Re: [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated Graphics Passthrough Solution from Intel

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > Another area of extension is how to expose a framebuffer to QEMU for
> > seamless integration into a SPICE/VNC channel.  For this I believe we
> > could use a new region, much like we've done to expose VGA access
> > through a vfio device file descriptor.  An area within this new
> > framebuffer region could be directly mappable in QEMU while a
> > non-mappable page, at a standard location with standardized format,
> > provides a description of framebuffer and potentially even a
> > communication channel to synchronize framebuffer captures.  This would
> > be new code for QEMU, but something we could share among all vGPU
> > implementations.
> 
> Now GVT-g already provides an interface to decode framebuffer information,
> w/ an assumption that the framebuffer will be further composited into 
> OpenGL APIs.

Can I have a pointer to docs / code?

iGVT-g_Setup_Guide.txt mentions a "Indirect Display Mode", but doesn't
explain how the guest framebuffer can be accessed then.

> So the format is defined according to OpenGL definition.
> Does that meet SPICE requirement?

Yes and no ;)

Some more background:  We basically have two rendering paths in qemu.
The classic one, without opengl, and a new, still emerging one, using
opengl and dma-bufs (gtk support merged for qemu 2.5, sdl2 support will
land in 2.6, spice support still WIP, hopefully 2.6 too).  For best
performance you probably want use the new opengl-based rendering
whenever possible.  However I do *not* expect the classic rendering path
disappear, we'll continue to need that in various cases, most prominent
one being vnc support.

So, for non-opengl rendering qemu needs the guest framebuffer data so it
can feed it into the vnc server.  The vfio framebuffer region is meant
to support this use case.

> Another thing to be added. Framebuffers are frequently switched in
> reality. So either Qemu needs to poll or a notification mechanism is required.

The idea is to have qemu poll (and adapt poll rate, i.e. without vnc
client connected qemu will poll alot less frequently).

> And since it's dynamic, having framebuffer page directly exposed in the
> new region might be tricky.  We can just expose framebuffer information
> (including base, format, etc.) and let Qemu to map separately out of VFIO
> interface.

Allocate some memory, ask gpu to blit the guest framebuffer there, i.e.
provide a snapshot of the current guest display instead of playing
mapping tricks?

> And... this works fine with vGPU model since software knows all the
> detail about framebuffer. However in pass-through case, who do you expect
> to provide that information? Is it OK to introduce vGPU specific APIs in
> VFIO?

It will only be used in the vgpu case, not for pass-though.

We think it is better to extend the vfio interface to improve vgpu
support rather than inventing something new while vfio can satisfy 90%
of the vgpu needs already.  We want avoid vendor-specific extensions
though, the vgpu extension should work across vendors.

> Now there is no standard. We expose vGPU life-cycle mgmt. APIs through
> sysfs (under i915 node), which is very Intel specific. In reality different
> vendors have quite different capabilities for their own vGPUs, so not sure
> how standard we can define such a mechanism.

Agree when it comes to create vGPU instances.

> But this code should be
> minor to be maintained in libvirt.

As far I know libvirt only needs to discover those devices.  If they
look like sr/iov devices in sysfs this might work without any changes to
libvirt.

cheers,
  Gerd