linux-kernel - [PATCH 3/3] KVM/VMX: Use try_cmpxchg64() in posted

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Date:   Tue, 15 Dec 2020 19:28:05 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, 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 3/3] KVM/VMX: Use try_cmpxchg64() in posted_intr.c

Use try_cmpxchg64() instead of cmpxchg64() to reuse flags from
cmpxchg/cmpxchg8b instruction. For 64 bit targets flags reuse
avoids a CMP instruction, while for 32 bit targets flags reuse
avoids XOR/XOR/OR instruction sequence.

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>
---
 arch/x86/kvm/vmx/posted_intr.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index f02962dcc72c..47b47970fb46 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -60,8 +60,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 			new.ndst = (dest << 8) & 0xFF00;
 
 		new.sn = 0;
-	} while (cmpxchg64(&pi_desc->control, old.control,
-			   new.control) != old.control);
+	} while (!try_cmpxchg64(&pi_desc->control, &old.control, new.control));
 
 after_clear_sn:
 
@@ -111,8 +110,7 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
 
 		/* set 'NV' to 'notification vector' */
 		new.nv = POSTED_INTR_VECTOR;
-	} while (cmpxchg64(&pi_desc->control, old.control,
-			   new.control) != old.control);
+	} while (!try_cmpxchg64(&pi_desc->control, &old.control, new.control));
 
 	if (!WARN_ON_ONCE(vcpu->pre_pcpu == -1)) {
 		spin_lock(&per_cpu(blocked_vcpu_on_cpu_lock, vcpu->pre_pcpu));
@@ -181,8 +179,7 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
 
 		/* set 'NV' to 'wakeup vector' */
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
-	} while (cmpxchg64(&pi_desc->control, old.control,
-			   new.control) != old.control);
+	} while (!try_cmpxchg64(&pi_desc->control, &old.control, new.control));
 
 	/* We should not block the vCPU if an interrupt is posted for it.  */
 	if (pi_test_on(pi_desc) == 1)
-- 
2.26.2