[PATCH mm-unstable v2 05/10] kvm/arm64: add kvm_arch_test_clear_young()

[Yu Zhao]

Implement kvm_arch_test_clear_young() to support the fast path in
mmu_notifier_ops->test_clear_young().

It focuses on a simple case, i.e., hardware sets the accessed bit in
KVM PTEs and VMs are not protected, where it can rely on RCU and
cmpxchg to safely clear the accessed bit without taking
kvm->mmu_lock. Complex cases fall back to the existing slow path
where kvm->mmu_lock is then taken.

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
 arch/arm64/include/asm/kvm_host.h |  6 ++++++
 arch/arm64/kvm/mmu.c              | 36 +++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 7e7e19ef6993..da32b0890716 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1113,4 +1113,10 @@ static inline void kvm_hyp_reserve(void) { }
 void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu);
 bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu);
 
+#define kvm_arch_has_test_clear_young kvm_arch_has_test_clear_young
+static inline bool kvm_arch_has_test_clear_young(void)
+{
+	return cpu_has_hw_af() && !is_protected_kvm_enabled();
+}
+
 #endif /* __ARM64_KVM_HOST_H__ */
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index c3b3e2afe26f..26a8d955b49c 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1678,6 +1678,42 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
 					   range->start << PAGE_SHIFT);
 }
 
+static int stage2_test_clear_young(const struct kvm_pgtable_visit_ctx *ctx,
+				   enum kvm_pgtable_walk_flags flags)
+{
+	kvm_pte_t new = ctx->old & ~KVM_PTE_LEAF_ATTR_LO_S2_AF;
+
+	VM_WARN_ON_ONCE(!page_count(virt_to_page(ctx->ptep)));
+
+	if (!kvm_pte_valid(new))
+		return 0;
+
+	if (new == ctx->old)
+		return 0;
+
+	if (kvm_should_clear_young(ctx->arg, ctx->addr / PAGE_SIZE))
+		stage2_try_set_pte(ctx, new);
+
+	return 0;
+}
+
+bool kvm_arch_test_clear_young(struct kvm *kvm, struct kvm_gfn_range *range)
+{
+	u64 start = range->start * PAGE_SIZE;
+	u64 end = range->end * PAGE_SIZE;
+	struct kvm_pgtable_walker walker = {
+		.cb	= stage2_test_clear_young,
+		.arg	= range,
+		.flags	= KVM_PGTABLE_WALK_LEAF | KVM_PGTABLE_WALK_SHARED,
+	};
+
+	BUILD_BUG_ON(is_hyp_code());
+
+	kvm_pgtable_walk(kvm->arch.mmu.pgt, start, end - start, &walker);
+
+	return false;
+}
+
 phys_addr_t kvm_mmu_get_httbr(void)
 {
 	return __pa(hyp_pgtable->pgd);
-- 
2.41.0.rc0.172.g3f132b7071-goog