Re: [BUG] KVM: x86: kvmclock jumps ~253 years on Hyper-V nested virt due to cross-CPU raw TSC inconsistency

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Thomas Lefebvre <thomas.lefebvre3@gmail.com> writes:

...

>
> Under Hyper-V, raw RDTSC values are not consistent across vCPUs.
> The hypervisor corrects them only through the TSC page scale/offset.
> If pvclock_update_vm_gtod_copy() runs on CPU 0 and __get_kvmclock()
> later runs on CPU 1 where the raw TSC is lower, the unsigned
> subtraction wraps.
>

According to the TLFS, reference TSC page is partition wide:

"The hypervisor provides a partition-wide virtual reference TSC page
which is overlaid on the partition’s GPA space. A partition’s reference
time stamp counter page is accessed through the Reference TSC MSR."

so if as you say RAW rdtsc value is inconsistent across vCPUs, I can
hardly see how we can use this time source at all, even without
KVM. scale/offset are the same for all vCPUs.

I think the fix here is to avoid setting up Hyper-V TSC page clocksource
in L1. Unfortunately, with unsynchronized TSCs this will leave us the
only choice for a sane clocksource: raw HV_X64_MSR_TIME_REF_COUNT MSR
reads.

-- 
Vitaly