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

[Catalin Marinas]

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?

-- 
Catalin