Re: [Qemu-devel] [PATCH] opengl rendering in the sdl window

[Jamie Lokier <jamie@shareable.org>]

Stefano Stabellini wrote:
> > A good quality guest _expects_ that reading its video RAM will be
> > slow, and only writes to its video RAM, and possible by preparing images
> > off-screen then wide block transfers. :-)
> 
> That is a really good point.
> Usually the safe way to handle the video ram VS main memory problem is
> to write to main memory then copy the whole buffer to video ram every so
> often.
> But in our case the guest thinks that he is writing to video ram
> already, so in theory he is already using all precautions needed to
> handle the latency problem.
> It is actually probable that the guest is handling another buffer in his
> own memory and syncing that one with what he thinks to be the video ram.

Yes, especially with any guest in the last 15 years :-), so I would
imagine that mapping to video RAM as directly as you can would give
the best results (e.g. using an OpenGL memory-mapped texture or X11
MIT-SHM image if they're possible to protect safely).  That's also
saving power, by the way, even if your machine is plenty fast enough
that extra full-screen image copies aren't humanly visible.

That said, numbers talk better than hand-waving.

-- Jamie