Rick C. Petty wrote:
> On Thu, Apr 12, 2007 at 10:54:17AM +0200, Dag-Erling
Sm?rgrav wrote:
>> Our native atomic operations are all defined as
either macros or
>> static inline functions in machine/atomic.h, so we
can easily make
>> this choice at compile time based on a config
option.
>
> Is there any way we could make the choice at boot time,
by checking for
> presence of the CX8 feature? Either as something
like:
>
> extern int feature_cx8; /* or MIB variable */
> #define CMPXCHG8(a) (feature_cx8 ? { _asm
"..." } : emulate_cmpxch8(a))
>
In Linux,
Two copies of code are compiled: one with cmpxch8, one
without.
They are placed into different sections.
When the computer boot up, it choose the best code,
update the pointers then free the unused codes memory.
> Otherwise something like ZFS which utilizes this
feature a lot could
> check the MIB variable and set different fn ptr in its
device structure,
> or something along those lines. Of course, that would
require compiling
> the same code twice essentially, but it had the
advantage that it would
> work on non-CX8 systems and that it would be fast on
systems with CX8.
>
> -- Rick C. Petty
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