[Skiboot] [RFC 1/1] OPAL : Support spr save-restore using new opal call

[Vaidyanathan Srinivasan]

* Nicholas Piggin <npiggin at gmail.com> [2018-07-24 15:36:48]:

> On Tue, 24 Jul 2018 10:06:12 +1000
> Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:
> 
> > On Mon, 2018-07-23 at 23:41 +1000, Nicholas Piggin wrote:
> > > Right, so we should make OPAL do the whole thing (except catch the
> > > SRESET wakeup which Linux has to do and has well architected SRR1
> > > bits).
> > >   
> > > > 
> > > > We could have a separate OPAL entry point for the wakeup case but
> > > > putting the entirety of the enter/exit into OPAL is complicated.
> > > > 
> > > > We have to arbitrate between multiple threads entering different
> > > > states. We don't know until we exit how deep the core actually went and
> > > > what actually needs to be restored. It's a function of the SRR1 bits on
> > > > return *and* the state that was requested.
> > > > 
> > > > There's also the need to synchronize/rendez-vous threads on wakeup in
> > > > some cases such as TB resync, and dealing with big vs. small core.
> > > > 
> > > > Doing it all in OPAL would require *all* stop states to go to OPAL, I
> > > > don't think that's a great idea for performance.  
> > > 
> > > Oh you want Linux native and OPAL drivers to be used simultaneously?
> > > Is that the right thing to optimise for?  
> > 
> > We want Linux native for states that are known to work (hopefully
> > STOP0/1) and OPAL for things that might need additional resources saved
> > or restored and/or workarounds.
> > 
> > At least that's how I thought of it so far... you prefer having an "all
> > or nothing" approach ?
> 
> As an option, yes. And once you have that option I don't think there is
> much point requiring kernel to know anything about deep states or even
> the STOP instruction or PSSCR register.
> 
> I would not object to an OPAL API that requires all deep states to be
> entered via OPAL but allows shallow states (GPR loss only) to be used
> natively by the kernel concurrently. The shallow states are pretty well
> architected and the deep states not nearly so performance critical.

I agree with your design point.  However based on our experience
across different platforms, we did not want to draw a line for shallow
vs deep for this API.  The framework can be used as fallback for any
state if required.  But in practical situations, we expect to hook
onto this one from the deepest state like STOP11 where we still have
few quirks to deal with.  The STOP11 case is a clear win with OPAL
path because it is so slow and kernel's context management will not
speed it up.

--Vaidy