ifneq ($(CONFIG_DRM_I915_GVT_KVMGT),)
+i915-y += intel_gvt.o
+endif
 
 kvmgt-y += gvt/kvmgt.o \
 	gvt/gvt.o \
diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index 745e84c72c90..20e7522fed84 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -328,6 +328,11 @@ void __intel_context_do_unpin(struct intel_context *ce, int sub)
 	intel_context_put(ce);
 }
 
+void intel_context_unpin(struct intel_context *ce)
+{
+	_intel_context_unpin(ce);
+}
+
 static void __intel_context_retire(struct i915_active *active)
 {
 	struct intel_context *ce = container_of(active, typeof(*ce), active);
diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
index c41098950746..f942cbf6300a 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.h
+++ b/drivers/gpu/drm/i915/gt/intel_context.h
@@ -131,7 +131,7 @@ static inline void intel_context_sched_disable_unpin(struct intel_context *ce)
 	__intel_context_do_unpin(ce, 2);
 }
 
-static inline void intel_context_unpin(struct intel_context *ce)
+static inline void _intel_context_unpin(struct intel_context *ce)
 {
 	if (!ce->ops->sched_disable) {
 		__intel_context_do_unpin(ce, 1);
@@ -150,6 +150,7 @@ static inline void intel_context_unpin(struct intel_context *ce)
 		}
 	}
 }
+void intel_context_unpin(struct intel_context *ce);
 
 void intel_context_enter_engine(struct intel_context *ce);
 void intel_context_exit_engine(struct intel_context *ce);
diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
index 806ad688274b..2c6a8bcef7c1 100644
--- a/drivers/gpu/drm/i915/i915_trace.h
+++ b/drivers/gpu/drm/i915/i915_trace.h
@@ -17,6 +17,7 @@
 
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM i915
+#undef TRACE_INCLUDE_FILE
 #define TRACE_INCLUDE_FILE i915_trace
 
 /* watermark/fifo updates */
-- 
2.32.0
---8<---



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Re: refactor the i915 GVT support

[Joonas Lahtinen]

Quoting Zhenyu Wang (2021-08-19 11:29:29)
> On 2021.08.17 13:22:03 +0800, Zhenyu Wang wrote:
> > > On 2021.08.16 19:34:58 +0200, Christoph Hellwig wrote:
> > > > Any updates on this?  I'd really hate to miss this merge window.
> > > 
> > > I'm still waiting for our validation team's report on this. I'm afraid
> > > it might be missing for next version as i915 merge window is mostly
> > > till rc5...and for any change outside of gvt, it still needs to be
> > > acked by i915 maintainers.
> > 
> > Looks our validation team did have problem against recent i915 change.
> > If you like to try, we have a gvt-staging branch on
> > https://github.com/intel/gvt-linux which is generated against drm-tip
> > with gvt changes for testing, currently it's broken.
> > 
> > One issue is with i915 export that intel_context_unpin has been
> > changed into static inline function. Another is that intel_gvt.c
> > should be part of i915 for gvt interface instead of depending on KVMGT
> > config.
> 
> I'm working on below patch to resolve this. But I met a weird issue in
> case when building i915 as module and also kvmgt module, it caused
> busy wait on request_module("kvmgt") when boot, it doesn't happen if
> building i915 into kernel. I'm not sure what could be the reason?
> 
> > But the problem I see is that after moving gvt device model (gvt/*.c
> > except kvmgt.c) into kvmgt module, we'll have issue with initial mmio
> > state which current gvt relies on, that is in design supposed to get
> > initial HW state before i915 driver has taken any operation.

As mentioned in some past discussions, I think it would be best rely on
golden MMIO located in /lib/firmware or elsewhere. This way we will better
isolate the guest system from host system updates/changes.

This should also hopefully allow enabling kvmgt module after i915 has
already loaded, as the initialization would not be conditional to
capture the MMIO.

Regards, Joonas

> > Before
> > we can ensure that, I think we may only remove MPT part first but
> > still keep gvt device model as part of i915 with config. I'll try to
> > split that out.
> 
> Sorry I misread the code that as we always request kvmgt module when
> gvt init, so it should still apply original method that this isn't a
> problem. Our current validation result has shown no regression as well.
> 
> ---8<---
> From 58ff84572f1a0f9d79ca1d7ec0cff5ecbe78d280 Mon Sep 17 00:00:00 2001
> From: Zhenyu Wang <zhenyuw@linux.intel.com>
> Date: Thu, 19 Aug 2021 16:36:33 +0800
> Subject: [PATCH] TESTONLY:drm/i915/gvt: potential fix for refactor against
>  current tip
> 
> ---
>  drivers/gpu/drm/i915/Makefile           | 4 +++-
>  drivers/gpu/drm/i915/gt/intel_context.c | 5 +++++
>  drivers/gpu/drm/i915/gt/intel_context.h | 3 ++-
>  drivers/gpu/drm/i915/i915_trace.h       | 1 +
>  4 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> index c4f953837f72..2248574428a1 100644
> --- a/drivers/gpu/drm/i915/Makefile
> +++ b/drivers/gpu/drm/i915/Makefile
> @@ -296,7 +296,9 @@ i915-$(CONFIG_DRM_I915_SELFTEST) += \
>  
>  # virtual gpu code
>  i915-y += i915_vgpu.o
> -i915-$(CONFIG_DRM_I915_GVT_KVMGT) += intel_gvt.o
> +ifneq ($(CONFIG_DRM_I915_GVT_KVMGT),)
> +i915-y += intel_gvt.o
> +endif
>  
>  kvmgt-y += gvt/kvmgt.o \
>         gvt/gvt.o \
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index 745e84c72c90..20e7522fed84 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -328,6 +328,11 @@ void __intel_context_do_unpin(struct intel_context *ce, int sub)
>         intel_context_put(ce);
>  }
>  
> +void intel_context_unpin(struct intel_context *ce)
> +{
> +       _intel_context_unpin(ce);
> +}
> +
>  static void __intel_context_retire(struct i915_active *active)
>  {
>         struct intel_context *ce = container_of(active, typeof(*ce), active);
> diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> index c41098950746..f942cbf6300a 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.h
> +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> @@ -131,7 +131,7 @@ static inline void intel_context_sched_disable_unpin(struct intel_context *ce)
>         __intel_context_do_unpin(ce, 2);
>  }
>  
> -static inline void intel_context_unpin(struct intel_context *ce)
> +static inline void _intel_context_unpin(struct intel_context *ce)
>  {
>         if (!ce->ops->sched_disable) {
>                 __intel_context_do_unpin(ce, 1);
> @@ -150,6 +150,7 @@ static inline void intel_context_unpin(struct intel_context *ce)
>                 }
>         }
>  }
> +void intel_context_unpin(struct intel_context *ce);
>  
>  void intel_context_enter_engine(struct intel_context *ce);
>  void intel_context_exit_engine(struct intel_context *ce);
> diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> index 806ad688274b..2c6a8bcef7c1 100644
> --- a/drivers/gpu/drm/i915/i915_trace.h
> +++ b/drivers/gpu/drm/i915/i915_trace.h
> @@ -17,6 +17,7 @@
>  
>  #undef TRACE_SYSTEM
>  #define TRACE_SYSTEM i915
> +#undef TRACE_INCLUDE_FILE
>  #define TRACE_INCLUDE_FILE i915_trace
>  
>  /* watermark/fifo updates */
> -- 
> 2.32.0
> ---8<---
> 
> 

