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Re: [Qemu-devel] Re: QEMU-KVM and video performance

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
Date: Wed, 21 Apr 2010 22:19:35 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

Avi Kivity wrote:
> On 04/21/2010 09:39 PM, Jamie Lokier wrote:
> >Avi Kivity wrote:
> >   
> >>Writes to vga in 16-color mode don't change set a memory location to a
> >>value, instead they change multiple memory locations.
> >>     
> >While code is just writing to the VGA memory, not reading(*) and not
> >touching the VGA I/O register that control the write latches, is it
> >possible in principle to swizzle the format around in memory to make
> >regular writes work?
> >   
> Not in software.  We can map pages, not cross address lines.

Hence "swizzle".  You rearrange the data inside the page for the
crossed address lines, and undo the swizzle later on demand.  That
doesn't work for other VGA magic though.

> Guests that use 16 color vga are usually of little interest.

Fair enough.  We can move on :-)

It's been said that the super-slow VGA writes triggering this thread
are in 256-colour mode, so there's a different problem.  That should
be fast, shouldn't it?

I vaguely recall extremely slow OS installs I've seen in KVM, which
were fast in QEMU (and fast in KVM after installing), were using text
mode.  Possibly it was Windows 2000, or Windows Server 2003.  Text
mode should be fast too, shouldn't it?  I suppose it's possible that
it just looked like text mode and was really 16-colour mode.

-- Jamie

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