Re: [GIT PULL] Native Linux KVM tool for 3.1

Paolo Bonzini
2011-07-25T12:59:08Z

On 07/25/2011 09:50 AM, Sasha Levin wrote:
> Anthony had a talk on last years KVM forum regarding the QEMU threading
> model (slide:
> http://www.linux-kvm.org/wiki/images/7/70/2010-forum-threading-qemu.pdf) .
>
> It was suggested that the KVM part of QEMU is having a hard time
> achieving the ideal threading model due to its need to support TCG -
> something which has nothing to do with KVM itself.

No, it is not having a hard time. The "foot in the door" that Anthony
mentions has been part of QEMU and qemu-kvm for a long time
(multi-threading is necessary to support SMP!) and works quite well.

Historically, there were three main loops:

1) QEMU single-threaded;

2) QEMU multi-threaded; clean, but buggy, untested and bitrotting;

3) qemu-kvm multi-threaded, forked from (1), ugly but robust and widely
deployed.

In 0.15 the two multi-threaded versions have been unified by Jan Kiszka.
I have even ported (2) to Windows with little or no pain; porting to
Mac OS X interestingly is harder than Windows, because non-portable
Linux assumptions about signal handling have crept in the code (to
preempt the objections: they weren't just non-portabilities, they were
latent bugs). Windows just does not have signals. :)

So, right now, the only difference is that QEMU is still defaulting to
the single-threaded main loop, while qemu-kvm enables multi-threading by
default. In some time even QEMU will switch.

Yes, this is of course worse than getting it right in the first place;
Nobody is saying the opposite.

Paolo