Re: [Qemu-devel] [PATCH] [RFC] Variable video ram size option

[Gerd Hoffmann <kraxel@redhat.com>]

Jamie Lokier wrote:
> I don't know why "ypan" is off by default;

Performance.  When reaching the end of video memory ypan does a memcpy
on the framebuffer.  ywrap doesn't need the memcpy, but doesn't work on
all hardware.  The memcpy is painful slow because framebuffer *read*
access isn't something gfx cards are optimized for.

> If you're using Linux guest with VESA framebuffer console, rather than
> a text-mode console, I'd recommend kernel boot option "video=vesafb:ypan",
> to save considerable CPU when scrolling the console.

Should work inside qemu because you should not face the slow-reads issue
then.  On real hardware it very likely makes scrolling noticeable slower.

cheers,
  Gerd