Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

Andy Lutomirski
2017-02-13T19:06:41Z

On Sun, Feb 12, 2017 at 11:49 PM, Dexuan Cui <decui@microsoft.com> wrote:
>> From: Thomas Gleixner [mailto:tglx@linutronix.de]
>> Sent: Saturday, February 11, 2017 02:02
>> ...
>> That's important if the stuff happens cross CPU. If the update happens on
>> the same CPU then this is a different story and as there are VMexits
>> involved they might provide the required ordering already. But I can't tell
>> as I have no idea how that host side thing is done.
>>
>> tglx
>
> IMO Hyper-V TSC page clocksource here seems pretty similar to KVM's pvclock,
> So I would guess "the structure is only updated just before reentering the guest
> after some VM event" (https://rwmj.wordpress.com/2010/10/15/kvm-pvclock/),
> that is, the update should happen on the same CPU, I guess.

If the patch is correct, there is one of these shared by all vCPUs, so
this is not a sufficient explanation.