linux-kernel - Re: Linux 2.6.32-rc1

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Date:	Wed, 30 Sep 2009 17:57:25 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	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

Arjan van de Ven a écrit :
> On Tue, 29 Sep 2009 14:56:28 -0700 (PDT)
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>>
>> On Tue, 29 Sep 2009, Arjan van de Ven wrote:
>>> can't we use alternatives() for this, to patch cmpxchg64 in ?
>>> I mean.. it'll be commonly supported nowadays.. the fallback to it
>>> not being supported could be a bit slower by now...
>> Yes, we could. It would limit us to some fixed address format,
>> probably
>>
>> 	cmpxchg8b (%esi)
>>
>> or something. Use something like this as a starting point, perhaps?
>>
>> NOTE! Totally untested! And you'd actually need to implement the 
>> "cmpxchg8b_emu" function that takes it's arguments in %eax:%edx,
>> %ebx:%ecx and %esi and doesn't trash any other registers..
> 
> so I debugged this guy (had a few bugs ;-)
> 
> patch, including a new cmpxchg8b_emu below:
> 
>>>From 5a76986c5dd272ea16a9b8abb7349ff3d6791c2b Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <arjan@...ux.intel.com>
> Date: Wed, 30 Sep 2009 17:04:35 +0200
> Subject: [PATCH] x86: Provide an alternative() based cmpxchg64()
> 
> Based on Linus' patch, this patch provides cmpxchg64() using
> the alternative() infrastructure.
> 
> Note: the fallback is NOT smp safe, just like the current fallback
> is not SMP safe.
> 
> Signed-off-by: Arjan van de Ven <arjan@...ux.intel.com>

> +#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 ?

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