Re: [PATCH 1/2] KVM: x86/mmu: Skip tlb flush if it has been done in zap_gfn_range()

[Sean Christopherson <seanjc@google.com>]

On Wed, Nov 17, 2021, Paolo Bonzini wrote:
> On 11/17/21 10:20, Hou Wenlong wrote:
> > If the parameter flush is set, zap_gfn_range() would flush remote tlb
> > when yield, then tlb flush is not needed outside. So use the return
> > value of zap_gfn_range() directly instead of OR on it in
> > kvm_unmap_gfn_range() and kvm_tdp_mmu_unmap_gfn_range().
> > 
> > Fixes: 3039bcc744980 ("KVM: Move x86's MMU notifier memslot walkers to generic code")
> > Signed-off-by: Hou Wenlong <houwenlong93@linux.alibaba.com>
> > ---
> >   arch/x86/kvm/mmu/mmu.c     | 2 +-
> >   arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
> >   2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 354d2ca92df4..d57319e596a9 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1582,7 +1582,7 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> >   		flush = kvm_handle_gfn_range(kvm, range, kvm_unmap_rmapp);
> >   	if (is_tdp_mmu_enabled(kvm))
> > -		flush |= kvm_tdp_mmu_unmap_gfn_range(kvm, range, flush);
> > +		flush = kvm_tdp_mmu_unmap_gfn_range(kvm, range, flush);
> >   	return flush;
> >   }
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 7c5dd83e52de..9d03f5b127dc 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1034,8 +1034,8 @@ bool kvm_tdp_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range,
> >   	struct kvm_mmu_page *root;
> >   	for_each_tdp_mmu_root(kvm, root, range->slot->as_id)
> > -		flush |= zap_gfn_range(kvm, root, range->start, range->end,
> > -				       range->may_block, flush, false);
> > +		flush = zap_gfn_range(kvm, root, range->start, range->end,
> > +				      range->may_block, flush, false);
> >   	return flush;
> >   }
> > 
> 
> Queued both, thanks.

Please replace patch 02 with the below.  Hou's patch isn't wrong, but it's nowhere
near agressive enough in purging the unecessary flush.  I was too slow in writing
a changelog for this patch in my local repo.

From 001a1b9f5f71c0d8115875b26dbac7694431c3dd Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Wed, 17 Nov 2021 08:53:57 -0800
Subject: [PATCH] KVM: x86/mmu: Remove spurious TLB flushes in TDP MMU zap
 collapsible path

Drop the "flush" param and return values to/from the TDP MMU's helper for
zapping collapsible SPTEs.  Because the helper runs with mmu_lock held
for read, not write, it uses tdp_mmu_zap_spte_atomic(), and the atomic
zap handles the necessary remote TLB flush.

Similarly, because mmu_lock is dropped and re-acquired between zapping
legacy MMUs and zapping TDP MMUs, kvm_mmu_zap_collapsible_sptes() must
handle remote TLB flushes from the legacy MMU before calling into the TDP
MMU.

Opportunistically drop the local "flush" variable in the common helper.

Fixes: e2209710ccc5d ("KVM: x86/mmu: Skip rmap operations if rmaps not allocated")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/mmu.c     |  9 ++-------
 arch/x86/kvm/mmu/tdp_mmu.c | 22 +++++++---------------
 arch/x86/kvm/mmu/tdp_mmu.h |  5 ++---
 3 files changed, 11 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d57319e596a9..b659787b7398 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5853,8 +5853,6 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
 void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 				   const struct kvm_memory_slot *slot)
 {
-	bool flush = false;
-
 	if (kvm_memslots_have_rmaps(kvm)) {
 		write_lock(&kvm->mmu_lock);
 		/*
@@ -5862,17 +5860,14 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
 		 * logging at a 4k granularity and never creates collapsible
 		 * 2m SPTEs during dirty logging.
 		 */
-		flush = slot_handle_level_4k(kvm, slot, kvm_mmu_zap_collapsible_spte, true);
-		if (flush)
+		if (slot_handle_level_4k(kvm, slot, kvm_mmu_zap_collapsible_spte, true))
 			kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
 		write_unlock(&kvm->mmu_lock);
 	}

 	if (is_tdp_mmu_enabled(kvm)) {
 		read_lock(&kvm->mmu_lock);
-		flush = kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot, flush);
-		if (flush)
-			kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
+		kvm_tdp_mmu_zap_collapsible_sptes(kvm, slot);
 		read_unlock(&kvm->mmu_lock);
 	}
 }
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7ac0c4f29c8e..b7ace8b9c019 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1364,10 +1364,9 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
  * Clear leaf entries which could be replaced by large mappings, for
  * GFNs within the slot.
  */
-static bool zap_collapsible_spte_range(struct kvm *kvm,
+static void zap_collapsible_spte_range(struct kvm *kvm,
 				       struct kvm_mmu_page *root,
-				       const struct kvm_memory_slot *slot,
-				       bool flush)
+				       const struct kvm_memory_slot *slot)
 {
 	gfn_t start = slot->base_gfn;
 	gfn_t end = start + slot->npages;
@@ -1378,10 +1377,8 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,

 	tdp_root_for_each_pte(iter, root, start, end) {
 retry:
-		if (tdp_mmu_iter_cond_resched(kvm, &iter, flush, true)) {
-			flush = false;
+		if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
 			continue;
-		}

 		if (!is_shadow_present_pte(iter.old_spte) ||
 		    !is_last_spte(iter.old_spte, iter.level))
@@ -1393,6 +1390,7 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,
 							    pfn, PG_LEVEL_NUM))
 			continue;

+		/* Note, a successful atomic zap also does a remote TLB flush. */
 		if (!tdp_mmu_zap_spte_atomic(kvm, &iter)) {
 			/*
 			 * The iter must explicitly re-read the SPTE because
@@ -1401,30 +1399,24 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,
 			iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
 			goto retry;
 		}
-		flush = true;
 	}

 	rcu_read_unlock();
-
-	return flush;
 }

 /*
  * Clear non-leaf entries (and free associated page tables) which could
  * be replaced by large mappings, for GFNs within the slot.
  */
-bool kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
-				       const struct kvm_memory_slot *slot,
-				       bool flush)
+void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
+				       const struct kvm_memory_slot *slot)
 {
 	struct kvm_mmu_page *root;

 	lockdep_assert_held_read(&kvm->mmu_lock);

 	for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, true)
-		flush = zap_collapsible_spte_range(kvm, root, slot, flush);
-
-	return flush;
+		zap_collapsible_spte_range(kvm, root, slot);
 }

 /*
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index 476b133544dd..3899004a5d91 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -64,9 +64,8 @@ void kvm_tdp_mmu_clear_dirty_pt_masked(struct kvm *kvm,
 				       struct kvm_memory_slot *slot,
 				       gfn_t gfn, unsigned long mask,
 				       bool wrprot);
-bool kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
-				       const struct kvm_memory_slot *slot,
-				       bool flush);
+void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
+				       const struct kvm_memory_slot *slot);

 bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn,
--
2.34.0.rc1.387.gb447b232ab-goog