linux-kernel - [PATCH] KVM: x86/mmu: Use try_cmpxchg64 in fast_pf_fix_direct

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Message-Id: <20220520144635.63134-1-ubizjak@gmail.com>
Date:   Fri, 20 May 2022 16:46:35 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH] 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@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Wanpeng Li <wanpengli@...cent.com>
Cc: Jim Mattson <jmattson@...gle.com>
Cc: Joerg Roedel <joro@...tes.org>
---

The patch is against tip tree.
---
 arch/x86/kvm/mmu/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 311e4e1d7870..347bfd8bc5a3 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3073,7 +3073,7 @@ 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))
-- 
2.35.3