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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Mon, 01 Feb 2016 14:10:46 +0100
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Subject: Re: [Qemu-devel] [iGVT-g] VFIO based vGPU(was Re: [Announcement]
 2015-Q3 release of XenGT - a Mediated ...)
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To: Alex Williamson <alex.williamson@redhat.com>
Cc: Yang Zhang <yang.zhang.wz@gmail.com>, "igvt-g@lists.01.org" <igvt-g@ml01.01.org>, Jike Song <jike.song@intel.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>

  Hi,

> > Unfortunately it's not the only one. Another example is, device-model
> > may want to write-protect a gfn (RAM). In case that this request goes
> > to VFIO .. how it is supposed to reach KVM MMU?
>=20
> Well, let's work through the problem.  How is the GFN related to the
> device?  Is this some sort of page table for device mappings with a base
> register in the vgpu hardware?

IIRC this is needed to make sure the guest can't bypass execbuffer
verification and works like this:

  (1) guest submits execbuffer.
  (2) host makes execbuffer readonly for the guest
  (3) verify the buffer (make sure it only accesses resources owned by
      the vm).
  (4) pass on execbuffer to the hardware.
  (5) when the gpu is done with it make the execbuffer writable again.

cheers,
  Gerd