Commit aef95dac authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
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locking/atomic/x86: Introduce arch_try_cmpxchg64() for !CONFIG_X86_CMPXCHG64 



Commit:

  6d12c8d3 ("percpu: Wire up cmpxchg128")

improved emulated cmpxchg8b_emu() library function to return
success/failure in a ZF flag.

Define arch_try_cmpxchg64() for !CONFIG_X86_CMPXCHG64 targets
to override the generic archy_try_cmpxchg() with an optimized
target specific implementation that handles ZF flag.

The assembly code at the call sites improves from:

   bf56d:	e8 fc ff ff ff       	call   cmpxchg8b_emu
   bf572:	8b 74 24 28          	mov    0x28(%esp),%esi
   bf576:	89 c3                	mov    %eax,%ebx
   bf578:	89 d1                	mov    %edx,%ecx
   bf57a:	8b 7c 24 2c          	mov    0x2c(%esp),%edi
   bf57e:	89 f0                	mov    %esi,%eax
   bf580:	89 fa                	mov    %edi,%edx
   bf582:	31 d8                	xor    %ebx,%eax
   bf584:	31 ca                	xor    %ecx,%edx
   bf586:	09 d0                	or     %edx,%eax
   bf588:	0f 84 e3 01 00 00    	je     bf771 <...>

to:

   bf572:	e8 fc ff ff ff       	call   cmpxchg8b_emu
   bf577:	0f 84 b6 01 00 00    	je     bf733 <...>

Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20240408091547.90111-4-ubizjak@gmail.com
parent 7016cc5d

arch/x86/include/asm/cmpxchg_32.h

+28 −0
Original line number Diff line number Diff line
@@ -122,6 +122,34 @@ static __always_inline u64 arch_cmpxchg64_local(volatile u64 *ptr, u64 old, u64 
}

#define arch_cmpxchg64_local arch_cmpxchg64_local



#define __arch_try_cmpxchg64_emu(_ptr, _oldp, _new)			\

({									\

	union __u64_halves o = { .full = *(_oldp), },			\

			   n = { .full = (_new), };			\

	bool ret;							\

									\

	asm volatile(ALTERNATIVE(LOCK_PREFIX_HERE			\

				 "call cmpxchg8b_emu",			\

				 "lock; cmpxchg8b %[ptr]", X86_FEATURE_CX8) \

		     CC_SET(e)						\

		     : CC_OUT(e) (ret),					\

		       [ptr] "+m" (*(_ptr)),				\

		       "+a" (o.low), "+d" (o.high)			\

		     : "b" (n.low), "c" (n.high), "S" (_ptr)		\

		     : "memory");					\

									\

	if (unlikely(!ret))						\

		*(_oldp) = o.full;					\

									\

	likely(ret);							\

})



static __always_inline bool arch_try_cmpxchg64(volatile u64 *ptr, u64 *oldp, u64 new)

{

	return __arch_try_cmpxchg64_emu(ptr, oldp, new);

}

#define arch_try_cmpxchg64 arch_try_cmpxchg64



#endif



#define system_has_cmpxchg64()		boot_cpu_has(X86_FEATURE_CX8)