linux-kernel - Re: Linux 2.6.32-rc1

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Message-ID: <20090930161447.GC5235@lenovo>
Date:	Wed, 30 Sep 2009 20:14:47 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Linux 2.6.32-rc1

[Eric Dumazet - Wed, Sep 30, 2009 at 05:57:25PM +0200]
...
| 
| > +#define cmpxchg64(ptr, o, n)					\
| > +({								\
| > +	__typeof__(*(ptr)) __ret;				\
| > +	__typeof__(*(ptr)) __old = (o);				\
| > +	__typeof__(*(ptr)) __new = (n);				\
| > +	alternative_io("call cmpxchg8b_emu",			\
| > +			"lock; cmpxchg8b (%%esi)" ,		\
| > +		       X86_FEATURE_CX8,				\
| > +		       "=A" (__ret),				\
| > +		       "S" ((ptr)), "0" (__old),		\
| > +		       "b" ((unsigned int)__new),		\
| > +		       "c" ((unsigned int)(__new>>32)));	\
| 
| 
| Note:
| 
| lock; cmpxchg8b (%%esi)
| 
| gives 4 bytes opcode : f0 0f c7 0e
| Because alternative (call cmpxchg8b_emu) uses 5 bytes, a nop will be added.
| 
| Choosing ".byte 0xf0, 0x0f, 0xc7, 0x4e, 0x00"  aka "lock cmpxchg8b 0x0(%esi)" is a litle bit better ?
| 

Just curious why not "nop; lock; cmpxchg8b (%esi)"? lock itself is destructive
instruction with causes write buffers to flush data back and NOP itself will
be discarded by cpu internals so I suppose this form should be better. Though
I could miss something, and OTOH it's not a big deal. But still curious :)

	-- Cyrill
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