Re: [PATCH 2/3] KVM: x86/mmu: Drop a redundant remote TLB flush in kvm_zap_gfn_range()

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 22, 2021, Maciej S. Szmigiero wrote:
> On 22.10.2021 03:00, Sean Christopherson wrote:
> > Remove an unnecessary remote TLB flush in kvm_zap_gfn_range() now that
> > said function holds mmu_lock for write for its entire duration.  The
> > flush was added by the now-reverted commit to allow TDP MMU to flush while
> > holding mmu_lock for read, as the transition from write=>read required
> > dropping the lock and thus a pending flush needed to be serviced.
> > 
> > Fixes: 5a324c24b638 ("Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"")
> > Cc: Maxim Levitsky <mlevitsk@redhat.com>
> > Cc: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> > Cc: Ben Gardon <bgardon@google.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c | 3 ---
> >   1 file changed, 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index f82b192bba0b..e8b8a665e2e9 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -5700,9 +5700,6 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> >   						end - 1, true, flush);
> >   			}
> >   		}
> > -		if (flush)
> > -			kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> > -							   gfn_end - gfn_start);
> >   	}
> >   	if (is_tdp_mmu_enabled(kvm)) {
> > 
> 
> Unfortunately, it seems that a pending flush from __kvm_zap_rmaps()
> can be reset back to false by the following line:
> > flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start, gfn_end, flush);
> 
> kvm_tdp_mmu_zap_gfn_range() calls __kvm_tdp_mmu_zap_gfn_range with
> "can_yield" set to true, which passes it to zap_gfn_range, which has
> this code:
> > if (can_yield &&
> >     tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
> >       flush = false;
> >       continue;
> > }

That's working by design.  If the MMU (legacy or TDP) yields during zap, it _must_
flush before dropping mmu_lock so that any SPTE modifications are guaranteed to be
observed by all vCPUs.  Clearing "flush" is deliberate/correct as another is flush
is needed if and only if additional SPTE modifications are made.


static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
					     struct tdp_iter *iter, bool flush,
					     bool shared)
{
	/* Ensure forward progress has been made before yielding. */
	if (iter->next_last_level_gfn == iter->yielded_gfn)
		return false;

	if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
		rcu_read_unlock();

		if (flush)
			kvm_flush_remote_tlbs(kvm);  <------- ****** HERE ******

		if (shared)
			cond_resched_rwlock_read(&kvm->mmu_lock);
		else
			cond_resched_rwlock_write(&kvm->mmu_lock);

		rcu_read_lock();

		WARN_ON(iter->gfn > iter->next_last_level_gfn);

		tdp_iter_restart(iter);

		return true;
	}

	return false;
}