Re: [PATCH v4 1/4] KVM: Implement dirty quota-based throttling of vcpus

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On Fri, Jul 15, 2022 at 05:07:45PM +0000, Sean Christopherson wrote:
> On Fri, Jul 15, 2022, Peter Xu wrote:
> > On Fri, Jul 15, 2022 at 04:23:54PM +0000, Sean Christopherson wrote:
> > > And the reasoning behind not having kvm_run.dirty_count is that it's fully
> > > redundant if KVM provides a stat, and IMO such a stat will be quite helpful for
> > > things beyond dirty quotas, e.g. being able to see which vCPUs are dirtying memory
> > > from the command line for debug purposes.
> > 
> > Not if with overflow in mind?  Note that I totally agree the overflow may
> > not even happen, but I think it makes sense to consider as a complete
> > design of ceiling-based approach.  Think the Millennium bug, we never know
> > what will happen in the future..
> > 
> > So no objection too on having stats for dirty pages, it's just that if we
> > still want to cover the overflow issue we'd make dirty_count writable, then
> > it'd still better be in kvm_run, imho.
> 
> Yeah, never say never, but unless my math is wrong, overflow isn't happening anytime
> soon.  And if future CPUs can overflow the number of dirty pages, then they'll be
> able to overflow a number of stats, at which point I think we'll want a generic
> ioctl() to reset _all_ stats.

It's slightly different as this affects functionality, unlike most stats.
I'll leave that to Shivam to see his preference.  If to go that way, I hope
we can at least have some WARN_ON_ONCE() on detecting overflows of "count".

Thanks,

-- 
Peter Xu




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