[PATCH 7/7] Documentation/arch/x86/amd_hsmp: Document the SMN and MSR IOCTLs

[Muralidhara M K]

Describe HSMP_IOCTL_SMN_CMD and HSMP_IOCTL_MSR_CMD: the payload structs,
the CAP_SYS_RAWIO and open-mode requirements, and the rule that both are
read-only, so a request with @write set is refused with -EPERM even for a
fully privileged caller on an O_RDWR handle.

Also note that reading the protocol_version attribute can fail on the
Fam1Ah client platforms, because some client SMU builds reject the
interface-version query, which is why the telemetry ioctl rather than
that attribute is the way to reach the client telemetry layout.

Signed-off-by: Muralidhara M K <muralidhara.mk@xxxxxxx>
---
 Documentation/arch/x86/amd_hsmp.rst | 44 +++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/Documentation/arch/x86/amd_hsmp.rst b/Documentation/arch/x86/amd_hsmp.rst
index 8a9d6fe7d4cf..0f03201a9f68 100644
--- a/Documentation/arch/x86/amd_hsmp.rst
+++ b/Documentation/arch/x86/amd_hsmp.rst
@@ -229,6 +229,50 @@ The following IOCTLs are defined:
   ``sizeof(struct hsmp_metric_table)``.  Later version metrics table
   layout is documented in the Public PPR.
 
+  On the Fam1Ah client platforms reading ``protocol_version`` can fail,
+  because some client SMU builds reject the interface-version query.
+  Those parts have their own telemetry layout and this ioctl is the way
+  to read it.
+
+``ioctl(file, HSMP_IOCTL_SMN_CMD, struct hsmp_smn_message *smn)``
+  Read a 32-bit SMN register on the socket given by ``sock_ind``.  This
+  reaches registers outside the HSMP message ABI, so it requires
+  ``CAP_SYS_RAWIO`` as well as a handle opened for reading, the latter
+  because the value read is written back into the request struct.
+
+  The ioctl is read-only.  ``write`` must be zero, and a request that
+  sets it is rejected with ``-EPERM`` even with ``CAP_SYS_RAWIO`` and an
+  ``O_RDWR`` handle; the field exists only so that write support could be
+  added later without changing the layout.  ``reserved`` must be zero.
+  Argument::
+
+    struct hsmp_smn_message {
+        __u32 smn_address; /* SMN address to read */
+        __u32 value;       /* Value read, filled in by the kernel */
+        __u16 sock_ind;    /* Socket index */
+        __u8  write;       /* Must be zero */
+        __u8  reserved;    /* Reserved, must be zero */
+    };
+
+``ioctl(file, HSMP_IOCTL_MSR_CMD, struct hsmp_msr_message *msr)``
+  Read a 64-bit MSR on the logical CPU given by ``core_id``, or on
+  whichever CPU the ioctl runs on when ``core_id`` is
+  ``HSMP_MSR_ANY_CPU`` (``(__u32)-1``).  Any other ``core_id`` must refer
+  to an online CPU, otherwise the call fails with ``-EINVAL``.  Requires
+  ``CAP_SYS_RAWIO`` and a handle opened for reading.
+
+  Read-only on the same terms as ``HSMP_IOCTL_SMN_CMD``: a non-zero
+  ``write`` is rejected with ``-EPERM``, and every byte of ``reserved``
+  must be zero.  Argument::
+
+    struct hsmp_msr_message {
+        __u32 msr_address; /* MSR index to read */
+        __u32 core_id;     /* Logical CPU, or HSMP_MSR_ANY_CPU */
+        __u64 value;       /* Value read, filled in by the kernel */
+        __u8  write;       /* Must be zero */
+        __u8  reserved[3]; /* Reserved, must be zero */
+    };
+
 The ioctl would return a non-zero on failure; you can read errno to see
 what happened. The transaction returns 0 on success.
 
-- 
2.34.1