Re: [RFC PATCH 3/3] cpuidle/powernv: Conditionally save-restore sprs using opal

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

Hello Nicholas,

On Fri, Aug 03, 2018 at 12:05:47AM +1000, Nicholas Piggin wrote:
> On Thu,  2 Aug 2018 10:21:32 +0530
> Akshay Adiga <akshay.adiga@linux.vnet.ibm.com> wrote:
> 
> > From: Abhishek Goel <huntbag@linux.vnet.ibm.com>
> > 
> > If a state has "opal-supported" compat flag in device-tree, an opal call
> > needs to be made during the entry and exit of the stop state. This patch
> > passes a hint to the power9_idle_stop and power9_offline_stop.
> > 
> > This patch moves the saving and restoring of sprs for P9 cpuidle
> > from kernel to opal. This patch still uses existing code to detect
> > first thread in core.
> > In an attempt to make the powernv idle code backward compatible,
> > and to some extent forward compatible, add support for pre-stop entry
> > and post-stop exit actions in OPAL. If a kernel knows about this
> > opal call, then just a firmware supporting newer hardware is required,
> > instead of waiting for kernel updates.
> 
> Still think we should make these do-everything calls. Including
> executing nap/stop instructions, restoring timebase, possibly even
> saving and restoring SLB (although a return code could be used to
> tell the kernel to do that maybe if performance advantage is
enough).

So, if we execute the stop instruction in opal, the wakeup from stop
still happens at the hypervisor 0x100. On wake up, we need to check
SRR1 to see if we have lost state, in which case, the stop exit also
needs to be handled inside opal. On return from this opal call, we
need to unwind the extra stack frame that would have been created when
kernel entered opal to execute the stop from which there was no
return. In the case where a lossy stop state was requested, but wakeup
happened from a lossless stop state, this adds additional overhead.

Furthermore, the measurements show that the additional time taken to
perform the restore of the resources in OPAL vs doing so in Kernel on
wakeup from stop takes additional 5-10us. For the current stop states
that lose hypervisor state, since the latency is relatively high (100s
of us), this is a relatively small penalty (~1%) .

However, in future if we do have states that lose only a part of
hypervisor state to provide a wakeup latency in the order of few tens
of microseconds the additional latency caused by OPAL call would
become noticable, no ?

	
> 
> I haven't had a lot of time to go through it, I'm working on moving
> ~all of idle_book3s.S to C code, I'd like to do that before this
> OPAL idle driver if possible.
> 
> A minor thing I just noticed, you don't have to allocate the opal
> spr save space in Linux, just do it all in OPAL.

The idea was to not leave any state in OPAL, as OPAL is supposed to be
state-less. However, I agree, that if OPAL is not going to interpret
the contents of the save/area, it should be harmless to move that bit
into OPAL.

That said, if we are going to add the logic of determining the first
thread in the core waking up, etc, then we have no choice but to
maintain that state in OPAL.


> 
> Thanks,
> Nick
> 
--
Thanks and Regards
gautham.