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

From: Richard Henderson
Subject: Re: [RFC PATCH 4/5] Add migration support for KVM guest with MTE
Date: Mon, 22 Feb 2021 14:47:31 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

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.


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