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

> From: Gerd Hoffmann [mailto:kraxel@xxxxxxxxxx]
> Sent: Friday, November 20, 2015 4:26 PM
>   Hi,
> > > 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
> > that.
> drm is fine.  That header explains the format, but not how it can be
> accessed.  Is the guest fb exported as dma-buf?

Currently not, but per our previous discussion we should move to use
dma-buf. We have some demo code in user space. Not sure whether
they're public now. Jike could you help do a check?

> > > 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.
> Yes, that conversion needs to happen, qemu can't deal with tiled
> graphics.  Anything which pixman can handle will work.  Prefered would
> be PIXMAN_x8r8g8b8 (aka DRM_FORMAT_XRGB8888 on little endian host) which
> is the format used by the vnc server (and other places in qemu)
> internally.
> qemu can also use the opengl texture for the guest fb, then fetch the
> data with glReadPixels().  Which will probably do exactly the same
> conversion.  But it'll add a opengl dependency to the non-opengl
> rendering path in qemu, would be nice if we can avoid that.
> While being at it:  When importing a dma-buf with a tiled framebuffer
> into opengl (via eglCreateImageKHR + EGL_LINUX_DMA_BUF_EXT) I suspect we
> have to pass in the tile size as attribute to make it work.  Is that
> correct?

I'd guess so, but need double confirm later when reaching that level of detail. 
some homework on dma-buf is required first. :-)

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