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

[Jamie Lokier <jamie@shareable.org>]

Gerd Hoffmann wrote:
> 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.

Why read the framebuffer at all?  In fact why even redraw everything,
when you can redraw only the text which changes during scroll, which
is what vesafb does by default for normal scrolling?

> > 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.

My experience with real hardware is it makes scrolling noticably faster.

-- Jamie