RE: [PATCH] x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize hv_nmi_unknown()

[Michael Kelley]

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>