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authorUros Bizjak <ubizjak@gmail.com>2023-11-14 17:59:28 +0100
committerWei Liu <wei.liu@kernel.org>2023-11-22 03:47:44 +0000
commit18286883e779fb79b413a7462968ee3f6768f19c (patch)
tree12ea006d25a8ad18ff1eb8e31788f7679c707324 /arch/x86
parent7e8037b099c0bbe8f2109dc452dbcab8d400fc53 (diff)
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@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> Reviewed-by: Michael Kelley <mhklinux@outlook.com> Link: https://lore.kernel.org/r/20231114170038.381634-1-ubizjak@gmail.com Signed-off-by: Wei Liu <wei.liu@kernel.org> Message-ID: <20231114170038.381634-1-ubizjak@gmail.com>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/kernel/cpu/mshyperv.c5
1 files changed, 4 insertions, 1 deletions
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;