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Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()
To: Linus Torvalds <torvalds@linux-foundation.org>,
        "Michael S. Tsirkin" <mst@redhat.com>
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From: Andy Lutomirski <luto@kernel.org>
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Date: Tue, 12 Jan 2016 12:30:46 -0800
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On 01/12/2016 09:20 AM, Linus Torvalds wrote:
> On Tue, Jan 12, 2016 at 5:57 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> #ifdef xchgrz
>> /* same as xchg but poking at gcc red zone */
>> #define barrier() do { int ret; asm volatile ("xchgl %0, -4(%%" SP ");": "=r"(ret) :: "memory", "cc"); } while (0)
>> #endif
>
> That's not safe in general. gcc might be using its redzone, so doing
> xchg into it is unsafe.
>
> But..
>
>> Is this a good way to test it?
>
> .. it's fine for some basic testing. It doesn't show any subtle
> interactions (ie some operations may have different dynamic behavior
> when the write buffers are busy etc), but as a baseline for "how fast
> can things go" the stupid raw loop is fine. And while the xchg into
> the redzoen wouldn't be acceptable as a real implementation, for
> timing testing it's likely fine (ie you aren't hitting the problem it
> can cause).

I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 
64) was better because it avoided stomping on very-likely-to-be-hot 
write buffers.

--Andy