On Tue, Sep 18, 2018 at 12:41:57PM +0200, Thomas Gleixner wrote:
> On Tue, 18 Sep 2018, Thomas Gleixner wrote:
> > On Tue, 18 Sep 2018, Thomas Gleixner wrote:
> > > On Tue, 18 Sep 2018, Peter Zijlstra wrote:
> > > > > Your memory serves you right. That's indeed observable on CPUs which
> > > > > lack TSC_ADJUST.
> > > >
> > > > But, if the gtod code can observe this, then why doesn't the code that
> > > > checks the sync?
> > >
> > > Because it depends where the involved CPUs are in the topology. The sync
> > > code might just run on the same package an simply not see it. Yes, w/o
> > > TSC_ADJUST the TSC sync code can just fail to see the havoc.
> >
> > Even with TSC adjust the TSC can be slightly off by design on multi-socket
> > systems.
>
> Here are the gory details:
>
>
> https://lore.kernel.org/lkml/3c1737210708230408i7a8049a9m5db49e6c4d89a...@mail.gmail.com/
>
> The changelog has an explanation as well.
>
> d8bb6f4c1670 ("x86: tsc prevent time going backwards")
>
> I still have one of the machines which is affected by this.
Are we sure this isn't a load vs rdtsc reorder? Because if I look at the
current code:
notrace static u64 vread_tsc(void)
{
u64 ret = (u64)rdtsc_ordered();
u64 last = gtod->cycle_last;
if (likely(ret >= last))
return ret;
/*
* GCC likes to generate cmov here, but this branch is extremely
* predictable (it's just a function of time and the likely is
* very likely) and there's a data dependence, so force GCC
* to generate a branch instead. I don't barrier() because
* we don't actually need a barrier, and if this function
* ever gets inlined it will generate worse code.
*/
asm volatile ("");
return last;
}
That does:
lfence
rdtsc
load gtod->cycle_last
Which obviously allows us to observe a cycles_last that is later than
the rdtsc itself, and thus time can trivially go backwards.
The new code:
last = gtod->cycle_last;
cycles = vgetcyc(gtod->vclock_mode);
if (unlikely((s64)cycles < 0))
return vdso_fallback_gettime(clk, ts);
if (cycles > last)
ns += (cycles - last) * gtod->mult;
looks like:
load gtod->cycle_last
lfence
rdtsc
which avoids that possibility, the cycle_last load must have completed
before the rdtsc.
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