On 09/03/2021 11:01, Peter Maydell wrote:
On Mon, 1 Mar 2021 at 14:23, 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 | 37 ++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index aed52b0fc16e..1406ea138127 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -4939,6 +4939,23 @@ KVM_XEN_VCPU_ATTR_TYPE_VCPU_TIME_INFO
Allows Xen vCPU attributes to be read. For the structure and types,
see KVM_XEN_VCPU_SET_ATTR above.
+4.131 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
+
+Copies Memory Tagging Extension (MTE) tags to/from guest tag memory.
Mostly virt/kvm/api.rst seems to include documentation of the
associated structs, something like:
::
struct kvm_arm_copy_mte_tags {
__u64 guest_ipa;
__u64 length;
union {
void __user *addr;
__u64 padding;
};
__u64 flags;
};
which saves the reader having to cross-reference against the header file.
Good point - I'll add that.
It also means you can more naturally use the actual field names in the doc,
eg:
+The
+starting address and length of guest memory must be ``PAGE_SIZE`` aligned.
you could say "The guest_ipa and length fields" here.
Also "The addr field must point to a buffer which the tags will
be copied to or from." I assume.
Indeed - I'll add the clarification.
+The size of the buffer to store the tags is ``(length / MTE_GRANULE_SIZE)``
+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``.
What are the valid values for 'flags' ? It looks like they specify which
direction the copy is, which we definitely need to document here.
Yes either KVM_ARM_TAGS_TO_GUEST or KVM_ARM_TAGS_FROM_GUEST - again I'll
clarify that.
What happens if the caller requests a tag copy for an area of guest
address space which doesn't have tags (eg it has nothing mapped),
or for an area of guest addres space which has tags in some parts
but not in others ?
Guest memory either exists (and has tags) or doesn't exist (assuming MTE
is enabled for the guest). So the cases this can fail are:
* The region isn't completely covered with memslots
* The region isn't completely writable (and KVM_ARM_TAGS_TO_GUEST is
specified).
* User space doesn't have access to the memory (i.e. the memory would
SIGSEGV or similar if the VMM accessed it).
Currently all the above produce the error -ENOENT, which now I come to
enumerate the cases doesn't seem like a great error code (it's really
only appropriate for the first)! Perhaps -EFAULT would be better.
+
5. The kvm_run structure
========================
@@ -6227,6 +6244,25 @@ KVM_RUN_BUS_LOCK flag is used to distinguish between them.
This capability can be used to check / enable 2nd DAWR feature provided
by POWER10 processor.
+7.23 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
s/,/;/
Yep
+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
"guest."
Good spot.
+
8. Other capabilities.
======================
@@ -6716,3 +6752,4 @@ KVM_XEN_HVM_SET_ATTR, KVM_XEN_HVM_GET_ATTR, KVM_XEN_VCPU_SET_ATTR and
KVM_XEN_VCPU_GET_ATTR ioctls, as well as the delivery of exception vectors
for event channel upcalls when the evtchn_upcall_pending field of a vcpu's
vcpu_info is set.
+
--
2.20.1
Stray whitespace change ?
Not sure how that got there - but will remove.
Thanks,
Steve
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