From 18286883e779fb79b413a7462968ee3f6768f19c Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Tue, 14 Nov 2023 17:59:28 +0100 Subject: 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" Cc: Haiyang Zhang Cc: Wei Liu Cc: Dexuan Cui Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: Dave Hansen Cc: "H. Peter Anvin" Signed-off-by: Uros Bizjak Reviewed-by: Michael Kelley Link: https://lore.kernel.org/r/20231114170038.381634-1-ubizjak@gmail.com Signed-off-by: Wei Liu Message-ID: <20231114170038.381634-1-ubizjak@gmail.com> --- arch/x86/kernel/cpu/mshyperv.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'arch/x86') 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; -- cgit v1.2.3-58-ga151