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

[Vitaly Kuznetsov]

Andy Lutomirski <luto at amacapital.net> writes:

>> On Oct 3, 2018, at 2:22 AM, Vitaly Kuznetsov <vkuznets at redhat.com> wrote:
>> 
>> Andy Lutomirski <luto at kernel.org> writes:
>> 
>>> Hi Vitaly, Paolo, Radim, etc.,
>>> 
>> The notification you're talking about exists, it is called
>> Reenligntenment, see 0092e4346f49 "x86/kvm: Support Hyper-V
>> reenlightenment"). When TSC page changes (and this only happens when L1
>> is migrated to a different host with a different TSC frequency and TSC
>> scaling is not supported by the CPU) we receive an interrupt in L1 (at
>> this moment all TSC accesses are emulated which guarantees the
>> correctness of the readings), pause all L2 guests, update their kvmclock
>> structures with new data (we already know the new TSC frequency) and
>> then tell L0 that we're done and it can stop emulating TSC accesses.
>
> That’s delightful!  Does the emulation magic also work for L1 user
> mode?

As far as I understand - yes, all rdtsc* calls will trap into L0.

>  If so, couldn’t we drop the HyperV vclock entirely and just
> fold the adjustment into the core timekeeping data?  (Preferably the
> actual core data, which would require core changes, but it could
> plausibly be done in arch code, too.)

Not all Hyper-V hosts support reenlightenment notifications (and, if I'm
not mistaken, you need to enable nesting for the VM to get the feature -
and most VMs don't have this) so I think we'll have to keep Hyper-V
vclock for the time being.

-- 
Vitaly