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Date: Fri, 14 Nov 2008 04:03:11 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH][RFC] Split non-TCG bits out of exec.c
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To: qemu-devel@nongnu.org
Cc: Carsten Otte <cotte@de.ibm.com>, Anthony Liguori <aliguori@us.ibm.com>, Hollis Blanchard <hollisb@us.ibm.com>, kvm-devel <kvm@vger.kernel.org>, Paul Brook <paul@codesourcery.com>

Anthony Liguori wrote:
> Unlike kqemu, KVM does not use TCG at all when accelerating QEMU.  Having TCG
> present is not a problem when using KVM on x86.  x86 already has TCG host and
> target support and it's quite convenient to be able to disable/enable KVM and
> compare it to TCG when debugging.

I agree with removing/isolating the dependency on TCG, and there are good
reasons for it.

But does the fact KVM doesn't use TCG prevent KVM from running some
x86 modes correctly?  E.g. I gather 16-bit code is run by KVM using
VM86 mode, which is not exactly correct.  It would be nice to have KVM
acceleration but also complete and correct emulation, by switching to
TCG for those modes.

Also, an earlier thread pointed out that loops doing a lot of MMIO are
_slower_ with KVM than without - this manifested as very slow VGA
output for some guests.  Having KVM pass control to TCG for short runs
of guest instructions which do MMIO, or other instructions which need
to be emulated, would accelerate KVM in this respect.

-- Jamie