On 01/12/2016 09:20 AM, Linus Torvalds wrote:
On Tue, Jan 12, 2016 at 5:57 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> 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
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