[Xen-devel] [PATCH 0/2] x86: improve remote CPU wakeup

["Jan Beulich" <JBeulich@xxxxxxxx>]

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.

The two patches bring down the average to 250us on said hardware.

1: x86: suppress event check IPI to MWAITing CPUs
2: x86/HVM: batch vCPU wakeups

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>


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