Re: [PATCH v9 1/2] powerpc/64s: reimplement book3s idle code in C

[Michael Ellerman <patch-notifications@ellerman.id.au>]

On Fri, 2019-04-12 at 14:30:52 UTC, Nicholas Piggin 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.
> 
> - KVM secondary wakeup from offline sequence is moved entirely into
>   the offline wakeup, which avoids a hwsync in the normal idle wakeup
>   path.
> 
> Performance measured with context switch ping-pong on different
> threads or cores, is possibly improved a small amount, 1-3% depending
> on stop state and core vs thread test for shallow states. Deep states
> it's in the noise compared with other latencies.
> 
> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Applied to powerpc topic/ppc-kvm, thanks.

https://git.kernel.org/powerpc/c/10d91611f426d4bafd2a83d966c36da8

cheers