Re: [PATCH v6 1/5] KVM: Implement dirty quota-based throttling of vcpus

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 17, 2022, Shivam Kumar wrote:
> 
> On 10/10/22 11:11 am, Shivam Kumar wrote:
> > 
> > On 15/09/22 3:40 pm, Shivam Kumar wrote:
> > > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > > index 3ca3db020e0e..263a588f3cd3 100644
> > > --- a/include/linux/kvm_types.h
> > > +++ b/include/linux/kvm_types.h
> > > @@ -118,6 +118,7 @@ struct kvm_vcpu_stat_generic {
> > >       u64 halt_poll_fail_hist[HALT_POLL_HIST_COUNT];
> > >       u64 halt_wait_hist[HALT_POLL_HIST_COUNT];
> > >       u64 blocking;
> > > +    u64 pages_dirtied;
> > I am reworking the QEMU patches and I am not sure how I can access the
> > pages_dirtied info from the userspace side when the migration starts, i.e.
> > without a dirty quota exit.
> > 
> > I need this info to initialise the dirty quota. This is what I am looking
> > to do on the userspace side at the start of dirty quota migration:
> >      dirty_quota = pages_dirtied + some initial quota
> > 
> > Hoping if you could help, Sean. Thanks in advance.
> I think I can set dirty_quota initially to 1 and let the vpcu exit with exit
> reason KVM_EXIT_DIRTY_QUOTA_EXHAUSTED. Then, I can set the quota.

The vCPU doesn't need to be paused to read stats, pages_dirtied can be read while
the vCPU is running via KVM_GET_STATS_FD.  Though at a glance, QEMU doesn't yet
utilize KVM_GETS_STATS_FD, i.e. QEMU would a need a decent chunk of infrastructure
improvements to avoid the unnecessary exit.