Re: refactor the i915 GVT support

[Luis Chamberlain]

On Fri, Aug 20, 2021 at 04:17:24PM +0200, Christoph Hellwig wrote:
> On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
> > I'm working on below patch to resolve this. But I met a weird issue in
> > case when building i915 as module and also kvmgt module, it caused
> > busy wait on request_module("kvmgt") when boot, it doesn't happen if
> > building i915 into kernel. I'm not sure what could be the reason?
> 
> Luis, do you know if there is a problem with a request_module from
> a driver ->probe routine that is probably called by a module_init
> function itself?

Generally no, but you can easily foot yourself in the feet by creating
cross dependencies and not dealing with them properly. I'd make sure
to keep module initialization as simple as possible, and run whatever
takes more time asynchronously, then use a state machine to allow
you to verify where you are in the initialization phase or query it
or wait for a completion with a timeout.

It seems the code in question is getting some spring cleaning, and its
unclear where the code is I can inspect. If there's a tree somewhere I
can take a peak I'd be happy to review possible oddities that may stick
out.

My goto model for these sorts of problems is to abstract the issue
*outside* of the driver in question and implement new selftests to
try to reproduce. This serves two purposes, 1) helps with testing
2) may allow you to see the problem more clearly.

  Luis

Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.19 17:43:43 +0300, Joonas Lahtinen wrote:
> Quoting Zhenyu Wang (2021-08-19 11:29:29)
> > On 2021.08.17 13:22:03 +0800, Zhenyu Wang wrote:
> > > > On 2021.08.16 19:34:58 +0200, Christoph Hellwig wrote:
> > > > > Any updates on this?  I'd really hate to miss this merge window.
> > > > 
> > > > I'm still waiting for our validation team's report on this. I'm afraid
> > > > it might be missing for next version as i915 merge window is mostly
> > > > till rc5...and for any change outside of gvt, it still needs to be
> > > > acked by i915 maintainers.
> > > 
> > > Looks our validation team did have problem against recent i915 change.
> > > If you like to try, we have a gvt-staging branch on
> > > https://github.com/intel/gvt-linux which is generated against drm-tip
> > > with gvt changes for testing, currently it's broken.
> > > 
> > > One issue is with i915 export that intel_context_unpin has been
> > > changed into static inline function. Another is that intel_gvt.c
> > > should be part of i915 for gvt interface instead of depending on KVMGT
> > > config.
> > 
> > I'm working on below patch to resolve this. But I met a weird issue in
> > case when building i915 as module and also kvmgt module, it caused
> > busy wait on request_module("kvmgt") when boot, it doesn't happen if
> > building i915 into kernel. I'm not sure what could be the reason?
> > 
> > > But the problem I see is that after moving gvt device model (gvt/*.c
> > > except kvmgt.c) into kvmgt module, we'll have issue with initial mmio
> > > state which current gvt relies on, that is in design supposed to get
> > > initial HW state before i915 driver has taken any operation.
> 
> As mentioned in some past discussions, I think it would be best rely on
> golden MMIO located in /lib/firmware or elsewhere. This way we will better
> isolate the guest system from host system updates/changes.
> 
> This should also hopefully allow enabling kvmgt module after i915 has
> already loaded, as the initialization would not be conditional to
> capture the MMIO.
> 

I think the concern is that even for same GEN hw there could be many
variant platforms e.g APL with gen9, etc. To verify golden states for
them all might take too much effort...

> 
> > > Before
> > > we can ensure that, I think we may only remove MPT part first but
> > > still keep gvt device model as part of i915 with config. I'll try to
> > > split that out.
> > 
> > Sorry I misread the code that as we always request kvmgt module when
> > gvt init, so it should still apply original method that this isn't a
> > problem. Our current validation result has shown no regression as well.
> > 
> > ---8<---
> > From 58ff84572f1a0f9d79ca1d7ec0cff5ecbe78d280 Mon Sep 17 00:00:00 2001
> > From: Zhenyu Wang <zhenyuw@linux.intel.com>
> > Date: Thu, 19 Aug 2021 16:36:33 +0800
> > Subject: [PATCH] TESTONLY:drm/i915/gvt: potential fix for refactor against
> >  current tip
> > 
> > ---
> >  drivers/gpu/drm/i915/Makefile           | 4 +++-
> >  drivers/gpu/drm/i915/gt/intel_context.c | 5 +++++
> >  drivers/gpu/drm/i915/gt/intel_context.h | 3 ++-
> >  drivers/gpu/drm/i915/i915_trace.h       | 1 +
> >  4 files changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> > index c4f953837f72..2248574428a1 100644
> > --- a/drivers/gpu/drm/i915/Makefile
> > +++ b/drivers/gpu/drm/i915/Makefile
> > @@ -296,7 +296,9 @@ i915-$(CONFIG_DRM_I915_SELFTEST) += \
> >  
> >  # virtual gpu code
> >  i915-y += i915_vgpu.o
> > -i915-$(CONFIG_DRM_I915_GVT_KVMGT) += intel_gvt.o
> > +ifneq ($(CONFIG_DRM_I915_GVT_KVMGT),)
> > +i915-y += intel_gvt.o
> > +endif
> >  
> >  kvmgt-y += gvt/kvmgt.o \
> >         gvt/gvt.o \
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > index 745e84c72c90..20e7522fed84 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > @@ -328,6 +328,11 @@ void __intel_context_do_unpin(struct intel_context *ce, int sub)
> >         intel_context_put(ce);
> >  }
> >  
> > +void intel_context_unpin(struct intel_context *ce)
> > +{
> > +       _intel_context_unpin(ce);
> > +}
> > +
> >  static void __intel_context_retire(struct i915_active *active)
> >  {
> >         struct intel_context *ce = container_of(active, typeof(*ce), active);
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> > index c41098950746..f942cbf6300a 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> > @@ -131,7 +131,7 @@ static inline void intel_context_sched_disable_unpin(struct intel_context *ce)
> >         __intel_context_do_unpin(ce, 2);
> >  }
> >  
> > -static inline void intel_context_unpin(struct intel_context *ce)
> > +static inline void _intel_context_unpin(struct intel_context *ce)
> >  {
> >         if (!ce->ops->sched_disable) {
> >                 __intel_context_do_unpin(ce, 1);
> > @@ -150,6 +150,7 @@ static inline void intel_context_unpin(struct intel_context *ce)
> >                 }
> >         }
> >  }
> > +void intel_context_unpin(struct intel_context *ce);
> >  
> >  void intel_context_enter_engine(struct intel_context *ce);
> >  void intel_context_exit_engine(struct intel_context *ce);
> > diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> > index 806ad688274b..2c6a8bcef7c1 100644
> > --- a/drivers/gpu/drm/i915/i915_trace.h
> > +++ b/drivers/gpu/drm/i915/i915_trace.h
> > @@ -17,6 +17,7 @@
> >  
> >  #undef TRACE_SYSTEM
> >  #define TRACE_SYSTEM i915
> > +#undef TRACE_INCLUDE_FILE
> >  #define TRACE_INCLUDE_FILE i915_trace
> >  
> >  /* watermark/fifo updates */
> > -- 
> > 2.32.0
> > ---8<---
> > 
> > 

