WFE/SEV
- Wait for event
  - The Event Register
  - The Send Event instructions
- Further Reading

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?

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