Re: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing to VMs

["Jacob Pan (Jun)" <jacob.jun.pan@intel.com>]

Hi Jason,
On Thu, 17 Sep 2020 11:53:49 +0800, Jason Wang <jasowang@redhat.com>
wrote:

> On 2020/9/17 上午7:09, Jacob Pan (Jun) wrote:
> > Hi Jason,
> > On Wed, 16 Sep 2020 15:38:41 -0300, Jason Gunthorpe <jgg@nvidia.com>
> > wrote:
> >  
> >> On Wed, Sep 16, 2020 at 11:21:10AM -0700, Jacob Pan (Jun) wrote:  
> >>> Hi Jason,
> >>> On Wed, 16 Sep 2020 14:01:13 -0300, Jason Gunthorpe
> >>> <jgg@nvidia.com> wrote:
> >>>      
> >>>> On Wed, Sep 16, 2020 at 09:33:43AM -0700, Raj, Ashok wrote:  
> >>>>> On Wed, Sep 16, 2020 at 12:07:54PM -0300, Jason Gunthorpe
> >>>>> wrote:  
> >>>>>> On Tue, Sep 15, 2020 at 05:22:26PM -0700, Jacob Pan (Jun)
> >>>>>> wrote:  
> >>>>>>>> If user space wants to bind page tables, create the PASID
> >>>>>>>> with /dev/sva, use ioctls there to setup the page table
> >>>>>>>> the way it wants, then pass the now configured PASID to a
> >>>>>>>> driver that can use it.  
> >>>>>>> Are we talking about bare metal SVA?  
> >>>>>> What a weird term.  
> >>>>> Glad you noticed it at v7 :-)
> >>>>>
> >>>>> Any suggestions on something less weird than
> >>>>> Shared Virtual Addressing? There is a reason why we moved from
> >>>>> SVM to SVA.  
> >>>> SVA is fine, what is "bare metal" supposed to mean?
> >>>>      
> >>> What I meant here is sharing virtual address between DMA and host
> >>> process. This requires devices perform DMA request with PASID and
> >>> use IOMMU first level/stage 1 page tables.
> >>> This can be further divided into 1) user SVA 2) supervisor SVA
> >>> (sharing init_mm)
> >>>
> >>> My point is that /dev/sva is not useful here since the driver can
> >>> perform PASID allocation while doing SVA bind.  
> >> No, you are thinking too small.
> >>
> >> Look at VDPA, it has a SVA uAPI. Some HW might use PASID for the
> >> SVA. 
> > Could you point to me the SVA UAPI? I couldn't find it in the
> > mainline. Seems VDPA uses VHOST interface?  
> 
> 
> It's the vhost_iotlb_msg defined in uapi/linux/vhost_types.h.
> 
Thanks for the pointer, for complete vSVA functionality we would need
1 TLB flush (IOTLB and PASID cache etc.)
2 PASID alloc/free
3 bind/unbind page tables or PASID tables
4 Page request service

Seems vhost_iotlb_msg can be used for #1 partially. And the
proposal is to pluck out the rest into /dev/sda? Seems awkward as Alex
pointed out earlier for similar situation in VFIO.

> 
> >  
> >> When VDPA is used by DPDK it makes sense that the PASID will be SVA
> >> and 1:1 with the mm_struct.
> >>  
> > I still don't see why bare metal DPDK needs to get a handle of the
> > PASID.  
> 
> 
> My understanding is that it may:
> 
> - have a unified uAPI with vSVA: alloc, bind, unbind, free
Got your point, but vSVA needs more than these

> - leave the binding policy to userspace instead of the using a
> implied one in the kenrel
> 
Only if necessary.

> 
> > Perhaps the SVA patch would explain. Or are you talking about
> > vDPA DPDK process that is used to support virtio-net-pmd in the
> > guest? 
> >> When VDPA is used by qemu it makes sense that the PASID will be an
> >> arbitary IOVA map constructed to be 1:1 with the guest vCPU
> >> physical map. /dev/sva allows a single uAPI to do this kind of
> >> setup, and qemu can support it while supporting a range of SVA
> >> kernel drivers. VDPA and vfio-mdev are obvious initial targets.
> >>
> >> *BOTH* are needed.
> >>
> >> In general any uAPI for PASID should have the option to use either
> >> the mm_struct SVA PASID *OR* a PASID from /dev/sva. It costs
> >> virtually nothing to implement this in the driver as PASID is just
> >> a number, and gives so much more flexability.
> >>  
> > Not really nothing in terms of PASID life cycles. For example, if
> > user uses uacce interface to open an accelerator, it gets an
> > FD_acc. Then it opens /dev/sva to allocate PASID then get another
> > FD_pasid. Then we pass FD_pasid to the driver to bind page tables,
> > perhaps multiple drivers. Now we have to worry about If FD_pasid
> > gets closed before FD_acc(s) closed and all these race conditions.  
> 
> 
> I'm not sure I understand this. But this demonstrates the flexibility
> of an unified uAPI. E.g it allows vDPA and VFIO device to use the
> same PAISD which can be shared with a process in the guest.
> 
This is for user DMA not for vSVA. I was contending that /dev/sva
creates unnecessary steps for such usage.

For vSVA, I think vDPA and VFIO can potentially share but I am not
seeing convincing benefits.

If a guest process wants to do SVA with a VFIO assigned device and a
vDPA-backed virtio-net at the same time, it might be a limitation if
PASID is not managed via a common interface. But I am not sure how vDPA
SVA support will look like, does it support gIOVA? need virtio IOMMU?

> For the race condition, it could be probably solved with refcnt.
> 
Agreed but the best solution might be not to have the problem in the
first place :)

> Thanks
> 
> 
> >
> > If we do not expose FD_pasid to the user, the teardown is much
> > simpler and streamlined. Following each FD_acc close, PASID unbind
> > is performed. 
> >>> Yi can correct me but this set is is about VFIO-PCI, VFIO-mdev
> >>> will be introduced later.  
> >> Last patch is:
> >>
> >>    vfio/type1: Add vSVA support for IOMMU-backed mdevs
> >>
> >> So pretty hard to see how this is not about vfio-mdev, at least a
> >> little..
> >>
> >> Jason  
> >
> > Thanks,
> >
> > Jacob
> >  
> 


Thanks,

Jacob
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