Re: [Qemu-devel] [PATCH V1 1/1] tests: Add migration test for aarch64

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On 29 January 2018 at 09:53, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> > * Peter Maydell (peter.maydell@linaro.org) wrote:
> >> On 26 January 2018 at 19:46, Dr. David Alan Gilbert <dgilbert@redhat.com> wrote:
> >> > * Peter Maydell (peter.maydell@linaro.org) wrote:
> >> >> I think the correct fix here is that your test code should turn
> >> >> its MMU on. Trying to treat guest RAM as uncacheable doesn't work
> >> >> for Arm KVM guests (for the same reason that VGA device video memory
> >> >> doesn't work). If it's RAM your guest has to arrange to map it as
> >> >> Normal Cacheable, and then everything should work fine.
> >> >
> >> > Does this cause problems with migrating at just the wrong point during
> >> > a VM boot?
> >>
> >> It wouldn't surprise me if it did, but I don't think I've ever
> >> tried to provoke that problem...
> >
> > If you think it'll get the RAM contents wrong, it might be best to fail
> > the migration if you can detect the cache is disabled in the guest.
> 
> I guess QEMU could look at the value of the "MMU disabled/enabled" bit
> in the guest's system registers, and refuse migration if it's off...
> 
> (cc'd Marc, Christoffer to check that I don't have the wrong end
> of the stick about how thin the ice is in the period before the
> guest turns on its MMU...)

OK; I mean it's not nice either way - but a failed migration is better
than one with corrupt RAM.

It's not pretty for cloud users; they do host-evacuations and the like
fully automatically without any knowledge of the state of a VM and hope
for migrations to work in any state.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK