Re: [PATCH v12 8/8] KVM: arm64: Document MTE capability and ioctl

[Steven Price]

On 17/05/2021 19:09, Marc Zyngier wrote:
> On Mon, 17 May 2021 13:32:39 +0100,
> Steven Price <steven.price@xxxxxxx> wrote:
>>
>> A new capability (KVM_CAP_ARM_MTE) identifies that the kernel supports
>> granting a guest access to the tags, and provides a mechanism for the
>> VMM to enable it.
>>
>> A new ioctl (KVM_ARM_MTE_COPY_TAGS) provides a simple way for a VMM to
>> access the tags of a guest without having to maintain a PROT_MTE mapping
>> in userspace. The above capability gates access to the ioctl.
>>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>>  Documentation/virt/kvm/api.rst | 53 ++++++++++++++++++++++++++++++++++
>>  1 file changed, 53 insertions(+)
>>
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 22d077562149..a31661b870ba 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -5034,6 +5034,40 @@ see KVM_XEN_VCPU_SET_ATTR above.
>>  The KVM_XEN_VCPU_ATTR_TYPE_RUNSTATE_ADJUST type may not be used
>>  with the KVM_XEN_VCPU_GET_ATTR ioctl.
>>  
>> +4.130 KVM_ARM_MTE_COPY_TAGS
>> +---------------------------
>> +
>> +:Capability: KVM_CAP_ARM_MTE
>> +:Architectures: arm64
>> +:Type: vm ioctl
>> +:Parameters: struct kvm_arm_copy_mte_tags
>> +:Returns: 0 on success, < 0 on error
>> +
>> +::
>> +
>> +  struct kvm_arm_copy_mte_tags {
>> +	__u64 guest_ipa;
>> +	__u64 length;
>> +	union {
>> +		void __user *addr;
>> +		__u64 padding;
>> +	};
>> +	__u64 flags;
>> +	__u64 reserved[2];
>> +  };
> 
> This doesn't exactly match the structure in the previous patch :-(.

:( I knew there was a reason I didn't include it in the documentation
for the first 9 versions... I'll fix this up, thanks for spotting it.

>> +
>> +Copies Memory Tagging Extension (MTE) tags to/from guest tag memory. The
>> +``guest_ipa`` and ``length`` fields must be ``PAGE_SIZE`` aligned. The ``addr``
>> +fieldmust point to a buffer which the tags will be copied to or from.
>> +
>> +``flags`` specifies the direction of copy, either ``KVM_ARM_TAGS_TO_GUEST`` or
>> +``KVM_ARM_TAGS_FROM_GUEST``.
>> +
>> +The size of the buffer to store the tags is ``(length / MTE_GRANULE_SIZE)``
> 
> Should we add a UAPI definition for MTE_GRANULE_SIZE?

I wasn't sure whether to export this or not. The ioctl is based around
the existing ptrace interface (PTRACE_{PEEK,POKE}MTETAGS) which doesn't
expose a UAPI definition. Admittedly the documentation there also just
says "16-byte granule" rather than MTE_GRANULE_SIZE.

So I'll just remove the reference to MTE_GRANULE_SIZE in the
documentation unless you feel that we should have a UAPI definition.

>> +bytes (i.e. 1/16th of the corresponding size). Each byte contains a single tag
>> +value. This matches the format of ``PTRACE_PEEKMTETAGS`` and
>> +``PTRACE_POKEMTETAGS``.
>> +
>>  5. The kvm_run structure
>>  ========================
>>  
>> @@ -6362,6 +6396,25 @@ default.
>>  
>>  See Documentation/x86/sgx/2.Kernel-internals.rst for more details.
>>  
>> +7.26 KVM_CAP_ARM_MTE
>> +--------------------
>> +
>> +:Architectures: arm64
>> +:Parameters: none
>> +
>> +This capability indicates that KVM (and the hardware) supports exposing the
>> +Memory Tagging Extensions (MTE) to the guest. It must also be enabled by the
>> +VMM before the guest will be granted access.
>> +
>> +When enabled the guest is able to access tags associated with any memory given
>> +to the guest. KVM will ensure that the pages are flagged ``PG_mte_tagged`` so
>> +that the tags are maintained during swap or hibernation of the host; however
>> +the VMM needs to manually save/restore the tags as appropriate if the VM is
>> +migrated.
>> +
>> +When enabled the VMM may make use of the ``KVM_ARM_MTE_COPY_TAGS`` ioctl to
>> +perform a bulk copy of tags to/from the guest.
>> +
> 
> Missing limitation to AArch64 guests.

As mentioned previously it's not technically limited to AArch64, but
I'll expand this to make it clear that MTE isn't usable from a AArch32 VCPU.

Thanks,

Steve