arm64: mops: Document requirements for hypervisors

Add a mops.rst document to clarify in more detail what hypervisors need
to do to run a Linux guest on a system with FEAT_MOPS.

Signed-off-by: Kristina Martsenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Catalin Marinas <[e-mail]>

Documentation/arch/arm64/booting.rst

···
 
     - HCRX_EL2.MSCEn (bit 11) must be initialised to 0b1.
 
-    - HCRX_EL2.MCE2 (bit 10) must be initialised to 0b1. The exception
-      handler must set PSTATE.SS to 0b0.
+    - HCRX_EL2.MCE2 (bit 10) must be initialised to 0b1 and the hypervisor
+      must handle MOPS exceptions as described in :ref:`arm64_mops_hyp`.
 
   For CPUs with the Extended Translation Control Register feature (FEAT_TCR2):
 

Documentation/arch/arm64/index.rst

···
     legacy_instructions
     memory
     memory-tagging-extension
+    mops
     perf
     pointer-authentication
     ptdump

Documentation/arch/arm64/mops.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Memory copy/set instructions (MOPS)
+===================================
+
+A MOPS memory copy/set operation consists of three consecutive CPY* or SET*
+instructions: a prologue, main and epilogue (for example: CPYP, CPYM, CPYE).
+
+A main or epilogue instruction can take a MOPS exception for various reasons,
+for example when a task is migrated to a CPU with a different MOPS
+implementation, or when the instruction's alignment and size requirements are
+not met. The software exception handler is then expected to reset the registers
+and restart execution from the prologue instruction. Normally this is handled
+by the kernel.
+
+For more details refer to "D1.3.5.7 Memory Copy and Memory Set exceptions" in
+the Arm Architecture Reference Manual DDI 0487K.a (Arm ARM).
+
+.. _arm64_mops_hyp:
+
+Hypervisor requirements
+-----------------------
+
+A hypervisor running a Linux guest must handle all MOPS exceptions from the
+guest kernel, as Linux may not be able to handle the exception at all times.
+For example, a MOPS exception can be taken when the hypervisor migrates a vCPU
+to another physical CPU with a different MOPS implementation.
+
+To do this, the hypervisor must:
+
+  - Set HCRX_EL2.MCE2 to 1 so that the exception is taken to the hypervisor.
+
+  - Have an exception handler that implements the algorithm from the Arm ARM
+    rules CNTMJ and MWFQH.
+
+  - Set the guest's PSTATE.SS to 0 in the exception handler, to handle a
+    potential step of the current instruction.
+
+    Note: Clearing PSTATE.SS is needed so that a single step exception is taken
+    on the next instruction (the prologue instruction). Otherwise prologue
+    would get silently stepped over and the single step exception taken on the
+    main instruction. Note that if the guest instruction is not being stepped
+    then clearing PSTATE.SS has no effect.