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Date:   Wed, 22 Nov 2023 03:51:57 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     Michael Kelley <mhklinux@outlook.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86/hyperv: Use atomic_try_cmpxchg() to micro-optimize
 hv_nmi_unknown()
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On Wed, Nov 15, 2023 at 09:58:29PM +0100, Uros Bizjak wrote:
> On Wed, Nov 15, 2023 at 6:19 PM Michael Kelley <mhklinux@outlook.com> wrote:
> >
> > From: Uros Bizjak <ubizjak@gmail.com> 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@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>
> > > ---
> > >  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.
> 
> Yes, I was trying to make the function more easy to understand and
> similar to nmi_panic() from kernel/panic.c. I had also the idea of
> using CPU_INVALID #define instead of -1, but IMO, the above works as
> well.
> 
> > In any case,
> >
> > Reviewed-by: Michael Kelley <mhklinux@outlook.com>

Applied to hyperv-fixes.

Uros, just so you know, DKIM verification failed when I used b4 to apply
this patch. You may want to check your email setup.

For such a simple patch I'm not worried about spoofing authorship, and I
also checked the same email address had sent similar patches before.

Thanks,
Wei.

> 
> Thanks,
> Uros.