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Re: [Qemu-devel] [PATCH][RFC] Split non-TCG bits out of exec.c

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH][RFC] Split non-TCG bits out of exec.c
Date: Fri, 14 Nov 2008 07:58:23 -0600
User-agent: Thunderbird (X11/20080925)

Jamie Lokier wrote:
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.

That's just a limitation of Intel VT. AMD SVM runs 16-bit code natively. We're slowly improving our in-kernel emulator so eventually we'll be able to emulate 16-bit mode in the kernel.

Running 16-bit code in TCG is something that has been considered.

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.

It falls apart for SMP guests. TCG does not preserve atomicity of memory instructions so you could never have an SMP VCPU running on bare metal while TCG is running. There is a rather large initial cost for building the TBs too so in practice, there are few areas that benefit from this sort of hand off.

The VGA optimization actually addresses this problem in a much nicer way. KVM also supports MMIO batching which we'll eventually merge that covers the remaining cases pretty well.


Anthony Liguori

-- Jamie
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