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

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Date:   Thu, 19 Nov 2020 16:39:41 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Steven Price <steven.price@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v5 0/2] MTE support for KVM guest

On Thu, 19 Nov 2020 at 15:57, Steven Price <steven.price@....com> wrote:
> On 19/11/2020 15:45, Peter Maydell wrote:
> > I'm a bit dubious about requring the VMM to map the guest memory
> > PROT_MTE unless somebody's done at least a sketch of the design
> > for how this would work on the QEMU side. Currently QEMU just
> > assumes the guest memory is guest memory and it can access it
> > without special precautions...
>
> I agree this needs some investigation - I'm hoping Haibo will be able to
> provide some feedback here as he has been looking at the QEMU support.
> However the VMM is likely going to require some significant changes to
> ensure that migration doesn't break, so either way there's work to be done.
>
> Fundamentally most memory will need a mapping with PROT_MTE just so the
> VMM can get at the tags for migration purposes, so QEMU is going to have
> to learn how to treat guest memory specially if it wants to be able to
> enable MTE for both itself and the guest.

If the only reason the VMM needs tag access is for migration it
feels like there must be a nicer way to do it than by
requiring it to map the whole of the guest address space twice
(once for normal use and once to get the tags)...

Anyway, maybe "must map PROT_MTE" is workable, but it seems
a bit premature to fix the kernel ABI as working that way
until we are at least reasonably sure that it is the right
design.

thanks
-- PMM