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

Jeremy Fitzhardinge
2009-10-13T20:00:58Z

On 10/12/09 23:39, Avi Kivity wrote:
> On 10/12/2009 09:13 PM, Jeremy Fitzhardinge wrote:
>> On 10/12/09 11:29, Avi Kivity wrote:
>>
>>> Good catch. Doesn't that invalidate rdtscp based vgettimeofday on
>>> non-virt as well (assuming p == cpu)?
>>>
>> The tsc clocksource assumes the tsc is (mostly?) synced; it doesn't use
>> rdtscp or make any attempt at per-cpu corrections.
>>
>
> 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.

J