Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

[David Hildenbrand]

On 29.09.22 10:05, Emanuele Giuseppe Esposito wrote:
> Am 28/09/2022 um 22:41 schrieb Sean Christopherson:
> > On Wed, Sep 28, 2022, Paolo Bonzini wrote:
> > > On 9/28/22 17:58, Sean Christopherson wrote:
> > > > I don't disagree that the memslots API is lacking, but IMO that is somewhat
> > > > orthogonal to fixing KVM x86's "code fetch to MMIO" mess.  Such a massive new API
> > > > should be viewed and prioritized as a new feature, not as a bug fix, e.g. I'd
> > > > like to have the luxury of being able to explore ideas beyond "let userspace
> > > > batch memslot updates", and I really don't want to feel pressured to get this
> > > > code reviewed and merge.
> > >
> > > I absolutely agree that this is not a bugfix.  Most new features for KVM can
> > > be seen as bug fixes if you squint hard enough, but they're still features.
> >
> > I guess I'm complaining that there isn't sufficient justification for this new
> > feature.  The cover letter provides a bug that would be fixed by having batched
> > updates, but as above, that's really due to deficiencies in a different KVM ABI.
> >
> > Beyond that, there's no explanation of why this exact API is necessary, i.e. there
> > are no requirements given.
> >
> >    - Why can't this be solved in userspace?
>
> Because this would provide the "feature" only to QEMU, leaving each
> other hypervisor to implement its own.
>
> In addition (maybe you already answered this question but I couldn't
> find an answer in the email thread), does it make sense to stop all
> vcpus for a couple of memslot update? What if we have 100 cpus?
>
> >    - Is performance a concern?  I.e. are updates that need to be batched going to
> >      be high frequency operations?
>
> Currently they are limited to run only at boot. In an unmodified
> KVM/QEMU build, however, I count 86 memslot updates done at boot with
>
> ./qemu-system-x86_64 --overcommit cpu-pm=on --smp $v --accel kvm
> --display none

I *think* there are only ~3 problematic ones (split/resize), where we 
temporarily delete something we will re-add. At least that's what I 
remember from working on my prototype.

--
Thanks,

David / dhildenb