Re: [PATCH v3 0/2] MTE support for KVM guest

From: Andrew Jones <drjones@redhat.com>
To: Steven Price <steven.price@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Maydell <peter.maydell@linaro.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Haibo Xu <Haibo.Xu@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	qemu-devel@nongnu.org, Dave Martin <Dave.Martin@arm.com>,
	Juan Quintela <quintela@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 0/2] MTE support for KVM guest
Date: Fri, 2 Oct 2020 16:36:40 +0200	[thread overview]
Message-ID: <20201002143640.uzsz3nhr45payhlb@kamzik.brq.redhat.com> (raw)
In-Reply-To: <20200925093607.3051-1-steven.price@arm.com>

On Fri, Sep 25, 2020 at 10:36:05AM +0100, Steven Price wrote:
> Version 3 of adding MTE support for KVM guests. See the previous (v2)
> posting for background:
> 
>  https://lore.kernel.org/r/20200904160018.29481-1-steven.price%40arm.com
> 
> These patches add support to KVM to enable MTE within a guest. They are
> based on Catalin's v9 MTE user-space support series[1] (currently in
> next).
> 
> Changes since v2:
> 
>  * MTE is no longer a VCPU feature, instead it is a VM cap.
> 
>  * Being a VM cap means easier probing (check for KVM_CAP_ARM_MTE).
> 
>  * The cap must be set before any VCPUs are created, preventing any
>    shenanigans where MTE is enabled for the guest after memory accesses
>    have been performed.
> 
> [1] https://lore.kernel.org/r/20200904103029.32083-1-catalin.marinas@arm.com
> 
> Steven Price (2):
>   arm64: kvm: Save/restore MTE registers
>   arm64: kvm: Introduce MTE VCPU feature
> 
>  arch/arm64/include/asm/kvm_emulate.h       |  3 +++
>  arch/arm64/include/asm/kvm_host.h          |  7 +++++++
>  arch/arm64/include/asm/sysreg.h            |  3 ++-
>  arch/arm64/kvm/arm.c                       |  9 +++++++++
>  arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 14 ++++++++++++++
>  arch/arm64/kvm/mmu.c                       | 15 +++++++++++++++
>  arch/arm64/kvm/sys_regs.c                  | 20 +++++++++++++++-----
>  include/uapi/linux/kvm.h                   |  1 +
>  8 files changed, 66 insertions(+), 6 deletions(-)
> 
> -- 
> 2.20.1
> 
>

Hi Steven,

These patches look fine to me, but I'd prefer we have a working
implementation in QEMU before we get too excited about the KVM
bits. kvmtool isn't sufficient since it doesn't support migration
(at least afaik). In the past we've implemented features in KVM
that look fine, but then issues have been discovered when trying
to enable them from QEMU, where we also support migration. This
feature looks like there's risk of issues with the userspace side.
Although these two patches would probably stay the same, even if
userspace requires more support.

Thanks,
drew