From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Steven Price <steven.price@arm.com>, dgibson@redhat.com
Cc: Peter Maydell <peter.maydell@linaro.org>,
Haibo Xu <haibo.xu@linaro.org>,
lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
Juan Quintela <quintela@redhat.com>,
Marc Zyngier <maz@kernel.org>,
Richard Henderson <richard.henderson@linaro.org>,
QEMU Developers <qemu-devel@nongnu.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>,
kvmarm <kvmarm@lists.cs.columbia.edu>,
arm-mail-list <linux-arm-kernel@lists.infradead.org>,
Dave Martin <Dave.Martin@arm.com>
Subject: Re: [PATCH v5 0/2] MTE support for KVM guest
Date: Mon, 7 Dec 2020 16:44:28 +0000 [thread overview]
Message-ID: <20201207164428.GD3135@work-vm> (raw)
In-Reply-To: <b975422f-14fd-13b3-c8ca-e8b1a68c0837@arm.com>
* Steven Price (steven.price@arm.com) wrote:
> On 07/12/2020 15:27, Peter Maydell wrote:
> > On Mon, 7 Dec 2020 at 14:48, Steven Price <steven.price@arm.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".
>
> Sorry, I know I simplified it rather by saying it's similar to protected VM.
> Basically as I see it there are three types of memory access:
>
> 1) Debug case - has to go via a special case for decryption or ignoring the
> MTE tag value. Hopefully this can be abstracted in the same way.
>
> 2) Migration - for a protected VM there's likely to be a special method to
> allow the VMM access to the encrypted memory (AFAIK memory is usually kept
> inaccessible to the VMM). For MTE this again has to be special cased as we
> actually want both the data and the tag values.
>
> 3) Device DMA - for a protected VM it's usual to unencrypt a small area of
> memory (with the permission of the guest) and use that as a bounce buffer.
> This is possible with MTE: have an area the VMM purposefully maps with
> PROT_MTE. The issue is that this has a performance overhead and we can do
> better with MTE because it's trivial for the VMM to disable the protection
> for any memory.
Those all sound very similar to the AMD SEV world; there's the special
case for Debug that Peter mentioned; migration is ...complicated and
needs special case that's still being figured out, and as I understand
Device DMA also uses a bounce buffer (and swiotlb in the guest to make
that happen).
I'm not sure about the stories for the IBM hardware equivalents.
Dave
> The part I'm unsure on is how easy it is for QEMU to deal with (3) without
> the overhead of bounce buffers. Ideally there'd already be a wrapper for
> guest memory accesses and that could just be wrapped with setting TCO during
> the access. I suspect the actual situation is more complex though, and I'm
> hoping Haibo's investigations will help us understand this.
>
> Thanks,
>
> Steve
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
next prev parent reply other threads:[~2020-12-07 16:46 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-19 15:38 [PATCH v5 0/2] MTE support for KVM guest Steven Price
2020-11-19 15:39 ` [PATCH v5 1/2] arm64: kvm: Save/restore MTE registers Steven Price
2020-11-19 15:39 ` [PATCH v5 2/2] arm64: kvm: Introduce MTE VCPU feature Steven Price
2020-11-19 15:45 ` [PATCH v5 0/2] MTE support for KVM guest Peter Maydell
2020-11-19 15:57 ` Steven Price
2020-11-19 16:39 ` Peter Maydell
2020-11-19 18:42 ` Andrew Jones
2020-11-19 19:11 ` Marc Zyngier
2020-11-20 9:50 ` Steven Price
2020-11-20 9:56 ` Marc Zyngier
2020-11-20 9:58 ` Steven Price
2020-12-04 8:25 ` Haibo Xu
2020-12-07 14:48 ` Steven Price
2020-12-07 15:27 ` Peter Maydell
2020-12-07 15:45 ` Steven Price
2020-12-07 16:05 ` Marc Zyngier
2020-12-07 16:34 ` Catalin Marinas
2020-12-07 19:03 ` Marc Zyngier
2020-12-08 17:21 ` Catalin Marinas
2020-12-08 18:21 ` Marc Zyngier
2020-12-09 12:44 ` Catalin Marinas
2020-12-09 13:25 ` Marc Zyngier
2020-12-09 15:27 ` Catalin Marinas
2020-12-09 18:27 ` Richard Henderson
2020-12-09 18:39 ` Catalin Marinas
2020-12-09 20:13 ` Richard Henderson
2020-12-09 20:20 ` Peter Maydell
2020-12-07 16:44 ` Dr. David Alan Gilbert [this message]
2020-12-07 17:10 ` Peter Maydell
2020-12-07 17:44 ` Dr. David Alan Gilbert
2020-12-08 10:05 ` Haibo Xu
2020-12-08 9:51 ` Haibo Xu
2020-12-08 10:01 ` Marc Zyngier
2020-12-08 10:10 ` Haibo Xu
2020-12-16 7:31 ` Haibo Xu
2020-12-16 10:22 ` Steven Price
2020-12-17 1:47 ` Haibo Xu
2020-11-23 12:16 ` Dr. David Alan Gilbert
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