Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Sean Christopherson]

On Tue, Jul 27, 2021, Ben Gardon wrote:
> On Tue, Jul 27, 2021 at 6:06 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
> >
> > On Thu, 2021-07-22 at 19:06 +0000, Sean Christopherson wrote:
> > > The elevated mmu_notifier_count and/or changed mmu_notifier_seq will cause vCPU1
> > > to bail and resume the guest without fixing the #NPF.  After acquiring mmu_lock,
> > > vCPU1 will see the elevated mmu_notifier_count (if kvm_zap_gfn_range() is about
> > > to be called, or just finised) and/or a modified mmu_notifier_seq (after the
> > > count was decremented).
> > >
> > > This is why kvm_zap_gfn_range() needs to take mmu_lock for write.  If it's allowed
> > > to run in parallel with the page fault handler, there's no guarantee that the
> > > correct apic_access_memslot_enabled will be observed.
> >
> > I understand now.
> >
> > So, Paolo, Ben Gardon, what do you think. Do you think this approach is feasable?
> > Do you agree to revert the usage of the read lock?
> >
> > I will post a new series using this approach very soon, since I already have
> > msot of the code done.
> >
> > Best regards,
> >         Maxim Levitsky
> 
> From reading through this thread, it seems like switching from read
> lock to write lock is only necessary for a small range of GFNs, (i.e.
> the APIC access page) is that correct?

For the APICv case, yes, literally a single GFN (the default APIC base).

> My initial reaction was that switching kvm_zap_gfn_range back to the
> write lock would be terrible for performance, but given its only two
> callers, I think it would actually be fine.

And more importantly, the two callers are gated by kvm_arch_has_noncoherent_dma()
and are very rare flows for the guest (updating MTRRs, toggling CR0.CD).

> If you do that though, you should pass shared=false to
> kvm_tdp_mmu_zap_gfn_range in that function, so that it knows it's
> operating with exclusive access to the MMU lock.

Ya, my suggested revert was to drop @shared entirely since kvm_zap_gfn_range() is
the only caller that passes @shared=true.