Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

Jeremy Fitzhardinge
2009-10-15T19:18:13Z

On 10/14/09 05:32, Avi Kivity wrote:
> On 10/14/2009 05:00 AM, Jeremy Fitzhardinge wrote:
>>
>>> So it's broken or disabled when that assumption is wrong? We could
>>> easily fix that now. Might even reuse the pvclock structures.
>>>
>> Well, the kernel internally makes more or less the same assumption; the
>> vsyscall clocksource is the same as the kernel's internal one. I think
>> idea is that it just drops back to something like hpet if the tsc
>> doesn't have very simple SMP characteristics.
>>
>> If the kernel could characterize the per-cpu properties of the tsc more
>> accurately, then it could use the pvclock mechanism on native.
>>
>
> It does - that's how kvm implements pvclock on the host side. See
> kvmclock_cpufreq_notifier() in arch/x86/kvm/x86.c.

The tsc clocksource currently seems a fair bit more picky though; it
doesn't attempt to sync tscs or work out their relative offsets or
rates. Which seems a bit defeatist.

J