[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Message-Id: <20200617123844.29960-1-steven.price@arm.com>
Date: Wed, 17 Jun 2020 13:38:42 +0100
From: Steven Price <steven.price@....com>
To: Catalin Marinas <catalin.marinas@....com>,
Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>
Cc: Steven Price <steven.price@....com>,
James Morse <james.morse@....com>,
Julien Thierry <julien.thierry.kdev@...il.com>,
Suzuki K Poulose <suzuki.poulose@....com>,
kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
Mark Rutland <mark.rutland@....com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: [RFC PATCH 0/2] MTE support for KVM guest
These patches add support to KVM to enable MTE within a guest. It is
based on Catalin's v4 MTE user space series[1].
[1] http://lkml.kernel.org/r/20200515171612.1020-1-catalin.marinas%40arm.com
Posting as an RFC as I'd like feedback on the approach taken. First a
little background on how MTE fits within the architecture:
The stage 2 page tables have limited scope for controlling the
availability of MTE. If a page is mapped as Normal and cached in stage 2
then it's the stage 1 tables that get to choose whether the memory is
tagged or not. So the only way of forbidding tags on a page from the
hypervisor is to change the cacheability (or make it device memory)
which would cause other problems. Note this restriction fits the
intention that a system should have all (general purpose) memory
supporting tags if it support MTE, so it's not too surprising.
However, the upshot of this is that to enable MTE within a guest all
pages of memory mapped into the guest as normal cached pages in stage 2
*must* support MTE (i.e. we must ensure the tags are appropriately
sanitised and save/restore the tags during swap etc).
My current approach is that KVM transparently upgrades any pages
provided by the VMM to be tag-enabled when they are faulted in (i.e.
sets the PG_mte_tagged flag on the page) which has the benefit of
requiring fewer changes in the VMM. However, save/restore of the VM
state still requires the VMM to have a PROT_MTE enabled mapping so that
it can access the tag values. A VMM which 'forgets' to enable PROT_MTE
would lose the tag values when saving/restoring (tags are RAZ/WI when
PROT_MTE isn't set).
An alternative approach would be to enforce the VMM provides PROT_MTE
memory in the first place. This seems appealing to prevent the above
potentially unexpected gotchas with save/restore, however this would
also extend to memory that you might not expect to have PROT_MTE (e.g. a
shared frame buffer for an emulated graphics card).
Comments on the approach (or ideas for alternative approaches) are very
welcome.
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 | 9 ++++++++-
arch/arm64/include/uapi/asm/kvm.h | 1 +
arch/arm64/kvm/hyp/sysreg-sr.c | 12 ++++++++++++
arch/arm64/kvm/reset.c | 8 ++++++++
arch/arm64/kvm/sys_regs.c | 8 ++++++++
virt/kvm/arm/mmu.c | 11 +++++++++++
7 files changed, 51 insertions(+), 1 deletion(-)
--
2.20.1
Powered by blists - more mailing lists