Messages in this thread | | | Date | Mon, 12 Oct 2009 20:29:57 +0200 | From | Avi Kivity <> | Subject | Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation |
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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.
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