Subject: Re: [PATCH v5 0/2] MTE support for KVM guest

From: Marc Zyngier <[email-masked]>
Date: Wed, 09 Dec 2020 13:25:18 +0000

On 2020-12-09 12:44, Catalin Marinas wrote:
> On Tue, Dec 08, 2020 at 06:21:12PM +0000, Marc Zyngier wrote:
>> On 2020-12-08 17:21, Catalin Marinas wrote:
>> > On Mon, Dec 07, 2020 at 07:03:13PM +0000, Marc Zyngier wrote:
>> > > I wonder whether we will have to have something kernel side to
>> > > dump/reload tags in a way that matches the patterns used by live
>> > > migration.
>> >
>> > We have something related - ptrace dumps/resores the tags. Can the same
>> > concept be expanded to a KVM ioctl?
>> 
>> Yes, although I wonder whether we should integrate this deeply into
>> the dirty-log mechanism: it would be really interesting to dump the
>> tags at the point where the page is flagged as clean from a dirty-log
>> point of view. As the page is dirtied, discard the saved tags.
> 
> From the VMM perspective, the tags can be treated just like additional
> (meta)data in a page. We'd only need the tags when copying over. It can
> race with the VM dirtying the page (writing tags would dirty it) but I
> don't think the current migration code cares about this. If dirtied, it
> copies it again.
> 
> The only downside I see is an extra syscall per page both on the origin
> VMM and the destination one to dump/restore the tags. Is this a
> performance issue?

I'm not sure. Migrating VMs already has a massive overhead, so an extra
syscall per page isn't terrifying. But that's the point where I admit
not knowing enough about what the VMM expects, nor whether that matches
what happens on other architectures that deal with per-page metadata.

Would this syscall operate on the guest address space? Or on the VMM's
own mapping?

         M.
-- 
Jazz is not dead. It just smells funny...

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