[Qemu-devel] Re: [PATCH] hook cpu running at a higher level.

[Avi Kivity <avi@redhat.com>]

Ian Jackson wrote:
> The Xen qemu process runs only in one thread which is fine because it
> doesn't need to be involved with actual processor execution.  In
> theory parallel execution (in different threads and thus on different
> physical cpus) of IO emulations requested by different guest vcpus
> might make some small performance difference but I doubt it would be
> worth our while.  So I think the Xen setup will still from qemu's
> point of view look like a single vcpu no matter how many vcpus the
> guest aactually has.
>   

I thing it makes sense for you to have multiple vcpus -- that is 
multiple CPUState objects.  You can still have just one thread (which 
would be the iothread in kvm's terminology).

So:

  tcg - multiplexes all vcpus and io on one thread
  kvm - iothread + per-vcpu thread
  xen - iothread, vcpus scheduled by hypervisor

tcg also ought to move to an iothread model, so it can take advantage of 
multicore for things like display updates.  For x86-on-x86, it may even 
be possible to have vcpus running in parallel (with some kind of 
shared/exclusive lock on softmmu).

-- 
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