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

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Message-ID: <20201209124443.GB13566@gaia>
Date:   Wed, 9 Dec 2020 12:44:43 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Steven Price <steven.price@....com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <haibo.xu@...aro.org>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Juan Quintela <quintela@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v5 0/2] MTE support for KVM guest

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