linux-kernel - [PATCH] x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize hv_nmi

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Message-ID: <20231114170038.381634-1-ubizjak@gmail.com>
Date:   Tue, 14 Nov 2023 17:59:28 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-hyperv@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize hv_nmi_unknown()

Use atomic_try_cmpxchg() instead of atomic_cmpxchg(*ptr, old, new) == old
in hv_nmi_unknown(). On x86 the CMPXCHG instruction returns success in
the ZF flag, so this change saves a compare after CMPXCHG. The generated
asm code improves from:

  3e:	65 8b 15 00 00 00 00 	mov    %gs:0x0(%rip),%edx
  45:	b8 ff ff ff ff       	mov    $0xffffffff,%eax
  4a:	f0 0f b1 15 00 00 00 	lock cmpxchg %edx,0x0(%rip)
  51:	00
  52:	83 f8 ff             	cmp    $0xffffffff,%eax
  55:	0f 95 c0             	setne  %al

to:

  3e:	65 8b 15 00 00 00 00 	mov    %gs:0x0(%rip),%edx
  45:	b8 ff ff ff ff       	mov    $0xffffffff,%eax
  4a:	f0 0f b1 15 00 00 00 	lock cmpxchg %edx,0x0(%rip)
  51:	00
  52:	0f 95 c0             	setne  %al

No functional change intended.

Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Wei Liu <wei.liu@...nel.org>
Cc: Dexuan Cui <decui@...rosoft.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 arch/x86/kernel/cpu/mshyperv.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e6bba12c759c..01fa06dd06b6 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -262,11 +262,14 @@ static uint32_t  __init ms_hyperv_platform(void)
 static int hv_nmi_unknown(unsigned int val, struct pt_regs *regs)
 {
 	static atomic_t nmi_cpu = ATOMIC_INIT(-1);
+	unsigned int old_cpu, this_cpu;
 
 	if (!unknown_nmi_panic)
 		return NMI_DONE;
 
-	if (atomic_cmpxchg(&nmi_cpu, -1, raw_smp_processor_id()) != -1)
+	old_cpu = -1;
+	this_cpu = raw_smp_processor_id();
+	if (!atomic_try_cmpxchg(&nmi_cpu, &old_cpu, this_cpu))
 		return NMI_HANDLED;
 
 	return NMI_DONE;
-- 
2.41.0