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From: Jason Wang <jasowang@redhat.com>
To: "Tian, Kevin" <kevin.tian@intel.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>
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Subject: Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver
Date: Wed, 12 Aug 2020 11:28:12 +0800
Message-ID: <b59ce5b0-5530-1f30-9852-409f7c9f630a@redhat.com> (raw)
In-Reply-To: <MWHPR11MB16452EBE866E330A7E000AFC8C440@MWHPR11MB1645.namprd11.prod.outlook.com>


On 2020/8/10 下午3:32, Tian, Kevin wrote:
>> From: Jason Gunthorpe <jgg@nvidia.com>
>> Sent: Friday, August 7, 2020 8:20 PM
>>
>> On Wed, Aug 05, 2020 at 07:22:58PM -0600, Alex Williamson wrote:
>>
>>> If you see this as an abuse of the framework, then let's identify those
>>> specific issues and come up with a better approach.  As we've discussed
>>> before, things like basic PCI config space emulation are acceptable
>>> overhead and low risk (imo) and some degree of register emulation is
>>> well within the territory of an mdev driver.
>> What troubles me is that idxd already has a direct userspace interface
>> to its HW, and does userspace DMA. The purpose of this mdev is to
>> provide a second direct userspace interface that is a little different
>> and trivially plugs into the virtualization stack.
> No. Userspace DMA and subdevice passthrough (what mdev provides)
> are two distinct usages IMO (at least in idxd context). and this might
> be the main divergence between us, thus let me put more words here.
> If we could reach consensus in this matter, which direction to go
> would be clearer.
>
> First, a passthrough interface requires some unique requirements
> which are not commonly observed in an userspace DMA interface, e.g.:
>
> - Tracking DMA dirty pages for live migration;
> - A set of interfaces for using SVA inside guest;
> 	* PASID allocation/free (on some platforms);
> 	* bind/unbind guest mm/page table (nested translation);
> 	* invalidate IOMMU cache/iotlb for guest page table changes;
> 	* report page request from device to guest;
> 	* forward page response from guest to device;
> - Configuring irqbypass for posted interrupt;
> - ...
>
> Second, a passthrough interface requires delegating raw controllability
> of subdevice to guest driver, while the same delegation might not be
> required for implementing an userspace DMA interface (especially for
> modern devices which support SVA). For example, idxd allows following
> setting per wq (guest driver may configure them in any combination):
> 	- put in dedicated or shared mode;
> 	- enable/disable SVA;
> 	- Associate guest-provided PASID to MSI/IMS entry;
> 	- set threshold;
> 	- allow/deny privileged access;
> 	- allocate/free interrupt handle (enlightened for guest);
> 	- collect error status;
> 	- ...
>
> We plan to support idxd userspace DMA with SVA. The driver just needs
> to prepare a wq with a predefined configuration (e.g. shared, SVA,
> etc.), bind the process mm to IOMMU (non-nested) and then map
> the portal to userspace. The goal that userspace can do DMA to
> associated wq doesn't change the fact that the wq is still *owned*
> and *controlled* by kernel driver. However as far as passthrough
> is concerned, the wq is considered 'owned' by the guest driver thus
> we need an interface which can support low-level *controllability*
> from guest driver. It is sort of a mess in uAPI when mixing the
> two together.


So for userspace drivers like DPDK, it can use both of the two uAPIs?


>
> Based on above two reasons, we see distinct requirements between
> userspace DMA and passthrough interfaces, at least in idxd context
> (though other devices may have less distinction in-between). Therefore,
> we didn't see the value/necessity of reinventing the wheel that mdev
> already handles well to evolve an simple application-oriented usespace
> DMA interface to a complex guest-driver-oriented passthrough interface.
> The complexity of doing so would incur far more kernel-side changes
> than the portion of emulation code that you've been concerned about...
>   
>> I don't think VFIO should be the only entry point to
>> virtualization. If we say the universe of devices doing user space DMA
>> must also implement a VFIO mdev to plug into virtualization then it
>> will be alot of mdevs.
> Certainly VFIO will not be the only entry point. and This has to be a
> case-by-case decision.


