On 10/10/09 11:10, Avi Kivity wrote:
On 10/10/2009 02:24 AM, Jeremy Fitzhardinge wrote:There's a second problem: If the time_info gets updated between the
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
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.
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.
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.