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 03:12:31PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > 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?
> 
> Not requiring gl is certainly a bonus, people might want build qemu
> without opengl support to reduce the attach surface and/or package
> dependency chain.
> 
> And, yes, requirements for the non-gl rendering path are pretty low.
> qemu needs something it can mmap, and which it can ask pixman to handle.
> Preferred format is PIXMAN_x8r8g8b8 (qemu uses that internally in alot
> of places so this avoids conversions).
> 
> Current plan is to have a special vfio region (not visible to the guest)
> where the framebuffer lives, with one or two pages at the end for meta
> data (format and size).  Status field is there too and will be used by
> qemu to request updates and the kernel to signal update completion.
> Guess I should write that down as vfio rfc patch ...
> 
> I don't think it makes sense to have fields to notify qemu about which
> framebuffer regions have been updated, I'd expect with full-screen
> composing we have these days this information isn't available anyway.
> Maybe a flag telling whenever there have been updates or not, so qemu
> can skip update processing in case we have the screensaver showing a
> black screen all day long.

GL, wayland, X, EGL and soonish Android's surface flinger (hwc already has
it afaik) all track damage. There's plans to add the same to the atomic
kms api too. But if you do damage tracking you really don't want to
support (maybe allow for perf reasons if the guest is stupid) frontbuffer
rendering, which means you need buffer handles + damage, and not a static
region.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch