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[Xen-devel] Concern about c/s 21404: Remove special-case paths for start

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Concern about c/s 21404: Remove special-case paths for start-of-day SMP bringup
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 19 May 2010 09:24:39 -0700 (PDT)
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"Notably ganged TSC calibration is removed"

Really?  We have recently seen machines which generated
the removed message "CPUX had N usecs TSC skew, fixed it up."

I'm not sure I fully understand the scope or the purpose
of all the patches, but since the TSC is the foundation for
Xen system time which is the foundation for virtually all time
handling in all guests (because even the non-TSC emulated
timers are built on Xen system time), I think you are introducing
a big risk here for an unknown number of existing systems.
Some of the problems may be obvious because a guest often
won't boot when clocks are screwy, but other problems may
not manifest until "later" when under a heavy load that
causes more frequent vcpu->pcpu switching and may not
be at all obviously related to clock problems.  (We've
recently seen that also with a Solaris HV guest running
on a pre-4.0 Xen where rdtsc is not yet emulated.)

Note that Linux is *adding* code to attempt to make TSC more
reliable, while it appears this Xen patch is taking code away.

If I understand, the purpose is to provide a better
way to online CPUs so as to handle CPU hotplug better?
While hotplug is a nice feature, am I correct that the
feature is supported in a very small number of
systems?  And is actually used in an even smaller

My two cents: Doesn't seem like a good tradeoff to me...

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