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Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.20 16:17:24 +0200, Christoph Hellwig wrote:
> On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
> > I'm working on below patch to resolve this. But I met a weird issue in
> > case when building i915 as module and also kvmgt module, it caused
> > busy wait on request_module("kvmgt") when boot, it doesn't happen if
> > building i915 into kernel. I'm not sure what could be the reason?
> 
> Luis, do you know if there is a problem with a request_module from
> a driver ->probe routine that is probably called by a module_init
> function itself?
> 
> In the meantime I'll try to reproduce it locally, but I always had a
> hard time getting useful results out of a modular i915, especially
> when combined with module paramters. (no blame on i915, just the problem
> with modules needed early on).
> 
> > 
> > > But the problem I see is that after moving gvt device model (gvt/*.c
> > > except kvmgt.c) into kvmgt module, we'll have issue with initial mmio
> > > state which current gvt relies on, that is in design supposed to get
> > > initial HW state before i915 driver has taken any operation.  Before
> > > we can ensure that, I think we may only remove MPT part first but
> > > still keep gvt device model as part of i915 with config. I'll try to
> > > split that out.
> > 
> > Sorry I misread the code that as we always request kvmgt module when
> > gvt init, so it should still apply original method that this isn't a
> > problem. Our current validation result has shown no regression as well.
> 
> What does initial mmio state mean?  This is something new to me.  But
> as you said in this mail unless I missed something very big it should
> work the same as before.
>

It's gvt internal track for all gfx mmio state, and yes with your current
change it should still work as before.

> > -static inline void intel_context_unpin(struct intel_context *ce)
> > +static inline void _intel_context_unpin(struct intel_context *ce)
> >  {
> >  	if (!ce->ops->sched_disable) {
> >  		__intel_context_do_unpin(ce, 1);
> > @@ -150,6 +150,7 @@ static inline void intel_context_unpin(struct intel_context *ce)
> >  		}
> >  	}
> >  }
> > +void intel_context_unpin(struct intel_context *ce);
> 
> Looking at intel_context_unpin/_intel_context_unpin is there really
> a need to have this inline to start with?  It don't see much the compiler
> could optimize by inlining it.

I'll send patch to i915 for this, and get more comments there.

thanks

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Re: refactor the i915 GVT support

[Zhenyu Wang]

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On 2021.08.20 12:56:34 -0700, Luis Chamberlain wrote:
> On Fri, Aug 20, 2021 at 04:17:24PM +0200, Christoph Hellwig wrote:
> > On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
> > > I'm working on below patch to resolve this. But I met a weird issue in
> > > case when building i915 as module and also kvmgt module, it caused
> > > busy wait on request_module("kvmgt") when boot, it doesn't happen if
> > > building i915 into kernel. I'm not sure what could be the reason?
> > 
> > Luis, do you know if there is a problem with a request_module from
> > a driver ->probe routine that is probably called by a module_init
> > function itself?
> 
> Generally no, but you can easily foot yourself in the feet by creating
> cross dependencies and not dealing with them properly. I'd make sure
> to keep module initialization as simple as possible, and run whatever
> takes more time asynchronously, then use a state machine to allow
> you to verify where you are in the initialization phase or query it
> or wait for a completion with a timeout.
> 
> It seems the code in question is getting some spring cleaning, and its
> unclear where the code is I can inspect. If there's a tree somewhere I
> can take a peak I'd be happy to review possible oddities that may stick
> out.

I tried to put current patches under test here: https://github.com/intel/gvt-linux/tree/gvt-staging
The issue can be produced with CONFIG_DRM_I915=m and CONFIG_DRM_I915_GVT_KVMGT=m.

> 
> My goto model for these sorts of problems is to abstract the issue
> *outside* of the driver in question and implement new selftests to
> try to reproduce. This serves two purposes, 1) helps with testing
> 2) may allow you to see the problem more clearly.
> 

I'll see if can abstract that.

Thanks, Luis.

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Re: refactor the i915 GVT support

[Wang, Zhi A]

Hey guys:

After some investigation, I found the root cause this problem ("i915" 
module loading will be stuck with Christoph's refactor patches), which 
can be reproduced by building both i915 and kvmgt as kernel module and 
the loading i915.

The root cause is: in Linux kernel loading, before a kernel module 
loading is finished, its symbols can not be reached by other module when 
resolving the symbols (even they can be found in /proc/kallsyms). 
Because the status of the kernel module is MODULE_STATE_COMING and 
resolve_symbol() from another kernel module will check this and return a 
-EBUSY.

