linux-kernel - [PATCH] KVM: VMX: Use try_cmpxchg64 in pi_try_set

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Message-Id: <20220518134550.2358-1-ubizjak@gmail.com>
Date:   Wed, 18 May 2022 15:45:50 +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>
Subject: [PATCH] KVM: VMX: Use try_cmpxchg64 in pi_try_set_control

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

  b9:   88 44 24 60             mov    %al,0x60(%rsp)
  bd:   48 89 c8                mov    %rcx,%rax
  c0:   c6 44 24 62 f2          movb   $0xf2,0x62(%rsp)
  c5:   48 8b 74 24 60          mov    0x60(%rsp),%rsi
  ca:   f0 49 0f b1 34 24       lock cmpxchg %rsi,(%r12)
  d0:   48 39 c1                cmp    %rax,%rcx
  d3:   75 cf                   jne    a4 <vmx_vcpu_pi_load+0xa4>

patched:

  c1:   88 54 24 60             mov    %dl,0x60(%rsp)
  c5:   c6 44 24 62 f2          movb   $0xf2,0x62(%rsp)
  ca:   48 8b 54 24 60          mov    0x60(%rsp),%rdx
  cf:   f0 48 0f b1 13          lock cmpxchg %rdx,(%rbx)
  d4:   75 d5                   jne    ab <vmx_vcpu_pi_load+0xab>

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
Patch requires commits 0aa7be05d83cc584da0782405e8007e351dfb6cc
and c2df0a6af177b6c06a859806a876f92b072dc624 from tip.git
---
 arch/x86/kvm/vmx/posted_intr.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 3834bb30ce54..4d41d5994a26 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -34,7 +34,7 @@ static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
 	return &(to_vmx(vcpu)->pi_desc);
 }
 
-static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
+static int pi_try_set_control(struct pi_desc *pi_desc, u64 *pold, u64 new)
 {
 	/*
 	 * PID.ON can be set at any time by a different vCPU or by hardware,
@@ -42,7 +42,7 @@ static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
 	 * update must be retried with a fresh snapshot an ON change causes
 	 * the cmpxchg to fail.
 	 */
-	if (cmpxchg64(&pi_desc->control, old, new) != old)
+	if (!try_cmpxchg64(&pi_desc->control, pold, new))
 		return -EBUSY;
 
 	return 0;
@@ -111,7 +111,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
 		 * descriptor was modified on "put" to use the wakeup vector.
 		 */
 		new.nv = POSTED_INTR_VECTOR;
-	} while (pi_try_set_control(pi_desc, old.control, new.control));
+	} while (pi_try_set_control(pi_desc, &old.control, new.control));
 
 	local_irq_restore(flags);
 
@@ -161,7 +161,7 @@ static void pi_enable_wakeup_handler(struct kvm_vcpu *vcpu)
 
 		/* set 'NV' to 'wakeup vector' */
 		new.nv = POSTED_INTR_WAKEUP_VECTOR;
-	} while (pi_try_set_control(pi_desc, old.control, new.control));
+	} while (pi_try_set_control(pi_desc, &old.control, new.control));
 
 	/*
 	 * Send a wakeup IPI to this CPU if an interrupt may have been posted
-- 
2.35.1