Re: [PATCH 1/3] powerpc/powernv: Avoid the secondary hold spinloop for OPAL boot

[Nicholas Piggin <npiggin@gmail.com>]

On Tue, 10 Oct 2017 21:44:15 +1000
Nicholas Piggin <npiggin@gmail.com> wrote:

> On Tue, 10 Oct 2017 22:11:46 +1100
> Michael Ellerman <mpe@ellerman.id.au> wrote:
> 
> > Nicholas Piggin <npiggin@gmail.com> writes:
> >   
> > > OPAL boot does not insert secondaries at 0x60 to wait at the secondary
> > > hold spinloop. Instead it keeps them held in firmware until the
> > > opal_start_cpu call is made, which directs them where the caller
> > > specifies. Linux inserts them into generic_secondary_smp_init(), which
> > > is after the secondary hold spinloop (they go on to spin at the per-CPU
> > > paca loops, but that is another step).
> > >
> > > So avoid waiting on this spinloop when booting with OPAL firmware.
> > > It always just times out.
> > >
> > > This saves 100ms boot time on bare metal, and 10s of seconds when
> > > booting the simulator in SMP.    
> > 
> > Oh nice, that's real facepalm territory.
> > 
> > It'd be neater if we just inserted them at 0x60, but the sequence is
> > wrong.
> > 
> > Can we fix it just by making spinning_secondaries zero on OPAL?  
> 
> I had a look at that, but generic_secondary_smp_init() still
> decrements it, so it would underflow which I thought was
> uglier.
> 
> I actually have to look a bit further, because KVM guests are
> also having the loop time out too by the looks.

Ahh okay, pseries is using the start-cpu RTAS call to enter at
generic_secondary_smp_init() as well. So we can take it out for
pseries as well.

Thanks,
Nick