In this case, before i915 loading is finished, the requested module 
"kvmgt" cannot reach the symbols in module i915. Thus it kept waiting 
and left message like below in the dmesg:

[  644.152021] kvmgt: gave up waiting for init of module i915.
[  644.152039] kvmgt: Unknown symbol i915_gem_object_set_to_cpu_domain 
(err -16)
[  674.871409] kvmgt: gave up waiting for init of module i915.
[  674.871427] kvmgt: Unknown symbol intel_ring_begin (err -16)
[  705.590586] kvmgt: gave up waiting for init of module i915.
[  705.590604] kvmgt: Unknown symbol i915_vma_move_to_active (err -16)
[  736.310230] kvmgt: gave up waiting for init of module i915.
[  736.310248] kvmgt: Unknown symbol shmem_unpin_map (err -16)
...

The error message is from execution path below:

kernel/module.c:

[i915 module loading] -> 
request_module("kvmgt")->[modprobe]->init_module("kvmgt")->load_module()->simplify_symbols()->resolve_symbol_wait():

static const struct kernel_symbol *
resolve_symbol_wait(struct module *mod,
             const struct load_info *info,
             const char *name)
{
     const struct kernel_symbol *ksym;
     char owner[MODULE_NAME_LEN];

     if (wait_event_interruptible_timeout(module_wq,
             !IS_ERR(ksym = resolve_symbol(mod, info, name, owner))
             || PTR_ERR(ksym) != -EBUSY,
                          30 * HZ) <= 0) {
         pr_warn("%s: gave up waiting for init of module %s.\n",
             mod->name, owner);

}

code: 
https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L1452

In resolve_symbol_wait(), it calls resolve_symbol() to resolve the 
symbols in "i915". In resolve_symbol() -> ref_module() -> 
strong_try_module_get(), it will check the status of the module which 
owns the symbol.

static inline int strong_try_module_get(struct module *mod)
{
     BUG_ON(mod && mod->state == MODULE_STATE_UNFORMED);
     if (mod && mod->state == MODULE_STATE_COMING)
         return -EBUSY;
     if (try_module_get(mod))
         return 0;
     else
         return -ENOENT;
}

code:https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L318

But unfortunately, this execution path begins in i915 module loading, at 
this time, the status of kernel module "i915" is MODULE_STATE_COMING 
until loading of "kvmgt" is finished. Thus a -EBUSY is always returned 
when kernel is trying to resolve symbols for "kvmgt".

This patch below might need re-work:

Author: Christoph Hellwig <hch@lst.de>
Date:   Wed Jul 21 17:53:38 2021 +0200

     drm/i915/gvt: move the gvt code into kvmgt.ko

     Instead of having an option to build the gvt code into the main i915
     module, just move it into the kvmgt.ko module.  This only requires
     a new struct with three entries that the main i915 module needs to
     request before enabling VGPU passthrough operations.

     This also conveniently streamlines the GVT initialization and avoids
     the need for the global device pointer.

     Signed-off-by: Christoph Hellwig <hch@lst.de>
     Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
     Link: 
http://patchwork.freedesktop.org/patch/msgid/20210721155355.173183-5-hch@lst.de
     Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>

On 8/26/21 6:12 AM, Zhenyu Wang wrote:
> On 2021.08.20 12:56:34 -0700, Luis Chamberlain wrote:
>> On Fri, Aug 20, 2021 at 04:17:24PM +0200, Christoph Hellwig wrote:
>>> On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
>>>> I'm working on below patch to resolve this. But I met a weird issue in
>>>> case when building i915 as module and also kvmgt module, it caused
>>>> busy wait on request_module("kvmgt") when boot, it doesn't happen if
>>>> building i915 into kernel. I'm not sure what could be the reason?
>>> Luis, do you know if there is a problem with a request_module from
>>> a driver ->probe routine that is probably called by a module_init
>>> function itself?
>> Generally no, but you can easily foot yourself in the feet by creating
>> cross dependencies and not dealing with them properly. I'd make sure
>> to keep module initialization as simple as possible, and run whatever
>> takes more time asynchronously, then use a state machine to allow
>> you to verify where you are in the initialization phase or query it
>> or wait for a completion with a timeout.
>>
>> It seems the code in question is getting some spring cleaning, and its
>> unclear where the code is I can inspect. If there's a tree somewhere I
>> can take a peak I'd be happy to review possible oddities that may stick
>> out.
> I tried to put current patches under test here: https://github.com/intel/gvt-linux/tree/gvt-staging
> The issue can be produced with CONFIG_DRM_I915=m and CONFIG_DRM_I915_GVT_KVMGT=m.
>
>> My goto model for these sorts of problems is to abstract the issue
>> *outside* of the driver in question and implement new selftests to
>> try to reproduce. This serves two purposes, 1) helps with testing
>> 2) may allow you to see the problem more clearly.
>>
> I'll see if can abstract that.
>
> Thanks, Luis.

Re: refactor the i915 GVT support

[Luis Chamberlain]

On Tue, Sep 28, 2021 at 07:41:00AM +0000, Wang, Zhi A wrote:
> Hey guys:
> 
> After some investigation, I found the root cause this problem ("i915" 
> module loading will be stuck with Christoph's refactor patches), which 
> can be reproduced by building both i915 and kvmgt as kernel module and 
> the loading i915.

Thanks for looking into this!

> The root cause is: in Linux kernel loading, before a kernel module 
> loading is finished, its symbols can not be reached by other module when 
> resolving the symbols (even they can be found in /proc/kallsyms). 
> Because the status of the kernel module is MODULE_STATE_COMING and 
> resolve_symbol() from another kernel module will check this and return a 
> -EBUSY.

Well, it would seem that way but...

