[Xen-devel] [PATCH] [RFC] Building guests on monotonic Xen system time

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Currently, hvm guest platform timers are built on top of the tsc.
Even though the guest believes it is utilizing a monotonic timesource
(such as pit, hpet, or pmtimer), all of these plumb down to an
rdtsc instruction.

Since on many SMP platforms tsc's in different processors are not
synchronized, VMs re-scheduled from a "fast tsc" processor to
a "slow tsc" processor may experience "time going backwards".
This is discussed in the following thread:

http://lists.xensource.com/archives/html/xen-devel/2008-04/msg00277.html

The fix proposed by Keir is that "The logic in vpt.c should
be fixed to use Xen's concept of system time and everything,
guest TSC included, should be derived from that."

The attached patch is a first attempt to derive all
guest timers from Xen's system time... and also to ensure
that system time is non-decreasing. I don't believe the
patch is complete... and possibly not even correct.
For example, I'm concerned that Xen's system time uses
different units than a guest tsc and I don't recall if
all guest tsc accesses are trapped.

Dan

===================================
Thanks... for the memory
I really could use more / My throughput's on the floor
The balloon is flat / My swap disk's fat / I've OOM's in store
Overcommitted so much
(with apologies to the late great Bob Hope)

Attachment: hvmstime.patch
Description: Binary data

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