On Wed, Nov 22, 2023 at 4:52 AM Wei Liu [off-list ref] wrote:

On Wed, Nov 15, 2023 at 09:58:29PM +0100, Uros Bizjak wrote:

quoted

On Wed, Nov 15, 2023 at 6:19 PM Michael Kelley [off-list ref] wrote:

quoted

From: Uros Bizjak <redacted> Sent: Tuesday, November 14, 2023 8:59 AM

quoted

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 <redacted>
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 <redacted>
---
 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.

quoted

In any case,

Reviewed-by: Michael Kelley <redacted>

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.

Strange, because I didn't touch the mailer and git config for
months... and recently I have sent many patches this way without
problems.

Thanks,
Uros.