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 26 January 2018 at 15:47, Wei Huang <wei@redhat.com> wrote:
> >
> >
> > On 01/25/2018 02:05 PM, Dr. David Alan Gilbert wrote:
> >> * Wei Huang (wei@redhat.com) wrote:
> >>> innerloop:
> >>>         /* clean cache because el2 might still cache guest data under KVM */
> >>>         dc      civac, x2
> >>
> >> Can you explain a bit more about that please;  if it's guest
> >> visible acorss migration, doesn't that suggest we need the cache clear
> >> in KVM or QEMU?
> >
> > I think this is ARM specific. This is caused by the inconsistency
> > between guest VM's data accesses and userspace's accesses (in
> > check_guest_ram) at the destination:
> >
> > 1) Because uncacheable (guest) + cacheable (host) ==> uncacheable. So
> > the data accesses from guest VM go directly into memory.
> > 2) QEMU user space will use the cacheable version. So it is possible
> > that data can come from cache, instead of RAM. The difference between
> > (1) and (2) obviously can cause check_guest_ram() to fail sometimes.
> 
> 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?

Dave

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