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

[Daniel Vetter]

On Tue, Nov 24, 2015 at 01:38:55PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > Yes, vGPU may have additional features, like a framebuffer area, that
> > > aren't present or optional for direct assignment.  Obviously we support
> > > direct assignment of GPUs for some vendors already without this feature.
> > 
> > For exposing framebuffers for spice/vnc I highly recommend against
> > anything that looks like a bar/fixed mmio range mapping. First this means
> > the kernel driver needs to internally fake remapping, which isn't fun.
> 
> Sure.  I don't think we should remap here.  More below.
> 
> > My recoomendation is to build the actual memory access for underlying
> > framebuffers on top of dma-buf, so that it can be vacuumed up by e.g. the
> > host gpu driver again for rendering.
> 
> We want that too ;)
> 
> Some more background:
> 
> OpenGL support in qemu is still young and emerging, and we are actually
> building on dma-bufs here.  There are a bunch of different ways how
> guest display output is handled.  At the end of the day it boils down to
> only two fundamental cases though:
> 
>   (a) Where qemu doesn't need access to the guest framebuffer
>       - qemu directly renders via opengl (works today with virtio-gpu
>         and will be in the qemu 2.5 release)
>       - qemu passed on the dma-buf to spice client for local display
>         (experimental code exists).
>       - qemu feeds the guest display into gpu-assisted video encoder
>         to send a stream over the network (no code yet).
> 
>   (b) Where qemu must read the guest framebuffer.
>       - qemu's builtin vnc server.
>       - qemu writing screenshots to file.
>       - (non-opengl legacy code paths for local display, will
>          hopefully disappear long-term though ...)
> 
> So, the question is how to support (b) best.  Even with OpenGL support
> in qemu improving over time I don't expect this going away completely
> anytime soon.
> 
> I think it makes sense to have a special vfio region for that.  I don't
> think remapping makes sense there.  It doesn't need to be "live", it
> doesn't need support high refresh rates.  Placing a copy of the guest
> framebuffer there on request (and convert from tiled to linear while
> being at it) is perfectly fine.  qemu has a adaptive update rate and
> will stop doing frequent update requests when the vnc client
> disconnects, so there will be nothing to do if nobody wants actually see
> the guest display.
> 
> Possible alternative approach would be to import a dma-buf, then use
> glReadPixels().  I suspect when doing the copy in the kernel the driver
> could ask just the gpu to blit the guest framebuffer.  Don't know gfx
> hardware good enough to be sure though, comments are welcome.

Generally the kernel can't do gpu blts since the required massive state
setup is only in the userspace side of the GL driver stack. But
glReadPixels can do tricks for detiling, and if you use pixel buffer
objects or something similar it'll even be amortized reasonably.

But there's some work to add generic mmap support to dma-bufs, and for
really simple case (where we don't have a gl driver to handle the dma-buf
specially) for untiled framebuffers that would be all we need?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch