Re: Linux 2.6.32-rc1

Eric Dumazet
2009-09-30T15:58:18Z

Arjan van de Ven a écrit :
> On Tue, 29 Sep 2009 14:56:28 -0700 (PDT)
> Linus Torvalds <torvalds@linux-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@linux.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@linux.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 ?