Re: [PATCH v3 2/2] timer/linux/x86: override TSC freq if no tsc_known_freq

[Isaac Boukris <iboukris@gmail.com>]

On Tue, Oct 1, 2024 at 11:01 PM Bruce Richardson
<bruce.richardson@intel.com> wrote:
>
> On Tue, Oct 01, 2024 at 03:22:51AM +0300, Isaac Boukris wrote:
> > If the tsc_known_freq cpu flag is missing, it means the kernel doesn't
> > trust it and calculates its own. We should do the same to avoid drift.
> >
> > Signed-off-by: Isaac Boukris <iboukris@gmail.com>
> > ---
> >  lib/eal/common/eal_common_timer.c |  3 +-
> >  lib/eal/common/eal_private.h      |  2 +-
> >  lib/eal/freebsd/eal_timer.c       |  5 ++-
> >  lib/eal/linux/eal_timer.c         | 53 +++++++++++++++++++++++++++++--
> >  lib/eal/windows/eal_timer.c       |  5 ++-
> >  5 files changed, 60 insertions(+), 8 deletions(-)
> >
> > diff --git a/lib/eal/common/eal_common_timer.c b/lib/eal/common/eal_common_timer.c
> > index c5c4703f15..e00be0a5c8 100644
> > --- a/lib/eal/common/eal_common_timer.c
> > +++ b/lib/eal/common/eal_common_timer.c
> > @@ -66,8 +66,7 @@ set_tsc_freq(void)
> >       }
> >
> >       freq = get_tsc_freq_arch();
> > -     if (!freq)
> > -             freq = get_tsc_freq();
> > +     freq = get_tsc_freq(freq);
> >       if (!freq)
> >               freq = estimate_tsc_freq();
> >
> > diff --git a/lib/eal/common/eal_private.h b/lib/eal/common/eal_private.h
> > index af09620426..bb315dab04 100644
> > --- a/lib/eal/common/eal_private.h
> > +++ b/lib/eal/common/eal_private.h
> > @@ -374,7 +374,7 @@ void set_tsc_freq(void);
> >   *
> >   * This function is private to the EAL.
> >   */
> > -uint64_t get_tsc_freq(void);
> > +uint64_t get_tsc_freq(uint64_t arch_hz);
> >
> >  /**
> >   * Get TSC frequency if the architecture supports.
> > diff --git a/lib/eal/freebsd/eal_timer.c b/lib/eal/freebsd/eal_timer.c
> > index 3dd70e24ba..5a8aea03e1 100644
> > --- a/lib/eal/freebsd/eal_timer.c
> > +++ b/lib/eal/freebsd/eal_timer.c
> > @@ -26,12 +26,15 @@
> >  enum timer_source eal_timer_source = EAL_TIMER_TSC;
> >
> >  uint64_t
> > -get_tsc_freq(void)
> > +get_tsc_freq(uint64_t arch_hz)
> >  {
> >       size_t sz;
> >       int tmp;
> >       uint64_t tsc_hz;
> >
> > +     if (arch_hz)
> > +             return arch_hz;
> > +
> >       sz = sizeof(tmp);
> >       tmp = 0;
> >
>
> On FreeBSD I'm not sure this is the best behaviour. On BSD we read the TSC
> value from the kernel, which, one assumes, has measured it accurately.
> Therefore I'd tend toward just using the kernel value in all cases, maybe
> check the arch value (if non-zero) against that and warning if they have
> significant divergence. WDYT?

Makes sense, I'll add a patch for that. We could also use the arch
value if for some reason the sysctlbyname() failed.