Re: [PATCH 1/5] Add a global synchronization point for pvclock

Zachary Amsden
2010-04-19T18:35:55Z

On 04/19/2010 12:54 AM, Avi Kivity wrote:
> On 04/19/2010 01:51 PM, Peter Zijlstra wrote:
>>
>>>> Right, so on x86 we have:
>>>>
>>>> X86_FEATURE_CONSTANT_TSC, which only states that TSC is frequency
>>>> independent, not that it doesn't stop in C states and similar fun
>>>> stuff.
>>>>
>>>> X86_FEATURE_TSC_RELIABLE, which IIRC should indicate the TSC is
>>>> constant
>>>> and synced between cores.
>>>>
>>>>
>>> Sockets and boards too? (IOW, how reliable is TSC_RELIABLE)?
>> Not sure, IIRC we clear that when the TSC sync test fails, eg when we
>> mark the tsc clocksource unusable.
>
> Worrying. By the time we detect this the guest may already have
> gotten confused by clocks going backwards.

Upstream, we are marking the TSC unstable preemptively when hardware
which will eventually sync test is detected, so this should be fine.

Zach