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

[Jamie Lokier]

Paul Brook wrote:
> On Sunday 07 September 2008, Samuel Thibault wrote:
> > Jamie Lokier, le Sun 07 Sep 2008 00:27:23 +0100, a écrit :
> > > Samuel Thibault wrote:
> > > > Jamie Lokier, le Fri 05 Sep 2008 13:02:15 +0100, a écrit :
> > > > > Another way to map guest framebuffer to video memory is the XF86DGA
> > > > > extension, which is great when you can use it.
> > > >
> > > > It is being deprecated and probably never works nowadays anyway :)
> > >
> > > That's a shame as the alternatives are all slower on old video cards.
> >
> > Yes, and because alternatives don't provide so much performance.
> > Ideally, once switched to fullscreen you could just let the guest
> > directly write to the actual video memory (particularly interesting for
> > Xen and KVM).
> 
> You really don't want to have the guest writing directly to host video ram. 
> Video ram tends to be high-latency, so you want to write to regular memory, 
> then use a wide block transfer or DMA to copy to video ram.

Latency applies mainly when reading.

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. :-)

> If you really need zero-copy then you should have the renderer read the data 
> directly out of the guest framebuffer.

That still forces one extra copy compared with XF86DGA.  "Renderer
reads the data directly out of the guest framebuffer" is the extra copy.

It's negligable on modern 3d cards, but slow on machines or those
where fast blitting or 3d isn't supported.

-- Jamie