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

[Peter Maydell]

On 29 January 2018 at 09:53, Dr. David Alan Gilbert <address@hidden> wrote:
> * Peter Maydell (address@hidden) wrote:
>> On 26 January 2018 at 19:46, Dr. David Alan Gilbert <address@hidden> wrote:
>> > * Peter Maydell (address@hidden) 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...)

thanks
-- PMM