> In this case, before i915 loading is finished, the requested module 
> "kvmgt" cannot reach the symbols in module i915. Thus it kept waiting 
> and left message like below in the dmesg:
> 
> [  644.152021] kvmgt: gave up waiting for init of module i915.
> [  644.152039] kvmgt: Unknown symbol i915_gem_object_set_to_cpu_domain 
> (err -16)
> [  674.871409] kvmgt: gave up waiting for init of module i915.
> [  674.871427] kvmgt: Unknown symbol intel_ring_begin (err -16)
> [  705.590586] kvmgt: gave up waiting for init of module i915.
> [  705.590604] kvmgt: Unknown symbol i915_vma_move_to_active (err -16)
> [  736.310230] kvmgt: gave up waiting for init of module i915.
> [  736.310248] kvmgt: Unknown symbol shmem_unpin_map (err -16)
> ...
> 
> The error message is from execution path below:
> 
> kernel/module.c:
> 
> [i915 module loading] -> 
> request_module("kvmgt")->[modprobe]->init_module("kvmgt")->load_module()->simplify_symbols()->resolve_symbol_wait():
> 
> static const struct kernel_symbol *
> resolve_symbol_wait(struct module *mod,
>              const struct load_info *info,
>              const char *name)
> {
>      const struct kernel_symbol *ksym;
>      char owner[MODULE_NAME_LEN];
> 
>      if (wait_event_interruptible_timeout(module_wq,
>              !IS_ERR(ksym = resolve_symbol(mod, info, name, owner))
>              || PTR_ERR(ksym) != -EBUSY,
>                           30 * HZ) <= 0) {
>          pr_warn("%s: gave up waiting for init of module %s.\n",
>              mod->name, owner);
> 
> }

Commit 9bea7f23952d5 ("module: fix bne2 "gave up waiting for init of
module libcrc32c") is worth reviewing. It dealt with a similar issue,
and in particular it addressed the issue with -EBUSY being returned
by ref_module().

And so, in theory that case should be dealt with in resolve_symbol_wait()
already. And so can you try this just to verify something:

diff --git a/kernel/module.c b/kernel/module.c
index 40ec9a030eec..98f87cbb37de 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1459,7 +1459,7 @@ resolve_symbol_wait(struct module *mod,
 	if (wait_event_interruptible_timeout(module_wq,
 			!IS_ERR(ksym = resolve_symbol(mod, info, name, owner))
 			|| PTR_ERR(ksym) != -EBUSY,
-					     30 * HZ) <= 0) {
+					     160 * HZ) <= 0) {
 		pr_warn("%s: gave up waiting for init of module %s.\n",
 			mod->name, owner);
 	}

> code: 
> https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L1452
> 
> In resolve_symbol_wait(), it calls resolve_symbol() to resolve the 
> symbols in "i915". In resolve_symbol() -> ref_module() -> 
> strong_try_module_get(), it will check the status of the module which 
> owns the symbol.
> 
> static inline int strong_try_module_get(struct module *mod)
> {
>      BUG_ON(mod && mod->state == MODULE_STATE_UNFORMED);
>      if (mod && mod->state == MODULE_STATE_COMING)
>          return -EBUSY;
>      if (try_module_get(mod))
>          return 0;
>      else
>          return -ENOENT;
> }
> 
> code:https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L318
> 
> But unfortunately, this execution path begins in i915 module loading, at 
> this time, the status of kernel module "i915" is MODULE_STATE_COMING 
> until loading of "kvmgt" is finished. Thus a -EBUSY is always returned 
> when kernel is trying to resolve symbols for "kvmgt".
>
> 
> This patch below might need re-work:

If the above test patch still fails, well.. that might be telling of
another issue which is perhaps difficult to see at first glance. If
resolve_symbol_wait() won't succeed until request_module("kvmgt")
completes and if this means having kvmgt's init routine complete, that
could end up in some longer chain or in the worst case a sort of
circular dependency which is only implicated by module loading. It'd be
really odd... but I cannot rule it out.

This is one reason I hinted that you should strive to not do much on a
module's init. If you can punt work off for later that's best.

  Luis

> 
> Author: Christoph Hellwig <hch@lst.de>
> Date:   Wed Jul 21 17:53:38 2021 +0200
> 
>      drm/i915/gvt: move the gvt code into kvmgt.ko
> 
>      Instead of having an option to build the gvt code into the main i915
>      module, just move it into the kvmgt.ko module.  This only requires
>      a new struct with three entries that the main i915 module needs to
>      request before enabling VGPU passthrough operations.
> 
>      This also conveniently streamlines the GVT initialization and avoids
>      the need for the global device pointer.
> 
>      Signed-off-by: Christoph Hellwig <hch@lst.de>
>      Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
>      Link: 
> http://patchwork.freedesktop.org/patch/msgid/20210721155355.173183-5-hch@lst.de
>      Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
> 
> On 8/26/21 6:12 AM, Zhenyu Wang wrote:
> > On 2021.08.20 12:56:34 -0700, Luis Chamberlain wrote:
> >> On Fri, Aug 20, 2021 at 04:17:24PM +0200, Christoph Hellwig wrote:
> >>> On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
> >>>> I'm working on below patch to resolve this. But I met a weird issue in
> >>>> case when building i915 as module and also kvmgt module, it caused
> >>>> busy wait on request_module("kvmgt") when boot, it doesn't happen if
> >>>> building i915 into kernel. I'm not sure what could be the reason?
> >>> Luis, do you know if there is a problem with a request_module from
> >>> a driver ->probe routine that is probably called by a module_init
> >>> function itself?
> >> Generally no, but you can easily foot yourself in the feet by creating
> >> cross dependencies and not dealing with them properly. I'd make sure
> >> to keep module initialization as simple as possible, and run whatever
> >> takes more time asynchronously, then use a state machine to allow
> >> you to verify where you are in the initialization phase or query it
> >> or wait for a completion with a timeout.
> >>
> >> It seems the code in question is getting some spring cleaning, and its
> >> unclear where the code is I can inspect. If there's a tree somewhere I
> >> can take a peak I'd be happy to review possible oddities that may stick
> >> out.
> > I tried to put current patches under test here: https://github.com/intel/gvt-linux/tree/gvt-staging
> > The issue can be produced with CONFIG_DRM_I915=m and CONFIG_DRM_I915_GVT_KVMGT=m.
> >
> >> My goto model for these sorts of problems is to abstract the issue
> >> *outside* of the driver in question and implement new selftests to
> >> try to reproduce. This serves two purposes, 1) helps with testing
> >> 2) may allow you to see the problem more clearly.
> >>
> > I'll see if can abstract that.
> >
> > Thanks, Luis.
> 
> 

Re: refactor the i915 GVT support

[Wang, Zhi A]