The problem is that if we tie all controls via VFIO uAPI, the other 
subsystem like vDPA is likely to duplicate them. I wonder if there is a 
way to decouple the vSVA out of VFIO uAPI?


>   If an userspace DMA interface can be easily
> adapted to be a passthrough one, it might be the choice.


It's not that easy even for VFIO which requires a lot of new uAPIs and 
infrastructures(e.g mdev) to be invented.


> But for idxd,
> we see mdev a much better fit here, given the big difference between
> what userspace DMA requires and what guest driver requires in this hw.


A weak point for mdev is that it can't serve kernel subsystem other than 
VFIO. In this case, you need some other infrastructures (like [1]) to do 
this.

(For idxd, you probably don't need this, but it's pretty common in the 
case of networking or storage device.)

Thanks

[1] https://patchwork.kernel.org/patch/11280547/


>
>> I would prefer to see that the existing userspace interface have the
>> extra needed bits for virtualization (eg by having appropriate
>> internal kernel APIs to make this easy) and all the emulation to build
>> the synthetic PCI device be done in userspace.
> In the end what decides the direction is the amount of changes that
> we have to put in kernel, not whether we call it 'emulation'. For idxd,
> adding special passthrough requirements (guest SVA, dirty tracking,
> etc.) and raw controllability to the simple userspace DMA interface
> is for sure making kernel more complex than reusing the mdev
> framework (plus some degree of emulation mockup behind). Not to
> mention the merit of uAPI compatibility with mdev...
>
> Thanks
> Kevin
>


