linux-kernel - [PATCH v2] KVM: VMX: Use try_cmpxchg64 in pi_try_set

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Date:   Fri, 20 May 2022 16:37:37 +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 v2] 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>

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>
---
v2:
- use address of local variable &old

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

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 3834bb30ce54..259315ea5e44 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -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, &old, new))
 		return -EBUSY;
 
 	return 0;
-- 
2.35.3