Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW

Peter Zijlstra
2018-03-14T13:12:44Z

On Wed, Mar 14, 2018 at 12:55:20PM +0000, Jason Vas Dias wrote:
> > While CPU_CLOCK is TSC based, there is no guarantee it has any
> > correlation to CLOCK_MONOTONIC_RAW (even if that is also TSC based).
> >
> > (although, I think I might have fixed that recently and it might just
> > work, but it's very much not guaranteed).
>
> Yes, I believe the CPU_CLOCK is effectively the converted TSC -
> it does appear to correlate well with the new CLOCK_MONOTONIC_RAW
> values from the patched VDSO.

It (now) runs at the same rate, but there is no guarantee for this, in
fact it didn't for a very long time.

Relying on this is broken.