Re: [PATCH v12 6/8] arm64: kvm: Expose KVM_ARM_CAP_MTE

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

On 20/05/2021 11:09, Marc Zyngier wrote:
> On Wed, 19 May 2021 14:26:31 +0100,
> Steven Price <steven.price@arm.com> wrote:
>>
>> On 17/05/2021 18:40, Marc Zyngier wrote:
>>> On Mon, 17 May 2021 13:32:37 +0100,
>>> Steven Price <steven.price@arm.com> wrote:
>>>>
>>>> It's now safe for the VMM to enable MTE in a guest, so expose the
>>>> capability to user space.
>>>>
>>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>>> ---
>>>>  arch/arm64/kvm/arm.c      | 9 +++++++++
>>>>  arch/arm64/kvm/sys_regs.c | 3 +++
>>>>  2 files changed, 12 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>>>> index 1cb39c0803a4..e89a5e275e25 100644
>>>> --- a/arch/arm64/kvm/arm.c
>>>> +++ b/arch/arm64/kvm/arm.c
>>>> @@ -93,6 +93,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>>>  		r = 0;
>>>>  		kvm->arch.return_nisv_io_abort_to_user = true;
>>>>  		break;
>>>> +	case KVM_CAP_ARM_MTE:
>>>> +		if (!system_supports_mte() || kvm->created_vcpus)
>>>> +			return -EINVAL;
>>>> +		r = 0;
>>>> +		kvm->arch.mte_enabled = true;
>>>
>>> As far as I can tell from the architecture, this isn't valid for a
>>> 32bit guest.
>>
>> Indeed, however the MTE flag is a property of the VM not of the vCPU.
>> And, unless I'm mistaken, it's technically possible to create a VM where
>> some CPUs are 32 bit and some 64 bit. Not that I can see much use of a
>> configuration like that.
> 
> It looks that this is indeed a bug, and I'm on my way to squash it.
> Can't believe we allowed that for so long...

Ah, well if you're going to kill off mixed 32bit/64bit VMs then...

> But the architecture clearly states:
> 
> <quote>
> These features are supported in AArch64 state only.
> </quote>
> 
> So I'd expect something like:
> 
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 956cdc240148..50635eacfa43 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -220,7 +220,8 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  	switch (vcpu->arch.target) {
>  	default:
>  		if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
> -			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1)) {
> +			if (!cpus_have_const_cap(ARM64_HAS_32BIT_EL1) ||
> +			    vcpu->kvm->arch.mte_enabled) {
>  				ret = -EINVAL;
>  				goto out;
>  			}
> 
> that makes it completely impossible to create 32bit CPUs within a
> MTE-enabled guest.

... that makes complete sense, and I'll include this hunk in my next
posting.

Thanks,

Steve