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

[Dor Laor]

On 11/23/2010 08:41 AM, Avi Kivity wrote:
> 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.

Why not use cgroup for that?

> > 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.