[PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[James Houghton]

Replace the MMU write locks (taken in the memslot iteration loop) for
read locks.

Grabbing the read lock instead of the write lock is safe because the
only requirement we have is that the stage-2 page tables do not get
deallocated while we are walking them. The stage2_age_walker() callback
is safe to race with itself; update the comment to reflect the
synchronization change.

Signed-off-by: James Houghton <jthoughton at google.com>
---
 arch/arm64/kvm/Kconfig       |  1 +
 arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++------
 arch/arm64/kvm/mmu.c         | 30 ++++++++++++++++++++++--------
 3 files changed, 32 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 58f09370d17e..7a1af8141c0e 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -22,6 +22,7 @@ menuconfig KVM
 	select KVM_COMMON
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select KVM_GENERIC_MMU_NOTIFIER
+	select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
 	select HAVE_KVM_CPU_RELAX_INTERCEPT
 	select KVM_MMIO
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 9e2bbee77491..a24a2a857456 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1319,10 +1319,10 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
 	data->young = true;
 
 	/*
-	 * stage2_age_walker() is always called while holding the MMU lock for
-	 * write, so this will always succeed. Nonetheless, this deliberately
-	 * follows the race detection pattern of the other stage-2 walkers in
-	 * case the locking mechanics of the MMU notifiers is ever changed.
+	 * This walk is not exclusive; the PTE is permitted to change from
+	 * under us. If there is a race to update this PTE, then the GFN is
+	 * most likely young, so failing to clear the AF is likely to be
+	 * inconsequential.
 	 */
 	if (data->mkold && !stage2_try_set_pte(ctx, new))
 		return -EAGAIN;
@@ -1345,10 +1345,13 @@ bool kvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr,
 	struct kvm_pgtable_walker walker = {
 		.cb		= stage2_age_walker,
 		.arg		= &data,
-		.flags		= KVM_PGTABLE_WALK_LEAF,
+		.flags		= KVM_PGTABLE_WALK_LEAF |
+				  KVM_PGTABLE_WALK_SHARED,
 	};
+	int r;
 
-	WARN_ON(kvm_pgtable_walk(pgt, addr, size, &walker));
+	r = kvm_pgtable_walk(pgt, addr, size, &walker);
+	WARN_ON_ONCE(r && r != -EAGAIN);
 	return data.young;
 }
 
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 6981b1bc0946..e37765f6f2a1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1912,29 +1912,43 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
 bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 {
 	u64 size = (range->end - range->start) << PAGE_SHIFT;
+	bool young = false;
+
+	read_lock(&kvm->mmu_lock);
 
 	if (!kvm->arch.mmu.pgt)
-		return false;
+		goto out;
 
-	return kvm_pgtable_stage2_test_clear_young(kvm->arch.mmu.pgt,
-						   range->start << PAGE_SHIFT,
-						   size, true);
+	young = kvm_pgtable_stage2_test_clear_young(kvm->arch.mmu.pgt,
+						    range->start << PAGE_SHIFT,
+						    size, true);
 	/*
 	 * TODO: Handle nested_mmu structures here using the reverse mapping in
 	 * a later version of patch series.
 	 */
+
+out:
+	read_unlock(&kvm->mmu_lock);
+	return young;
 }
 
 bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 {
 	u64 size = (range->end - range->start) << PAGE_SHIFT;
+	bool young = false;
+
+	read_lock(&kvm->mmu_lock);
 
 	if (!kvm->arch.mmu.pgt)
-		return false;
+		goto out;
 
-	return kvm_pgtable_stage2_test_clear_young(kvm->arch.mmu.pgt,
-						   range->start << PAGE_SHIFT,
-						   size, false);
+	young = kvm_pgtable_stage2_test_clear_young(kvm->arch.mmu.pgt,
+						    range->start << PAGE_SHIFT,
+						    size, false);
+
+out:
+	read_unlock(&kvm->mmu_lock);
+	return young;
 }
 
 phys_addr_t kvm_mmu_get_httbr(void)
-- 
2.46.0.rc1.232.g9752f9e123-goog