Re: [PATCH 2/2] KVM: Don't take mmu_lock for range invalidation unless necessary

From: Sean Christopherson
Date: Mon Aug 02 2021 - 15:05:24 EST


On Tue, Jul 27, 2021, Paolo Bonzini wrote:
> @@ -605,8 +597,13 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
>
> /*
> * .change_pte() must be surrounded by .invalidate_range_{start,end}(),
> + * If mmu_notifier_count is zero, then start() didn't find a relevant
> + * memslot and wasn't forced down the slow path; rechecking here is
> + * unnecessary.

Critiquing my own comment...

Maybe elaborate on what's (not) being rechecked? And also clarify that rechecking
the memslots on a false positive (due to a second invalidation) is not problematic?

* If mmu_notifier_count is zero, then no in-progress invalidations,
* including this one, found a relevant memslot at start(); rechecking
* memslots here is unnecessary. Note, a false positive (count elevated
* by a different invalidation) is sub-optimal but functionally ok.
*/

Thanks for doing the heavy lifting!

> */
> WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
> + if (!kvm->mmu_notifier_count)
> + return;
>
> kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
> }