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

From: Tian, Kevin
Date: Fri Nov 20 2015 - 01:13:41 EST

> From: Gerd Hoffmann [mailto:kraxel@xxxxxxxxxx]
> Sent: Thursday, November 19, 2015 4:41 PM
> 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.

You can check "fb_decoder.h". One thing to clarify. Its format is
actually based on drm definition, instead of OpenGL. Sorry for

> > 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.

what's the format requirement on that framebuffer? If you are familiar
with Intel Graphics, there's a so-called tiling feature applied on frame
buffer so it can't be used as a raw input to vnc server. w/o opengl you
need do some conversion on CPU first.

> > 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?

yes it works but better to be completed in user level.

> > 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.

it's fine, as long as vgpu specific interface is allowed. :-)

> > 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