Conversation
jarkko

Jarkko Sakkinen

OK, so WFI on RISC-V is not detemistic! It can be same as HLT (x86) or WAIT (ARM) but could also be nop 🤷 Beyond me tbh. #riscv
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sdbbp@mastodon.social

sdbbp

Reply to @jarkko

@jarkko WFI on RISC-V is not that different from in ARMv8. In both cases, SW should treat it as a potentially-power-saving NOP. There are special cases around scenarios like CPU physical reset and power down. But, basically a NOP from an architectural point of view.

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edwintorok@discuss.systems

Edwin Török

Reply to @jarkko

@jarkko looks like WFI allows for spurious wakeups, if the code handles WFI==NOP, then it'll also work correctly for spurious wake-ups. A hypervisor may also trap WFI and switch to another guest.

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jarkko

Jarkko Sakkinen

Reply to @edwintorok@discuss.systems
@edwintorok @sdbbp right thanks for to great answers! i mean mostly hlt and similar are used to conserve energy (e.g. in PM an similar), and e.g. locks just reschedule so i could imagine that in e.g. linux nop should not cause any actual collateral damage.

i'm just used to specs where opcode does one and only one thing :-)
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