[PATCH v3 4/4] powerpc/64s: idle ESL=0 stop can avoid MSR and save/restore overhead

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> On Wed, 30 Aug 2017 21:25:59 +1000
> Michael Ellerman <mpe at ellerman.id.au> wrote:
>
>> Nicholas Piggin <npiggin at gmail.com> writes:
>> 
>> > When stop is executed with EC=ESL=0, it appears to execute like a
>> > normal instruction (resuming from NIP when woken by interrupt).
>> > So all the save/restore handling can be avoided completely. In
>> > particular NV GPRs do not have to be saved, and MSR does not have
>> > to be switched back to kernel MSR.
>> >
>> > So move the test for "lite" sleep states out to power9_idle_stop.
>> >
>> > Reviewed-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>
>> > Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> > ---
>> >  arch/powerpc/kernel/idle_book3s.S | 35 ++++++++++++++++++++++++-----------
>> >  1 file changed, 24 insertions(+), 11 deletions(-)  
>> 
>> This is blowing up for me on mambo:
>
> Oh this is a known bug in mambo that does not match the hardware.
>
> You need >= Mambo.7.8.21, or this firmware patch to work around
> the issue for old mambos.

As discussed elsewhere this still breaks on new mambo with more than one
CPU. So I've dropped it for now.

cheers