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ARM

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WFE/SEV

Introduce WFE/WFET, SEV/SEVL instructions.
  • These instructions are used to implement the following mechanisms for power saving (Put a PE into low-power state that is IMPLEMENTATION DEFINED).
    • Wait for event (WFE/WFET, SEV/SEVL)
  • The working mechanism of WFE/WFET, SEV/SEVL is based on the Event register(1). Each PE has a bit in the Event register corresponding to it.
  • The Event register can only be cleared by WFE/WFET, but can be set by SEV/SEVL or other mechanisms (This means that the WFE/WFET can be completed when these cases occur.).
  • SEV/SEVL cannot wake up a PE that has been placed in a low power state by the WFI or WFIT.

Wait for event

The Event Register

When the PE executes WFE or WFET, all of the following apply.
  • If the Event register is clear, then the PE can enter the low-power state.
  • If the Event register is set, then all of the following apply:
    • The PE does not suspend operation.
    • The Event register is cleared.
    • The WFE or WFET instruction completes immediately.

The Event Register for a PE is set by any of the following:

  • A Send Event instruction, SEV, executed by any PE in the system.
  • A Send Event Local instruction, SEVL, executed by the PE.
  • An exception return.
  • The clearing of the global monitor for the PE.
  • An event from a Generic Timer event stream.
  • An event sent by some IMPLEMENTATION DEFINED mechanism.

The Send Event instructions

  • SEV, Send Event: Causes an event to be signaled to all PEs in a multiprocessor system.(2)
  • SEVL, Send Event Local: Causes an event to be signaled locally without requiring the event to be signaled to other PEs in a multiprocessor system.

Why need SEVL?

In ARMv8-A A64, the WFE instruction is unconditional. So I think there are two typical ways to implement the lock function, with or without SEVL.

The first way (without SEVL) is: 

func spin_lock
  MOV   w2, #1
loop:
  LDXR  w1, [x0]
  CBNZ  w1, sleep
  STXR  w1, w2, [x0]
  CBZ   w1, end
  B     loop
sleep:
  WFE
  B     loop
end:
  ret
endfunc spin_lock

The second way (with SEVL) is: 

func spin_lock
  MOV   w2, #1
  SEVL
l1:
  WFE
l2:
  LDXR  w1, [x0]
  CBNZ  w1, l1
  STXR  w1, w2, [x0]
  CBNZ  w1, l2
  RET
endfunc spin_lock

As the first way listed above, I think the lock function will still work fine with just WFE. Therefore, I still don't understand why ARM designed it. Maybe it can be used for efficiency or other scenarios.

Notes

(1) A conceptual register which cannot be explicitly accessed.
(2) Set only the Event register? Can it do something else by IMPLEMENTATION DEFINED?

Further Reading

Creation Date: 2023/05/17, Last modified: 2023/10/26, Author: Siyuan Liu

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