KVM: x86/mmu: Use try_cmpxchg64 in fast_pf_fix_direct_spte

Use try_cmpxchg64 instead of cmpxchg64 (*ptr, old, new) != old in
fast_pf_fix_direct_spte. cmpxchg returns success in ZF flag, so this
change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Message-Id: <20220520144635.63134-1-ubizjak@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0a44385..34785bc 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3093,7 +3093,7 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
         *
         * Compare with set_spte where instead shadow_dirty_mask is set.
         */
-       if (cmpxchg64(sptep, old_spte, new_spte) != old_spte)
+       if (!try_cmpxchg64(sptep, &old_spte, new_spte))
                return false;
 
        if (is_writable_pte(new_spte) && !is_writable_pte(old_spte))