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diff --git a/Documentation/arch/arm64/mops.rst b/Documentation/arch/arm64/mops.rst new file mode 100644 index 000000000000..2ef5b147f8dc --- /dev/null +++ b/Documentation/arch/arm64/mops.rst @@ -0,0 +1,44 @@ +.. 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. |