Re: [PATCH/RFC] KVM: track pid for VCPU only on KVM_RUN ioctl

David Hildenbrand
2014-08-19T14:23:11Z

> On 19/08/14 14:14, David Hildenbrand wrote:
> >> Il 19/08/2014 13:28, David Hildenbrand ha scritto:
> >>> Looking at the code, kvm_cpu_synchronize_state() seems to do these ioctls in
> >>> the vcpu thread (e.g. comming from cpu_synchronize_all_states()), any reasons
> >>> why kvm_cpu_synchronize_post_reset() doesn't do the same (e.g. called from
> >>> cpu_synchronize_all_post_reset())?
> >>
> >> No reason, feel free to post a patch for QEMU kvm-all.c.
> >> Documentation/virtual/kvm/api.txt clearly says:
> >>
> >> Only run vcpu ioctls from the same thread that was used to create the
> >> vcpu.
> >>
> >> Paolo
> >>
> >
> > Thanks! A little more tweaking in the other parts of s390x resets
> > and we should be able to reduce the number of "wrong" ioctls (I think I found
> > most cases that are responsible for the performance degradation).
>
> Hmm. We want to not only reduce, we want them be zero.
> In addition to a reworked MP_STATE patch set, we might be able to change the code to call "KVM_S390_INITIAL_RESET" only from the cpu thread itself.
> If that simplifies things, we could avoid doing KVM_S390_INITIAL_RESET on CPU creation, because we know that all kernel version will do an implicit cpu reset on cpu creation anyway. Can you have a try on this as well when reworking that code? We could then fix this rcu performance penalty independent from getting rid of that ioctl.
>
> Christian
>

Already working on it, only one ioctl left on vcpu creation that is called
from wrong context, trying to hide from me. Restarts and resets are already
blasting fast.

David