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Date:   Thu, 17 Sep 2020 14:31:07 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Jason Wang <jasowang@redhat.com>
CC:     "Jacob Pan (Jun)" <jacob.jun.pan@intel.com>,
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        <kvm@vger.kernel.org>, "Michael S. Tsirkin" <mst@redhat.com>,
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Subject: Re: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing
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On Thu, Sep 17, 2020 at 11:53:49AM +0800, Jason Wang wrote:
> > > 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.
> 
> For the race condition, it could be probably solved with refcnt.

Yep

Jason

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Date: Thu, 17 Sep 2020 14:31:07 -0300
From: Jason Gunthorpe <jgg@nvidia.com>
To: Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH v7 00/16] vfio: expose virtual Shared Virtual Addressing
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On Thu, Sep 17, 2020 at 11:53:49AM +0800, Jason Wang wrote:
> > > 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.
> 
> For the race condition, it could be probably solved with refcnt.

Yep

Jason
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