[Xen-devel] Xen clocksource and PV shim

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

I was debugging constant freezes of PV shim on AMD hardware
after going out of a long suspend. As it turned out the root cause
of this is platform time jumping forward to the amount of time
spent in suspended state. On Intel this issue is papered over
by CONSTANT_TSC being set which avoids CPU time sync with
platform time.

Upon further examination it appears that jumping is baked
into the implementation of L0 Xen and there is no seemingly
straight forward way to extract stable continuous rate out
of what we have.

I expect this is a known issue with Xen PV clock as I found
this almost immediately: https://wiki.debian.org/Xen/Clocksource
Currently I don't understand how in that case Xen clock source
could be suitable as a platform timer for nested Xen.

Is my understanding of the situation correct? Could it be
fixed in L0 Xen or it's already backed into the ABI? Should
we keep Xen platform timer in the source code then? Does using
alternative clock source for PV shim make sense?

Igor

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