[PATCH] powerpc/64s: relocation, register save fixes for system reset interrupt

[Nicholas Piggin]

On Thu, 3 Nov 2016 02:32:39 -0400
"Shreyas B. Prabhu" <shreyasbp at gmail.com> wrote:

> On Thu, Nov 3, 2016 at 2:17 AM, Nicholas Piggin <npiggin at gmail.com> wrote:
> > On Thu, 3 Nov 2016 01:56:46 -0400
> > "Shreyas B. Prabhu" <shreyasbp at gmail.com> wrote:
> >  
> >> On Thu, Nov 3, 2016 at 1:21 AM, Nicholas Piggin <npiggin at gmail.com> wrote:  
> >> > On Wed, 2 Nov 2016 14:15:48 +0530
> >> > Gautham R Shenoy <ego at linux.vnet.ibm.com> wrote:
> >> >  
> >> >> Hi Nick,
> >> >>
> >> >> On Wed, Nov 02, 2016 at 07:36:24PM +1100, Nicholas Piggin wrote:  
> >> >> >
> >> >> > Okay, I'll work with that. What's the best way to make a P8 do
> >> >> > winkle sleeps?  
> >> >>
> >> >> From the userspace, offlining the CPUs of the core will put them to
> >> >> winkle.  
> >> >
> >> > Thanks for this. Hum, that r13 manipulation throughout the idle
> >> > and exception code is a bit interesting. I'll do the minimal patch
> >> > for 4.9, but what's the reason not to just use the winkle state
> >> > in the PACA rather than storing it into HSPRG0 bit, can you (or
> >> > Shreyas) explain?
> >> >  
> >> Hi Nick,
> >>
> >> Before deep winkle, checking SRR1's wakeup bits (Bits 46:47) was enough to
> >> figure out which idle state we are waking up from. But in P8, SRR1's wakeup
> >> bits aren't enough since bits 46:47 are 0b11 for both fast sleep and
> >> deep winkle.
> >> So to distinguish bw fastsleep and deep winkle, we use the current HSPRG0/PORE
> >> trick. We program the PORE engine (which is used for state restore when waking
> >> up from deep winkle) to restore HSPRG0 with the last bit set (we do this in
> >> pnv_save_sprs_for_winkle()). R13 bit manipulation in pnv_restore_hyp_resource
> >> is related to this.  
> >
> > Right, I didn't realize how that exactly worked until I had to go read
> > the code just now. It's a neat little trick. I'm wondering can we use PACA_THREAD_IDLE_STATE==PNV_THREAD_WINKLE for this instead? It would just
> > make the early PACA usage in the exception handlers able to use more common
> > code.
> >  
> 
> PACA_THREAD_IDLE_STATE will have what was 'requested'. It may not be the
> state we are waking up from. For example, if 7 threads of the core execute
> winkle instruction while 1 thread of the same core executes sleep. Here
> the core only enters sleep whereas PACA_THREAD_IDLE_STATE for the 7 threads
> will have PNV_THREAD_WINKLE.

I see, that makes sense. Would it be possible to keep count of the number of
threads going into winkle in core_idle_state? Even if that is not a guarantee
if them requiring a PORE wakeup, would the restore case be harmful?

Thanks,
Nick