Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM

Peter Zijlstra
2009-05-11T14:42:17Z

On Mon, 2009-05-11 at 17:24 +0300, Avi Kivity wrote:

> > I.e. this is a somewhat poor solution as far as scheduling goes. But
> > i'm wondering what the CPU side does. Can REP-NOP really take
> > thousands of cycles? If yes, under what circumstances?
> >
>
> The guest is running rep-nop in a loop while trying to acquire a
> spinlock. The hardware detects this (most likely, repeated rep-nop with
> the same rip) and exits. We can program the loop count; obviously if
> we're spinning for only a short while it's better to keep spinning while
> hoping the lock will be released soon.
>
> The idea is to detect that the guest is not making forward progress and
> yield. If I could tell the scheduler, you may charge me a couple of
> milliseconds, I promise not to sue, that would be ideal. Other tasks
> can become eligible, hopefully the task holding the spinlock, and by the
> time we're scheduled back the long running task will have finished and
> released the lock.
>
> For newer Linux as a guest we're better off paravirtualizing this, so we
> can tell the host which vcpu holds the lock; in this case kvm will want
> to say, take a couple milliseconds off my account and transfer it to
> this task (so called directed yield). However there's no reason to
> paravirtualize all cpu_relax() calls.

So we're now officially giving up on (soft) realtime virtualization?