[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] HVM Migration of domU on Qemu-upstream DM causes stuck system clock with ACPI

--On 31 May 2013 12:49:18 +0100 George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:

No -- Linux is asking, "Can you give me an alarm in 5ns?"  And Xen is
saying, "No".  So Linux is saying, "OK, how about 5us?  10us? 20us?"  By
the time it reaches 4ms, Linux has had enough, and says, "If this timer
is so bad that it can't give me an event within 4ms it just won't use
timers at all, thank you very much."

The problem appears to be that Linux thinks it's asking for something in
the future, but is actually asking for something in the past.  It must
look at its watch just before the final domain pause, and then asks for
the time just after the migration resumes on the other side.  So it
doesn't realize that 10ms (or something) has already passed, and that
it's actually asking for a timer in the past.  The Xen timer driver in
Linux specifically asks Xen for times set in the past to return an error.
Xen is returning an error because the time is in the past, Linux thinks
it's getting an error because the time is too close in the future and
tries asking a little further away.

Unfortunately I think this is something which needs to be fixed on the
Linux side; I don't really see how we can work around it in Xen.

I don't think fixing it only on the Linux side is a great idea, not least as it makes any current Linux image not live migrateable reliably. That's pretty horrible.

What would happen if Xen simply lied, then delivered the event a bit late? (obviously post migrate only). Obviously this would mean events wouldn't be delivered at exactly the right time, but in most circumstances that would be better than the guest wallclock simply dying.

Alex Bligh

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.