On 9/28/21 2:00 PM, Luis Chamberlain wrote:
> On Tue, Sep 28, 2021 at 07:41:00AM +0000, Wang, Zhi A wrote:
>> Hey guys:
>>
>> After some investigation, I found the root cause this problem ("i915"
>> module loading will be stuck with Christoph's refactor patches), which
>> can be reproduced by building both i915 and kvmgt as kernel module and
>> the loading i915.
> Thanks for looking into this!
>
>> The root cause is: in Linux kernel loading, before a kernel module
>> loading is finished, its symbols can not be reached by other module when
>> resolving the symbols (even they can be found in /proc/kallsyms).
>> Because the status of the kernel module is MODULE_STATE_COMING and
>> resolve_symbol() from another kernel module will check this and return a
>> -EBUSY.
> Well, it would seem that way but...
>
>> In this case, before i915 loading is finished, the requested module
>> "kvmgt" cannot reach the symbols in module i915. Thus it kept waiting
>> and left message like below in the dmesg:
>>
>> [  644.152021] kvmgt: gave up waiting for init of module i915.
>> [  644.152039] kvmgt: Unknown symbol i915_gem_object_set_to_cpu_domain
>> (err -16)
>> [  674.871409] kvmgt: gave up waiting for init of module i915.
>> [  674.871427] kvmgt: Unknown symbol intel_ring_begin (err -16)
>> [  705.590586] kvmgt: gave up waiting for init of module i915.
>> [  705.590604] kvmgt: Unknown symbol i915_vma_move_to_active (err -16)
>> [  736.310230] kvmgt: gave up waiting for init of module i915.
>> [  736.310248] kvmgt: Unknown symbol shmem_unpin_map (err -16)
>> ...
>>
>> The error message is from execution path below:
>>
>> kernel/module.c:
>>
>> [i915 module loading] ->
>> request_module("kvmgt")->[modprobe]->init_module("kvmgt")->load_module()->simplify_symbols()->resolve_symbol_wait():
>>
>> static const struct kernel_symbol *
>> resolve_symbol_wait(struct module *mod,
>>               const struct load_info *info,
>>               const char *name)
>> {
>>       const struct kernel_symbol *ksym;
>>       char owner[MODULE_NAME_LEN];
>>
>>       if (wait_event_interruptible_timeout(module_wq,
>>               !IS_ERR(ksym = resolve_symbol(mod, info, name, owner))
>>               || PTR_ERR(ksym) != -EBUSY,
>>                            30 * HZ) <= 0) {
>>           pr_warn("%s: gave up waiting for init of module %s.\n",
>>               mod->name, owner);
>>
>> }
> Commit 9bea7f23952d5 ("module: fix bne2 "gave up waiting for init of
> module libcrc32c") is worth reviewing. It dealt with a similar issue,
> and in particular it addressed the issue with -EBUSY being returned
> by ref_module().
>
> And so, in theory that case should be dealt with in resolve_symbol_wait()
> already. And so can you try this just to verify something:
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 40ec9a030eec..98f87cbb37de 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -1459,7 +1459,7 @@ resolve_symbol_wait(struct module *mod,
>   	if (wait_event_interruptible_timeout(module_wq,
>   			!IS_ERR(ksym = resolve_symbol(mod, info, name, owner))
>   			|| PTR_ERR(ksym) != -EBUSY,
> -					     30 * HZ) <= 0) {
> +					     160 * HZ) <= 0) {
>   		pr_warn("%s: gave up waiting for init of module %s.\n",
>   			mod->name, owner);
>   	}
>
Hi Luis:

Thanks so much for the reply and patch.:)

I am afraid that this patch wouldn't work in this case as the 
request_module("kvmgt") happens in the init_module of i915. Before the 
initialization path is finished in i915, the i915 symbols are not 
available to be referenced. Unfortunately, It's matter of sequence, not 
of delay. :(

>> code:
>> https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L1452
>>
>> In resolve_symbol_wait(), it calls resolve_symbol() to resolve the
>> symbols in "i915". In resolve_symbol() -> ref_module() ->
>> strong_try_module_get(), it will check the status of the module which
>> owns the symbol.
>>
>> static inline int strong_try_module_get(struct module *mod)
>> {
>>       BUG_ON(mod && mod->state == MODULE_STATE_UNFORMED);
>>       if (mod && mod->state == MODULE_STATE_COMING)
>>           return -EBUSY;
>>       if (try_module_get(mod))
>>           return 0;
>>       else
>>           return -ENOENT;
>> }
>>
>> code:https://github.com/intel/gvt-linux/blob/bd950a66c7919d7121d2530f30984351534a96dc/kernel/module.c#L318
>>
>> But unfortunately, this execution path begins in i915 module loading, at
>> this time, the status of kernel module "i915" is MODULE_STATE_COMING
>> until loading of "kvmgt" is finished. Thus a -EBUSY is always returned
>> when kernel is trying to resolve symbols for "kvmgt".
>>
>>
>> This patch below might need re-work:
> If the above test patch still fails, well.. that might be telling of
> another issue which is perhaps difficult to see at first glance. If
> resolve_symbol_wait() won't succeed until request_module("kvmgt")
> completes and if this means having kvmgt's init routine complete, that
> could end up in some longer chain or in the worst case a sort of
> circular dependency which is only implicated by module loading. It'd be
> really odd... but I cannot rule it out.
>
> This is one reason I hinted that you should strive to not do much on a
> module's init. If you can punt work off for later that's best.

Yes. I was thinking of the possibility of putting off some work later so 
that we don't need to make a lot of changes. GVT-g needs to take a 
snapshot of GPU registers as the initial virtual states for other vGPUs, 
which requires the initialization happens at a certain early time of 
initialization of i915. I was thinking maybe we can take other patches 
from Christoph like "de-virtualize*" except this one because currently 
we have to maintain a TEST-ONLY patch on our tree to prevent i915 built 
as kernel module.

Zhi.

