From: Uros Bizjak <ubizjak@xxxxxxxxx> Sent: Tuesday, November 14, 2023 8:59 AM
>
> 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@xxxxxxxxxxxxx>
> Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Cc: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> ---
> 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
The change looks correct to me. But is there any motivation other
than saving 3 bytes of generated code? This is not a performance
sensitive path. And the change adds 3 lines of source code. So
I wonder if the change is worth the churn.
In any case,
Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
