Re: get_cycles() on i386

[Linus Torvalds]

On 4 Nov 2003, john stultz wrote:
> 
> CONFIG_X86_TSC be the devil. Personally, I'd much prefer dropping the
> compile time option and using dynamic detection. Something like (not
> recently tested and i believe against 2.5.something, but you get the
> idea):

Some of the users are really timing-critical (eg scheduler).

How about just using the "alternative()" infrastructure that we already 
have in 2.6.x for this? See <asm-i386/system.h> for details.

We don't have an "alternative_output()" available yet, but using that it
would look something like:

	static inline unsigned long long get_cycle(void)
	{
		unsigned long long tsc;

		alternative_output(
			"xorl %%eax,%%eax ; xorl %%edx,%%edx",
			"rdtsc",
			X86_FEATURE_TSC,
			"=A" (tsc));
		return tsc;
	 }

which should allow for "perfect" code (well, gcc tends to mess up 64-bit 
stuff, but you get the idea).

We use the "alternative_input()" thing for prefetch() handling (see 
<asm-i386/processor.h>).

		Linus

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