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

Avi Kivity
2009-10-12T18:31:37Z

On 10/12/2009 08:20 PM, Jeremy Fitzhardinge wrote:
> On 10/10/09 11:10, Avi Kivity wrote:
>
>> On 10/10/2009 02:24 AM, Jeremy Fitzhardinge wrote:
>>
>>> On 10/07/09 03:25, Avi Kivity wrote:
>>>
>>>
>>>> def try_pvclock_vtime():
>>>> tsc, p0 = rdtscp()
>>>> v0 = pvclock[p0].version
>>>> tsc, p = rdtscp()
>>>> t = pvclock_time(pvclock[p], tsc)
>>>> if p != p0 or pvclock[p].version != v0:
>>>> raise Exception("Processor or timebased change under our feet")
>>>> return t
>>>>
> There's a second problem: If the time_info gets updated between the
> first rdtscp and the first version fetch, then we won't have a
> consistent tsc,time_info pair. You could check if tsc_timestamp is>
> tsc, but that won't necessarily work on save/restore/migrate.
>

Good catch. Doesn't that invalidate rdtscp based vgettimeofday on
non-virt as well (assuming p == cpu)?

> I suppose that works if you assume that:
>
> 1. every task->vcpu migration is associated with a hv/guest context
> switch, and
> 2. every hv/guest context switch is a write barrier
>
> I guess 2 is a given, but I can at least imagine cases where 1 might not
> be true. Maybe. It all seems very subtle.
>

What is 1 exactly? task switching to another vcpu? that doesn't incur
hypervisor involvement. vcpu moving to another cpu? That does.

> And I don't really see a gain. You avoid maintaining a second version
> number, but at the cost of two rdtscps. In my measurements, the whole
> vsyscall takes around 100ns to run, and a single rdtsc takes about 30,
> so 30% of total. Unlike rdtsc, rdtscp is documented as being ordered in
> the instruction stream, and so will take at least as long; two of them
> will completely blow the vsyscall execution time.
>

I agree, let's stick with the rdtscpless implementation.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.