  parent reply index

Thread overview: 97+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-21 16:02 Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 01/18] platform-msi: Introduce platform_msi_ops Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 02/18] irq/dev-msi: Add support for a new DEV_MSI irq domain Dave Jiang
2020-07-21 16:13   ` Jason Gunthorpe
2020-07-22 16:50     ` Dey, Megha
2020-07-22 18:52   ` Marc Zyngier
2020-07-22 19:59     ` Jason Gunthorpe
2020-07-23  8:51       ` Marc Zyngier
2020-07-24  0:16         ` Jason Gunthorpe
2020-07-24  0:36           ` Thomas Gleixner
2020-08-05 19:18       ` Dey, Megha
2020-08-05 22:15         ` Jason Gunthorpe
2020-08-05 22:36           ` Dey, Megha
2020-08-05 22:53             ` Jason Gunthorpe
2020-08-06  0:13               ` Dey, Megha
2020-08-06  0:19                 ` Jason Gunthorpe
2020-08-06  0:32                   ` Dey, Megha
2020-08-06  0:46                     ` Jason Gunthorpe
2020-08-06 17:10                     ` Thomas Gleixner
2020-08-06 17:58                       ` Dey, Megha
2020-08-06 20:21                         ` Thomas Gleixner
2020-08-06 22:27                           ` Dey, Megha
2020-08-07  8:48                             ` Thomas Gleixner
2020-08-07 12:06                           ` Jason Gunthorpe
2020-08-07 12:38                             ` gregkh
2020-08-07 13:34                               ` Jason Gunthorpe
2020-08-07 16:47                                 ` Thomas Gleixner
2020-08-07 17:54                                   ` Dey, Megha
2020-08-07 18:39                                     ` Jason Gunthorpe
2020-08-07 20:31                                       ` Dey, Megha
2020-08-08 19:47                                     ` Thomas Gleixner
2020-08-10 21:46                                       ` Thomas Gleixner
2020-08-11  9:53                                         ` Thomas Gleixner
2020-08-11 18:46                                           ` Dey, Megha
2020-08-11 21:25                                             ` Thomas Gleixner
2020-08-11 18:39                                       ` Dey, Megha
2020-08-11 22:39                                         ` Thomas Gleixner
2020-08-07 15:22                             ` Thomas Gleixner
2020-08-05 18:55     ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 03/18] irq/dev-msi: Create IR-DEV-MSI " Dave Jiang
2020-07-21 16:21   ` Jason Gunthorpe
2020-07-22 17:03     ` Dey, Megha
2020-07-22 17:33       ` Jason Gunthorpe
2020-07-22 20:44   ` Thomas Gleixner
2020-08-05 19:02     ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 04/18] irq/dev-msi: Introduce APIs to allocate/free dev-msi interrupts Dave Jiang
2020-07-21 16:25   ` Jason Gunthorpe
2020-07-22 17:05     ` Dey, Megha
2020-07-22 17:35       ` Jason Gunthorpe
2020-08-05 20:19         ` Dey, Megha
2020-07-21 16:02 ` [PATCH RFC v2 05/18] dmaengine: idxd: add support for readonly config devices Dave Jiang
2020-07-21 16:02 ` [PATCH RFC v2 06/18] dmaengine: idxd: add interrupt handle request support Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 07/18] dmaengine: idxd: add DEV-MSI support in base driver Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 08/18] dmaengine: idxd: add device support functions in prep for mdev Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 09/18] dmaengine: idxd: add basic mdev registration and helper functions Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 10/18] dmaengine: idxd: add emulation rw routines Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 11/18] dmaengine: idxd: prep for virtual device commands Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 12/18] dmaengine: idxd: virtual device commands emulation Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 13/18] dmaengine: idxd: ims setup for the vdcm Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 14/18] dmaengine: idxd: add mdev type as a new wq type Dave Jiang
2020-07-21 16:03 ` [PATCH RFC v2 15/18] dmaengine: idxd: add dedicated wq mdev type Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 16/18] dmaengine: idxd: add new wq state for mdev Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 17/18] dmaengine: idxd: add error notification from host driver to mediated device Dave Jiang
2020-07-21 16:04 ` [PATCH RFC v2 18/18] dmaengine: idxd: add ABI documentation for mediated device support Dave Jiang
2020-07-21 16:28 ` [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver Greg KH
2020-07-21 17:17   ` Dave Jiang
2020-07-21 21:35   ` Dan Williams
2020-07-21 16:45 ` Jason Gunthorpe
2020-07-21 18:00   ` Dave Jiang
2020-07-22 17:31     ` Dey, Megha
2020-07-22 18:16       ` Jason Gunthorpe
2020-07-21 23:54   ` Tian, Kevin
2020-07-24  0:19     ` Jason Gunthorpe
2020-08-06  1:22       ` Alex Williamson
2020-08-07 12:19         ` Jason Gunthorpe
2020-08-10  7:32           ` Tian, Kevin
2020-08-11 17:00             ` Alex Williamson
2020-08-12  1:58               ` Tian, Kevin
2020-08-12  2:36                 ` Alex Williamson
2020-08-12  3:35                   ` Tian, Kevin
2020-08-12  3:28             ` Jason Wang [this message]
2020-08-12  4:05               ` Tian, Kevin
2020-08-13  4:33                 ` Jason Wang
2020-08-13  5:26                   ` Tian, Kevin
2020-08-13  6:01                     ` Jason Wang
2020-08-14 13:23                       ` Jason Gunthorpe
2020-08-17  2:24                         ` Tian, Kevin
2020-08-14 13:35             ` Jason Gunthorpe
2020-08-17  2:12               ` Tian, Kevin
2020-08-18  0:43                 ` Jason Gunthorpe
2020-08-18  1:09                   ` Tian, Kevin
2020-08-18 11:50                     ` Jason Gunthorpe
2020-08-18 16:27                       ` Paolo Bonzini
2020-08-18 16:49                         ` Jason Gunthorpe
2020-08-18 17:05                           ` Paolo Bonzini
2020-08-18 17:18                             ` Jason Gunthorpe
2020-08-19  7:29                       ` Tian, Kevin

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