Re: [RFC PATCH 4/5] Add migration support for KVM guest with MTE

[Haibo Xu <haibo.xu@linaro.org>]

++ more migration experts!

On Tue, 23 Feb 2021 at 06:47, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 2/22/21 1:46 AM, Haibo Xu wrote:
> > As I mentioned in the cover later, the reason to let the tag go with the
> > memory data together is to make it easier to sync with each other. I think
> > if we migratie them separately, it would be hard to keep the tags to sync
> > with the data.
> Well, maybe, maybe not.  See below.
>
>
> > Saying if we migration all the data first, then the tags. If the data got
> > dirty during the migration of the tag memory, we may need to send the data
> > again, or freeze the source VM after data migration? What's more, the
> > KVM_GET_DIRTY_LOG API may not be able to differentiate between a tag and
> > data changes.
> I would certainly expect KVM_GET_DIRTY_LOG to only care about the normal
> memory.  That is, pages as viewed by the guest.
>
> I would expect the separate tag_memory block to be private to the host.  If a
> normal page is dirty, then we would read the tags into the tag_memory and
> manually mark that dirty.  Migration would continue as normal, and eventually
> both normal and tag memory would all be clean and migrated.
>
> But I'll admit that it does require that we retain a buffer 1/16 the size of
> main memory, which is otherwise unused, and thus this is less than ideal.  So
> if we do it your way, we should arrange for tcg to migrate the tag data in the
> same way.
>
> I'll still wait for migration experts, of which I am not one.
>
>
> r~