>    Luis
>
>> Author: Christoph Hellwig <hch@lst.de>
>> Date:   Wed Jul 21 17:53:38 2021 +0200
>>
>>       drm/i915/gvt: move the gvt code into kvmgt.ko
>>
>>       Instead of having an option to build the gvt code into the main i915
>>       module, just move it into the kvmgt.ko module.  This only requires
>>       a new struct with three entries that the main i915 module needs to
>>       request before enabling VGPU passthrough operations.
>>
>>       This also conveniently streamlines the GVT initialization and avoids
>>       the need for the global device pointer.
>>
>>       Signed-off-by: Christoph Hellwig <hch@lst.de>
>>       Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
>>       Link:
>> http://patchwork.freedesktop.org/patch/msgid/20210721155355.173183-5-hch@lst.de
>>       Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
>>
>> On 8/26/21 6:12 AM, Zhenyu Wang wrote:
>>> On 2021.08.20 12:56:34 -0700, Luis Chamberlain wrote:
>>>> On Fri, Aug 20, 2021 at 04:17:24PM +0200, Christoph Hellwig wrote:
>>>>> On Thu, Aug 19, 2021 at 04:29:29PM +0800, Zhenyu Wang wrote:
>>>>>> I'm working on below patch to resolve this. But I met a weird issue in
>>>>>> case when building i915 as module and also kvmgt module, it caused
>>>>>> busy wait on request_module("kvmgt") when boot, it doesn't happen if
>>>>>> building i915 into kernel. I'm not sure what could be the reason?
>>>>> Luis, do you know if there is a problem with a request_module from
>>>>> a driver ->probe routine that is probably called by a module_init
>>>>> function itself?
>>>> Generally no, but you can easily foot yourself in the feet by creating
>>>> cross dependencies and not dealing with them properly. I'd make sure
>>>> to keep module initialization as simple as possible, and run whatever
>>>> takes more time asynchronously, then use a state machine to allow
>>>> you to verify where you are in the initialization phase or query it
>>>> or wait for a completion with a timeout.
>>>>
>>>> It seems the code in question is getting some spring cleaning, and its
>>>> unclear where the code is I can inspect. If there's a tree somewhere I
>>>> can take a peak I'd be happy to review possible oddities that may stick
>>>> out.
>>> I tried to put current patches under test here: https://github.com/intel/gvt-linux/tree/gvt-staging
>>> The issue can be produced with CONFIG_DRM_I915=m and CONFIG_DRM_I915_GVT_KVMGT=m.
>>>
>>>> My goto model for these sorts of problems is to abstract the issue
>>>> *outside* of the driver in question and implement new selftests to
>>>> try to reproduce. This serves two purposes, 1) helps with testing
>>>> 2) may allow you to see the problem more clearly.
>>>>
>>> I'll see if can abstract that.
>>>
>>> Thanks, Luis.
>>

Re: refactor the i915 GVT support

[Jason Gunthorpe]

On Tue, Sep 28, 2021 at 02:35:06PM +0000, Wang, Zhi A wrote:

> Yes. I was thinking of the possibility of putting off some work later so 
> that we don't need to make a lot of changes. GVT-g needs to take a 
> snapshot of GPU registers as the initial virtual states for other vGPUs, 
> which requires the initialization happens at a certain early time of 
> initialization of i915. I was thinking maybe we can take other patches 
> from Christoph like "de-virtualize*" except this one because currently 
> we have to maintain a TEST-ONLY patch on our tree to prevent i915 built 
> as kernel module.

How about just capture these registers in the main module/device and
not try so hard to isolate it to the gvt stuff?

Jason

Re: refactor the i915 GVT support

[Wang, Zhi A]

On 9/28/21 3:05 PM, Jason Gunthorpe wrote:
> On Tue, Sep 28, 2021 at 02:35:06PM +0000, Wang, Zhi A wrote:
>
>> Yes. I was thinking of the possibility of putting off some work later so
>> that we don't need to make a lot of changes. GVT-g needs to take a
>> snapshot of GPU registers as the initial virtual states for other vGPUs,
>> which requires the initialization happens at a certain early time of
>> initialization of i915. I was thinking maybe we can take other patches
>> from Christoph like "de-virtualize*" except this one because currently
>> we have to maintain a TEST-ONLY patch on our tree to prevent i915 built
>> as kernel module.
> How about just capture these registers in the main module/device and
> not try so hard to isolate it to the gvt stuff?

Hi Jason:

Thanks for the idea. I am not sure i915 guys would take this idea since 
that it's only for GVT-g, i915 doesn't use this at all. We need to take 
a snapshot of both PCI configuration space and MMIO registers before 
i915 driver starts to touch the HW.

One idea is we capture the registers in intel_gvt.c during the early 
initialization and do everything else when i915.ko is fully loaded. But 
how about dependence between i915.ko and kvmgt.ko? We cannot do 
request_module("kvmgt") in i915.ko.

Maybe Christoph can give more input on this and how we can deal with 
this? Before we figure out an solution, we have to pick that patch out 
since it blocks our pull request schedule.

Also I was thinking if moving gvt into kvmgt.ko is the right direction. 
It seems the module loading system in kernel is not designed for "module 
A loading module B, which needs symbols from module A, in the 
initialization path of module A".

Thanks,

Zhi.

> Jason

Re: refactor the i915 GVT support

[Jason Gunthorpe]

On Wed, Sep 29, 2021 at 06:27:16PM +0000, Wang, Zhi A wrote:
> On 9/28/21 3:05 PM, Jason Gunthorpe wrote:
> > On Tue, Sep 28, 2021 at 02:35:06PM +0000, Wang, Zhi A wrote:
> >
> >> Yes. I was thinking of the possibility of putting off some work later so
> >> that we don't need to make a lot of changes. GVT-g needs to take a
> >> snapshot of GPU registers as the initial virtual states for other vGPUs,
> >> which requires the initialization happens at a certain early time of
> >> initialization of i915. I was thinking maybe we can take other patches
> >> from Christoph like "de-virtualize*" except this one because currently
> >> we have to maintain a TEST-ONLY patch on our tree to prevent i915 built
> >> as kernel module.
> > How about just capture these registers in the main module/device and
> > not try so hard to isolate it to the gvt stuff?
> 
> Hi Jason:
> 
> Thanks for the idea. I am not sure i915 guys would take this idea since 
> that it's only for GVT-g, i915 doesn't use this at all. We need to take 
> a snapshot of both PCI configuration space and MMIO registers before 
> i915 driver starts to touch the HW.

Given the code is already linked into i915 I don't see there is much
to object to here. It can remain conditional on the kernel parameter
as today.

As a general philosophy this would all be much less strange if the
mdev .ko is truely optional. It should be cleanly seperate from its
base device and never request_module'd..

In this case auxiliary device might be a good option, have i915 create
one and the mdev module be loaded against it.

In the mean time is there some shortcut to get this series to move
ahead? Is patch 4 essential to the rest of the series?

A really awful hack would be to push the pci_driver_register into a
WQ so that the request_module is guarenteed to not be part of the
module_init callchain.

> Also I was thinking if moving gvt into kvmgt.ko is the right direction. 
> It seems the module loading system in kernel is not designed for "module 
> A loading module B, which needs symbols from module A, in the 
> initialization path of module A".

Of course not, that is a circular module dependency, it should not be
that way. The SW layers need to be clean and orderly - meaning the
i915 module needs to have the minimal amount of code to support the
mdev module.

