KVM: arm64: Add fast path to handle permission relaxation during dirty logging

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>
Tested-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220118015703.3630552-3-jingzhangos@google.com

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index cafd581..10df5d8 100644 (file)
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1080,6 +1080,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
        gfn_t gfn;
        kvm_pfn_t pfn;
        bool logging_active = memslot_is_logging(memslot);
+       bool logging_perm_fault = false;
        unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
        unsigned long vma_pagesize, fault_granule;
        enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
@@ -1114,6 +1115,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
        if (logging_active) {
                force_pte = true;
                vma_shift = PAGE_SHIFT;
+               logging_perm_fault = (fault_status == FSC_PERM && write_fault);
        } else {
                vma_shift = get_vma_page_shift(vma, hva);
        }
@@ -1212,7 +1214,15 @@ 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);
+       /*
+        * To reduce MMU contentions and enhance concurrency during dirty
+        * logging dirty logging, only acquire read lock for permission
+        * relaxation.
+        */
+       if (logging_perm_fault)
+               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 +1281,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
        }
 
 out_unlock:
-       write_unlock(&kvm->mmu_lock);
+       if (logging_perm_fault)
+               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;