Re: [PATCH v2 2/2] powerpc/64s: reimplement book3s idle code in C

On Fri, 10 Aug 2018 16:42:49 +1000
Nicholas Piggin [off-list ref] wrote:

Reimplement Book3S idle code in C, moving POWER7/8/9 implementation
speific HV idle code to the powernv platform code.

Book3S assembly stubs are kept in common code and used only to save
the stack frame and non-volatile GPRs before executing architected
idle instructions, and restoring the stack and reloading GPRs then
returning to C after waking from idle.

The complex logic dealing with threads and subcores, locking, SPRs,
HMIs, timebase resync, etc., is all done in C which makes it more
maintainable.

This is not a strict translation to C code, there are some
significant differences:

- Idle wakeup no longer uses the ->cpu_restore call to reinit SPRs,
  but saves and restores them itself.

- The optimisation where EC=ESL=0 idle modes did not have to save GPRs
  or change MSR is restored, because it's now simple to do. ESL=1
  sleeps that do not lose GPRs can use this optimization too.

- KVM secondary entry and cede is now more of a call/return style
  rather than branchy. nap_state_lost is not required because KVM
  always returns via NVGPR restoring path.

Reviewed-by: Gautham R. Shenoy <redacted>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Left to do:
- KVM could use more review, it's pretty tricky. Not sure if what
  I'm doing with the emergency stack is kosher. But it runs pretty fine
  here with a POWER9 SMP+SMT guest. Possible to streamline

That should be POWER8. POWER9 radix on hash with dependent threads
mode also seems to work okay.

Thanks,
Nick