Mailinglist Archive: opensuse-bugs (5385 mails)

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[Bug 623680] xen kernel freezes during boot when processor module is loaded

https://bugzilla.novell.com/show_bug.cgi?id=623680

https://bugzilla.novell.com/show_bug.cgi?id=623680#c59


--- Comment #59 from Jan Beulich <jbeulich@xxxxxxxxxx> 2011-04-13 07:10:07 UTC
---
(In reply to comment #57)
Hm, the whole point of this bug report (for me) was to enable C2/C3 on this
machine :-(

Sort of contrary to the bug title...

If I am reading the code correctly, the main difference between Xen and Linux
is that Linux completely avoids using the TSC where it is unstable or
otherwise unreliable. Xen, OTOH, always uses TSC, and just uses the "platform
timer" to calibrate and adjust TSC when it isn't stable.

Correct.

(In reply to comment #58)
Just a very crude idea: On CPUs such as mine, where writing the TSC zeroes out
the upper 32bits, wouldn't it be possible to treat the TSC as a 32bit timer
only? Similar code is already in place for the 32bit and 24bit platform
timers.

Here you sort of contradict your own observations noted in the previous
comment: The platform timer is used for calibration, and an overflow timer is
used as a helper to deal with it being narrow. The TSC, otoh, is the main
timer, and hence treating it as a 32-bit counter would involve quite some more
changes (but it's certainly doable afaict).

Since the (64-bit) TSC is part of the hypervisor/kernel interface, running
*all* guests (including Dom0) with emulated rdtsc would be a direct
consequence.

In any case, your machine not dying anymore is where we'll have to end this.
Enabling C2/C3 on such systems would (as far as we're concerned) presumably be
an enhancement request, not a bug report, and hence would be expected to get
addressed upstream first. So if you can talk anyone into doing this work, or if
you want to give this a try yourself...

One thing you might try though is whether at least C2 can be used on your
system ("lapic_timer_c2_ok" on the Xen command line if the APIC timer doesn't
stop in C2, similar to the identical native Linux option).

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