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Date: Wed, 21 Apr 2010 19:53:22 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
Message-ID: <20100421185322.GN27575@shareable.org>
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To: Gerhard Wiesinger <lists@wiesinger.com>
Cc: Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org, qemu-devel@nongnu.org

Gerhard Wiesinger wrote:
> >>Would it be possible to handle these writes through QEMU directly 
> >>(without
> >>KVM), because performance is there very well (looking at the code there
> >>is some pointer arithmetic and some memory write done)?
> >
> >I've noticed extremely slow VGA performance too, when installing OSes.
> >It makes the difference between installing in a few minutes, and
> >installing taking hours - just because of the slow VGA.
> >
> >So generally I use qemu for installing old versions of Windows, then
> >change to KVM to run them after installing.
> >
> >Switching between KVM and qemu automatically based on guest code
> >behaviour, and making both memory models and device models compatible
> >at run time, is a difficult thing.  I guess it's not worth the
> >difficulty just to speed up VGA.
> 
> I think this is very easy to distingish:
> 1.) VGA Segment A000 is legacy and should be handled through QEMU 
> and not through KVM (because it is much more faster). Also 16 color modes 
> should be fast enough there.
> 2.) All other flat PCI memory accesses should be handled through KVM 
> (there is a specialized driver loaded for that PCI device in the non 
> legacy OS).
> 
> Is that easily possible?

No it isn't.  Distingushing addresses is trivial.  You've ignored the
hard part, which is switching between different virtualisation
architectures...

-- Jamie