Messages in this thread | | | Subject | Re: [Intel-gfx] [Announcement] 2015-Q3 release of XenGT - a Mediated Graphics Passthrough Solution from Intel | From | Gerd Hoffmann <> | Date | Thu, 19 Nov 2015 09:40:52 +0100 |
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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
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