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

[Steven Price <steven.price@arm.com>]

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