Re: [PATCH v6 0/2] MTE support for KVM guest

Steven Price
2020-12-03T16:50:45Z

On 03/12/2020 16:09, Mark Rutland wrote:
> On Fri, Nov 27, 2020 at 03:21:11PM +0000, Steven Price wrote:
>> It's been a week, and I think the comments on v5 made it clear that
>> enforcing PROT_MTE requirements on the VMM was probably the wrong
>> approach. So since I've got swap working correctly without that I
>> thought I'd post a v6 which hopefully addresses all the comments so far.
>>
>> This series adds support for Arm's Memory Tagging Extension (MTE) to
>> KVM, allowing KVM guests to make use of it. This builds on the existing
>> user space support already in v5.10-rc4, see [1] for an overview.
>
>> arch/arm64/include/asm/kvm_emulate.h | 3 +++
>> arch/arm64/include/asm/kvm_host.h | 8 ++++++++
>> arch/arm64/include/asm/pgtable.h | 2 +-
>> arch/arm64/include/asm/sysreg.h | 3 ++-
>> arch/arm64/kernel/mte.c | 18 +++++++++++++-----
>> arch/arm64/kvm/arm.c | 9 +++++++++
>> arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 14 ++++++++++++++
>> arch/arm64/kvm/mmu.c | 16 ++++++++++++++++
>> arch/arm64/kvm/sys_regs.c | 20 +++++++++++++++-----
>> include/uapi/linux/kvm.h | 1 +
>> 10 files changed, 82 insertions(+), 12 deletions(-)
>
> I note that doesn't fixup arch/arm64/kvm/inject_fault.c, where in
> enter_exception64() we have:
>
> | // TODO: TCO (if/when ARMv8.5-MemTag is exposed to guests)
>
> ... and IIUC when MTE is present, TCO should be set when delivering an
> exception, so I believe that needs to be updated to set TCO.

Well spotted! As you say TCO should be set when delivering an exception,
so we need the following:

- // TODO: TCO (if/when ARMv8.5-MemTag is exposed to guests)
+ if (kvm_has_mte(vcpu->kvm))
+ new |= PSR_TCO_BIT;

> Given that MTE-capable HW does that unconditionally, this is going to be
> a mess for big.LITTLE. :/

I'm not sure I follow. Either all CPUs support MTE in which this isn't a
problem, or the MTE feature just isn't exposed. We don't support a mix
of MTE and non-MTE CPUs. There are several aspects of MTE which
effective mean it's an all-or-nothing feature for the system.

Thanks,

Steve