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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2013-05-31 at 14:16 +0100, Alex Bligh wrote:
> 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

There's no such thing as a "migration" on physical hardware and a
save/restore etc is under kernel control so it knows not to cache timer
values etc.

> or kvm live migrate?

I'm not sure how this works. *If* this is a kernel bug then I don't know
why they wouldn't also be effected.

Note that my comments were predicated on this being a guest kernel bug.
Obviously if this turns out to be a Xen bug then we should fix Xen.

> 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'.

What does "just" mean here? How do you determine it?

I said "filling the hypervisor with lots of quirky exceptions", this is
just one and in isolation maybe it isn't too bad. Now imagine we'd
accumulated a dozen over the last 10 years, the semantics of our timer
operation would be impossible to understand, do this unless A, otherwise
if not B do something else, etc etc.

>  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).

That would be changing the behaviour of an existing ABI AFAICT, which is
right out -- what if some other guest is relying on the current
behaviour?

But in any case until George (or someone else) has actually diagnosed
what is going on this entire discussion is premature.

>  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.

Are you saying this didn't happen with Xen 4.2 and earlier? That would
tend to lean towards this being a Xen bug.

Ian.


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