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x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize hv_nmi_unknown()

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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>
This commit is contained in:
Uros Bizjak 2023-11-14 17:59:28 +01:00 committed by Wei Liu
parent 7e8037b099
commit 18286883e7
1 changed files with 4 additions and 1 deletions
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5
arch/x86/kernel/cpu/mshyperv.c
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@ -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;