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.comSigned-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20231114170038.381634-1-ubizjak@gmail.com>