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

[Nicholas Piggin]

On Fri, 01 Sep 2017 19:39:41 +1000
Michael Ellerman <mpe at ellerman.id.au> wrote:

> 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.

This now seems to be properly fixed in mambo with commit 11783550ee11.

Skiboot also has a patch merged which disables the problematic state on
mambo so older versions won't crash.

d2a24406a49 ("idle: disable stop*_lite POWER9 idle states for Mambo platform")

So this could be re-applied now.

Thanks,
Nick