[PATCH v1 2/3] KVM: arm64: Add fast path to handle permission relaxation during dirty logging

[Jing Zhang <jingzhangos@google.com>]

To reduce MMU lock contention during dirty logging, all permission
relaxation operations would be performed under read lock.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 arch/arm64/kvm/mmu.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index cafd5813c949..15393cb61a3f 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1084,6 +1084,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	unsigned long vma_pagesize, fault_granule;
 	enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
 	struct kvm_pgtable *pgt;
+	bool use_mmu_readlock = false;
 
 	fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
 	write_fault = kvm_is_write_fault(vcpu);
@@ -1212,7 +1213,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (exec_fault && device)
 		return -ENOEXEC;
 
-	write_lock(&kvm->mmu_lock);
+	if (fault_status == FSC_PERM && fault_granule == PAGE_SIZE
+				     && logging_active && write_fault)
+		use_mmu_readlock = true;
+	/*
+	 * To reduce MMU contentions and enhance concurrency during dirty
+	 * logging dirty logging, only acquire read lock for permission
+	 * relaxation. This fast path would greatly reduce the performance
+	 * degradation of guest workloads.
+	 */
+	if (use_mmu_readlock)
+		read_lock(&kvm->mmu_lock);
+	else
+		write_lock(&kvm->mmu_lock);
 	pgt = vcpu->arch.hw_mmu->pgt;
 	if (mmu_notifier_retry(kvm, mmu_seq))
 		goto out_unlock;
@@ -1271,7 +1284,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	}
 
 out_unlock:
-	write_unlock(&kvm->mmu_lock);
+	if (use_mmu_readlock)
+		read_unlock(&kvm->mmu_lock);
+	else
+		write_unlock(&kvm->mmu_lock);
 	kvm_set_pfn_accessed(pfn);
 	kvm_release_pfn_clean(pfn);
 	return ret != -EAGAIN ? ret : 0;
-- 
2.34.1.703.g22d0c6ccf7-goog