Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

[Rasmus Villemoes]

On 2018-09-19 00:46, Thomas Gleixner wrote:
> On Tue, 18 Sep 2018, Andy Lutomirski wrote:
>>>
>>
>> Do we do better if we use signed arithmetic for the whole calculation?
>> Then a small backwards movement would result in a small backwards result.
>> Or we could offset everything so that weâd have to go back several
>> hundred ms before we cross zero.
> 
> That would be probably the better solution as signed math would be
> problematic when the resulting ns value becomes negative. As the delta is
> really small, otherwise the TSC sync check would have caught it, the caller
> should never be able to observe time going backwards.
> 
> I'll have a look into that. It needs some thought vs. the fractional part
> of the base time, but it should be not rocket science to get that
> correct. Famous last words...

Does the sentinel need to be U64_MAX? What if vgetcyc and its minions
returned gtod->cycle_last-1 (for some value of 1), and the caller just
does "if ((s64)cycles - (s64)last < 0) return fallback; ns +=
(cycles-last)* ...". That should just be a "sub ; js ; ". It's an extra
load of ->cycle_last, but only on the path where we're heading for the
fallback anyway. The value of 1 can be adjusted so that in the "js"
path, we could detect and accept an rdtsc_ordered() call that's just a
few 10s of cycles behind last and treat that as 0 and continue back on
the normal path. But maybe it's hard to get gcc to generate the expected
code.

Rasmus