x86/HVM: batch vCPU wakeups

x86/HVM: batch vCPU wakeups

Mass wakeups (via vlapic_ipi()) can take enormous amounts of time,
especially when many of the remote pCPU-s are in deep C-states. For
64-vCPU Windows Server 2012 R2 guests on Ivybridge hardware,
accumulated times of over 2ms were observed (average 1.1ms).
Considering that Windows broadcasts IPIs from its timer interrupt,
which at least at certain times can run at 1kHz, it is clear that this
can't result in good guest behavior. In fact, on said hardware guests
with significantly beyond 40 vCPU-s simply hung when e.g. ServerManager
gets started.

This isn't just helping to reduce the number of ICR writes when the
host APICs run in clustered mode, it also reduces them by suppressing
the sends altogether when - by the time
cpu_raise_softirq_batch_finish() is reached - the remote CPU already
managed to handle the softirq. Plus - when using MONITOR/MWAIT - the
update of softirq_pending(cpu), being on the monitored cache line -
should make the remote CPU wake up ahead of the ICR being sent,
allowing the wait-for-ICR-idle latencies to be reduced (perhaps to a
large part due to overlapping the wakeups of multiple CPUs).

With this alone (i.e. without the IPI avoidance patch in place),
average broadcast times for a 64-vCPU guest went down to a measured
maximum of 310us. With that other patch in place, improvements aren't
as clear anymore (short term averages only went down from 255us to
250us, which clearly is within the error range of the measurements),
but longer term an improvement of the averages is still visible.
Depending on hardware, long term maxima were observed to go down quite
a bit (on aforementioned hardware), while they were seen to go up
again on a (single core) Nehalem (where instead the improvement on the
average values was more visible).

Of course this necessarily increases the latencies for the remote
CPU wakeup at least slightly. To weigh between the effects, the
condition to enable batching in vlapic_ipi() may need further tuning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>