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

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Date:   Mon, 7 Dec 2020 15:27:40 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Steven Price <steven.price@....com>
Cc:     Haibo Xu <haibo.xu@...aro.org>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Juan Quintela <quintela@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Richard Henderson <richard.henderson@...aro.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Catalin Marinas <catalin.marinas@....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 Mon, 7 Dec 2020 at 14:48, Steven Price <steven.price@....com> wrote:
> Sounds like you are making good progress - thanks for the update. Have
> you thought about how the PROT_MTE mappings might work if QEMU itself
> were to use MTE? My worry is that we end up with MTE in a guest
> preventing QEMU from using MTE itself (because of the PROT_MTE
> mappings). I'm hoping QEMU can wrap its use of guest memory in a
> sequence which disables tag checking (something similar will be needed
> for the "protected VM" use case anyway), but this isn't something I've
> looked into.

It's not entirely the same as the "protected VM" case. For that
the patches currently on list basically special case "this is a
debug access (eg from gdbstub/monitor)" which then either gets
to go via "decrypt guest RAM for debug" or gets failed depending
on whether the VM has a debug-is-ok flag enabled. For an MTE
guest the common case will be guests doing standard DMA operations
to or from guest memory. The ideal API for that from QEMU's
point of view would be "accesses to guest RAM don't do tag
checks, even if tag checks are enabled for accesses QEMU does to
memory it has allocated itself as a normal userspace program".

thanks
-- PMM