Re: get_cycles() on i386

[Marcelo Tosatti]

On Tue, 4 Nov 2003, Linus Torvalds wrote:

> 
> 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>).

I'm not confident this is something for 2.4.

The "if (cpu_has_tsc)" fix from John sounds fine. 

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