Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support

[Andy Lutomirski]

> On Oct 4, 2018, at 5:00 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
>> On 04/10/2018 09:54, Vitaly Kuznetsov wrote:
>> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling
>> is supported).
> 
> Not if you want to migrate to pre-Skylake systems.
> 
>> - Check if non-masterclock mode is still needed. E.g. HyperV's TSC page
>> clocksource is a single page for the whole VM, not a per-cpu thing. Can
>> we think that all the buggy hardware is already gone?
> 
> No. :(  We still get reports whenever we break 2007-2008 hardware.
> 
> 

Does the KVM non-masterclock mode actually help?  Itâs not clear to me exactly how itâs supposed to work, but it seems like itâs trying to expose per-vCPU adjustments to the guest. Which is dubious at best, since the guest canât validly use them for anything other than sched_clock, since they arenât fully corrected by anything KVM can do.