linux-kernel - [PATCH v7 15/18] KVM: x86/mmu: Locklessly harvest access information from shadow MMU

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Message-ID: <20240926013506.860253-16-jthoughton@google.com>
Date: Thu, 26 Sep 2024 01:35:03 +0000
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, James Houghton <jthoughton@...gle.com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v7 15/18] KVM: x86/mmu: Locklessly harvest access information
 from shadow MMU

Move where the lock is taken for the shadow MMU case to only take the
lock when !range->arg.fast_only (i.e., for the non-fast_only aging MMU
notifiers).

Signed-off-by: James Houghton <jthoughton@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a63497bbcc61..f47bd88b55e3 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1822,16 +1822,24 @@ static bool __kvm_rmap_age_gfn_range(struct kvm *kvm,
 static bool kvm_rmap_age_gfn_range(struct kvm *kvm,
 				   struct kvm_gfn_range *range, bool test_only)
 {
+	bool young;
+
 	/*
 	 * We can always locklessly test if an spte is young. Because marking
 	 * non-A/D sptes for access tracking without holding the mmu_lock is
 	 * not currently supported, we cannot always locklessly clear.
+	 *
+	 * For fast_only, we must not take the mmu_lock, so locklessly age in
+	 * that case even though we will not be able to clear the age for
+	 * non-A/D sptes.
 	 */
-	if (test_only)
+	if (test_only || range->arg.fast_only)
 		return kvm_rmap_age_gfn_range_lockless(kvm, range, test_only);
 
-	lockdep_assert_held_write(&kvm->mmu_lock);
-	return __kvm_rmap_age_gfn_range(kvm, range, test_only);
+	write_lock(&kvm->mmu_lock);
+	young = __kvm_rmap_age_gfn_range(kvm, range, test_only);
+	write_unlock(&kvm->mmu_lock);
+	return young;
 }
 
 static bool kvm_has_shadow_mmu_sptes(struct kvm *kvm)
@@ -1846,11 +1854,8 @@ bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 	if (tdp_mmu_enabled)
 		young = kvm_tdp_mmu_age_gfn_range(kvm, range);
 
-	if (kvm_has_shadow_mmu_sptes(kvm)) {
-		write_lock(&kvm->mmu_lock);
+	if (kvm_has_shadow_mmu_sptes(kvm))
 		young |= kvm_rmap_age_gfn_range(kvm, range, false);
-		write_unlock(&kvm->mmu_lock);
-	}
 
 	return young;
 }
@@ -1862,11 +1867,11 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 	if (tdp_mmu_enabled)
 		young = kvm_tdp_mmu_test_age_gfn(kvm, range);
 
-	if (!young && kvm_has_shadow_mmu_sptes(kvm)) {
-		write_lock(&kvm->mmu_lock);
+	if (young)
+		return young;
+
+	if (kvm_has_shadow_mmu_sptes(kvm))
 		young |= kvm_rmap_age_gfn_range(kvm, range, true);
-		write_unlock(&kvm->mmu_lock);
-	}
 
 	return young;
 }
-- 
2.46.0.792.g87dc391469-goog