Re: [PATCH] ensure guest's kvmclock never goes backwards when TSC jumps backward

Paolo Bonzini
2014-07-16T10:18:45Z

Il 16/07/2014 11:52, Igor Mammedov ha scritto:
> There are buggy hosts in the wild that advertise invariant
> TSC and as result host uses TSC as clocksource, but TSC on
> such host sometimes sporadically jumps backwards.
>
> This causes kvmclock to go backwards if host advertises
> PVCLOCK_TSC_STABLE_BIT, which turns off aggregated clock
> accumulator and returns:
> pvclock_vcpu_time_info.system_timestamp + offset
> where 'offset' is calculated using TSC.
> Since TSC is not virtualized in KVM, it makes guest see
> TSC jumped backwards and leads to kvmclock going backwards
> as well.
>
> This is defensive patch that keeps per CPU last clock value
> and ensures that clock will never go backwards even with
> using PVCLOCK_TSC_STABLE_BIT enabled path.

I'm not sure that a per-CPU value is enough; your patch can make the
problem much less frequent of course, but I'm not sure neither detection
nor correction are 100% reliable. Your addition is basically a faster
but less reliable version of the last_value logic.

If may be okay to have detection that is faster but not 100% reliable.
However, once you find that the host is buggy I think the correct thing
to do is to write last_value and kill PVCLOCK_TSC_STABLE_BIT from
valid_flags.

Did you check that the affected host has the latest microcode?
Alternatively, could we simply blacklist some CPU steppings? I'm not
sure who we could ask at AMD :( but perhaps there is an erratum.

Paolo