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

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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