Jason

Re: refactor the i915 GVT support

[Wang, Zhi A]

On 9/29/21 6:55 PM, Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 06:27:16PM +0000, Wang, Zhi A wrote:
>> On 9/28/21 3:05 PM, Jason Gunthorpe wrote:
>>> On Tue, Sep 28, 2021 at 02:35:06PM +0000, Wang, Zhi A wrote:
>>>
>>>> Yes. I was thinking of the possibility of putting off some work later so
>>>> that we don't need to make a lot of changes. GVT-g needs to take a
>>>> snapshot of GPU registers as the initial virtual states for other vGPUs,
>>>> which requires the initialization happens at a certain early time of
>>>> initialization of i915. I was thinking maybe we can take other patches
>>>> from Christoph like "de-virtualize*" except this one because currently
>>>> we have to maintain a TEST-ONLY patch on our tree to prevent i915 built
>>>> as kernel module.
>>> How about just capture these registers in the main module/device and
>>> not try so hard to isolate it to the gvt stuff?
>> Hi Jason:
>>
>> Thanks for the idea. I am not sure i915 guys would take this idea since
>> that it's only for GVT-g, i915 doesn't use this at all. We need to take
>> a snapshot of both PCI configuration space and MMIO registers before
>> i915 driver starts to touch the HW.
> Given the code is already linked into i915 I don't see there is much
> to object to here. It can remain conditional on the kernel parameter
> as today.
>
> As a general philosophy this would all be much less strange if the
> mdev .ko is truely optional. It should be cleanly seperate from its
> base device and never request_module'd..
>
> In this case auxiliary device might be a good option, have i915 create
> one and the mdev module be loaded against it.
>
> In the mean time is there some shortcut to get this series to move
> ahead? Is patch 4 essential to the rest of the series?
>
> A really awful hack would be to push the pci_driver_register into a
> WQ so that the request_module is guarenteed to not be part of the
> module_init callchain.

Hi Jason and folks:

Thanks so much for the ideas. That sounds great and I was keeping 
thinking how to make progress on this. How about we do like this: We 
don't do request_module("kvmgt") in i915.ko, which resolves the circular 
module dependency. We keep the code of doing snapshot of registers in 
intel_gvt.c. When i915.enable_gvt=1, we do the snapshot. Then we export 
functions for kvmgt.ko in intel_gvt.c to check if gvt in i915 is enabled 
or not and get the snapshots.

How does that sounds? I just need to write another patch and put it on 
top of Christoph's series.

Thanks,

Zhi.

>> Also I was thinking if moving gvt into kvmgt.ko is the right direction.
>> It seems the module loading system in kernel is not designed for "module
>> A loading module B, which needs symbols from module A, in the
>> initialization path of module A".
> Of course not, that is a circular module dependency, it should not be
> that way. The SW layers need to be clean and orderly - meaning the
> i915 module needs to have the minimal amount of code to support the
> mdev module.
>
> Jason

Re: refactor the i915 GVT support

[Wang, Zhi A]

Hi folks:

It seems we haven't reached a possible solution of this refactor patch 
series. The current patch series needs to be re-worked because of the 
module/symbol dependency(The root cause has been discussed in another 
email). I have to get them off from our gvt-next repo so that we can 
continue our development and pull-request to upstream. Thanks so much 
for the patch and the discussion.

Thanks,
Zhi.

On 10/1/21 1:01 PM, Wang, Zhi A wrote:
> On 9/29/21 6:55 PM, Jason Gunthorpe wrote:
>> On Wed, Sep 29, 2021 at 06:27:16PM +0000, Wang, Zhi A wrote:
>>> On 9/28/21 3:05 PM, Jason Gunthorpe wrote:
>>>> On Tue, Sep 28, 2021 at 02:35:06PM +0000, Wang, Zhi A wrote:
>>>>
>>>>> Yes. I was thinking of the possibility of putting off some work 
>>>>> later so
>>>>> that we don't need to make a lot of changes. GVT-g needs to take a
>>>>> snapshot of GPU registers as the initial virtual states for other 
>>>>> vGPUs,
>>>>> which requires the initialization happens at a certain early time of
>>>>> initialization of i915. I was thinking maybe we can take other 
>>>>> patches
>>>>> from Christoph like "de-virtualize*" except this one because 
>>>>> currently
>>>>> we have to maintain a TEST-ONLY patch on our tree to prevent i915 
>>>>> built
>>>>> as kernel module.
>>>> How about just capture these registers in the main module/device and
>>>> not try so hard to isolate it to the gvt stuff?
>>> Hi Jason:
>>>
>>> Thanks for the idea. I am not sure i915 guys would take this idea since
>>> that it's only for GVT-g, i915 doesn't use this at all. We need to take
>>> a snapshot of both PCI configuration space and MMIO registers before
>>> i915 driver starts to touch the HW.
>> Given the code is already linked into i915 I don't see there is much
>> to object to here. It can remain conditional on the kernel parameter
>> as today.
>>
>> As a general philosophy this would all be much less strange if the
>> mdev .ko is truely optional. It should be cleanly seperate from its
>> base device and never request_module'd..
>>
>> In this case auxiliary device might be a good option, have i915 create
>> one and the mdev module be loaded against it.
>>
>> In the mean time is there some shortcut to get this series to move
>> ahead? Is patch 4 essential to the rest of the series?
>>
>> A really awful hack would be to push the pci_driver_register into a
>> WQ so that the request_module is guarenteed to not be part of the
>> module_init callchain.
>
> Hi Jason and folks:
>
> Thanks so much for the ideas. That sounds great and I was keeping 
> thinking how to make progress on this. How about we do like this: We 
> don't do request_module("kvmgt") in i915.ko, which resolves the 
> circular module dependency. We keep the code of doing snapshot of 
> registers in intel_gvt.c. When i915.enable_gvt=1, we do the snapshot. 
> Then we export functions for kvmgt.ko in intel_gvt.c to check if gvt 
> in i915 is enabled or not and get the snapshots.
>
> How does that sounds? I just need to write another patch and put it on 
> top of Christoph's series.
>
> Thanks,
>
> Zhi.
>
>>> Also I was thinking if moving gvt into kvmgt.ko is the right direction.
>>> It seems the module loading system in kernel is not designed for 
>>> "module
>>> A loading module B, which needs symbols from module A, in the
>>> initialization path of module A".
>> Of course not, that is a circular module dependency, it should not be
>> that way. The SW layers need to be clean and orderly - meaning the
>> i915 module needs to have the minimal amount of code to support the
>> mdev module.
>>
>> Jason
>
>