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

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH] [RFC] Variable video ram size option
Date: Thu, 15 Jan 2009 18:36:12 +0100
User-agent: Thunderbird (X11/20090105)

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.


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