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

[Alex Bligh <alex@xxxxxxxxxxx>]

Ian,

--On 31 May 2013 13:40:31 +0100 Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
wrote:
> Ultimately though a guest bug is a guest bug, we don't really want to be
> filling the hypervisor with lots of quirky exceptions to interfaces in
> order to work around them, otherwise where does it end?
I'm presuming you're saying this solely a problem with the Xen timer
driver under Linux, rather than a general timer bug under Linux. If
it were a general Linux guest bug, wouldn't it be appearing in
other circumstances than Xen live-migrate? EG on physical hardware or
kvm live migrate?

If that's correct, and I've understood what George said, then
I /think/ the only quirky fix that needs doing is this is to change
the API between kernel driver and xen so that 'don't give me a time
in the past' means 'don't give me a time in the past unless you've
just done a live migrate'. If you really want giving a time in the
past to error under some circumstances, you can signal that another
way ('really don't give me a time in the past).

IE make VCPU_SSHOTTMR_future mean 'error if the time is in the past
if not post a migrate', VCPU_SSHOTTMR_reallyfuture means 'error if the
time is in the past under any circumstances'.

A less messy change might just be a VM config value to ignore
VCPU_SSHOTTMR_future.

> A kernel side fix can be pushed to the distros fairly aggressively (it's
> mostly just a case of getting an upstream stable backport then filing
> bugs with the main ones, we've done it before) and for users upgrading
> the kernel via the distros is really not so hard and mostly reuses the
> process they must have in place for guest kernel security updates and
> other important kernel bugs anyway.
The issue with this from a practical point of view is that in a service
provider environment you need to support guest of all shapes and sizes.
And the migrate is often done by the SP with no notice to the guests.
Guests that partly hang as a result do not make people popular with their
customers. Yes, it would be lovely if everyone always applied the latest
patches to their kernel and rebooted, but they don't.

Otherwise the net result will be Xen4.3 does not reliably live migrate
a pile of Linux OS's unless running with a patched kernel. That is not
a great conclusion.

--
Alex Bligh

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