linux-kernel - Re: [PATCH v2 13/17] x86/tsc: Rename native_read_tsc() to rdtsc

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Date:	Sat, 13 Jun 2015 11:25:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...e.de>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>, Huang Rui <ray.huang@....com>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH v2 13/17] x86/tsc: Rename native_read_tsc() to rdtsc_unordered()

On Sat, Jun 13, 2015 at 1:57 AM, Borislav Petkov <bp@...e.de> wrote:
> On Fri, Jun 12, 2015 at 04:44:53PM -0700, Andy Lutomirski wrote:
>> Now that there is no paravirt TSC, the "native" is inappropriate.
>> The fact that rdtsc is not ordered can catch people by surprise, so
>> call it rdtsc_unordered().
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>
> ...
>
>> @@ -109,7 +109,16 @@ notrace static inline int native_write_msr_safe(unsigned int msr,
>>  extern int rdmsr_safe_regs(u32 regs[8]);
>>  extern int wrmsr_safe_regs(u32 regs[8]);
>>
>> -static __always_inline unsigned long long native_read_tsc(void)
>> +/**
>> + * rdtsc_unordered() - returns the current TSC without ordering constraints
>> + *
>> + * rdtsc_unordered() returns the result of RDTSC as a 64-bit integer.  The
>> + * only ordering constraint it supplies is the ordering implied by
>> + * "asm volatile": it will put the RDTSC in the place you expect.  The
>> + * CPU can and will speculatively execute that RDTSC, though, so the
>> + * results can be non-monotonic if compared on different CPUs.
>> + */
>> +static __always_inline unsigned long long rdtsc_unordered(void)
>
> I like the rdtsc_ordered() thing because it wraps the barrier and people
> cannot just forget it. But let's call this not rdtsc_unordered() but
> simply
>
>         rdtsc()
>
> The "_unordered" suffix is unnecessary IMO since this function is a
> simple wrapper around the hw insn and we do that naming scheme with all
> such wrappers.

I could go either way here.  rdtsc() is weird because people seem to
forget about the barrier.  Maybe I'm being unnecessarily paranoid.

--Andy

>
> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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