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Subject[PATCH] x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize hv_nmi_unknown()
Date
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@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.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
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