Re: [Qemu-devel] [PATCH] qemu-kvm: introduce cpu_start/cpu_stop commands

[Avi Kivity <avi@redhat.com>]

On 11/23/2010 01:00 AM, Anthony Liguori wrote:
> qemu-kvm vcpu threads don't response to SIGSTOP/SIGCONT.  Instead of teaching
> them to respond to these signals, introduce monitor commands that stop and start
> individual vcpus.
>
> The purpose of these commands are to implement CPU hard limits using an external
> tool that watches the CPU consumption and stops the CPU as appropriate.
>
> The monitor commands provide a more elegant solution that signals because it
> ensures that a stopped vcpu isn't holding the qemu_mutex.
>

 From signal(7):

   The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

Perhaps this is a bug in kvm?

If we could catch SIGSTOP, then it would be easy to unblock it only 
while running in guest context. It would then stop on exit to userspace.

Using monitor commands is fairly heavyweight for something as high 
frequency as this.  What control period do you see people using?  Maybe 
we should define USR1 for vcpu start/stop.

What happens if one vcpu is stopped while another is running?  Spin 
loops, synchronous IPIs will take forever.  Maybe we need to stop the 
entire process.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.