linux-kernel - Re: [PATCH] powerpc/powernv: Initialise nest mmu

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Date:	Tue, 16 Aug 2016 10:37:58 +1000
From:	Alistair Popple <alistair@...ple.id.au>
To:	bsingharora@...il.com
Cc:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org, stewart@...ux.vnet.ibm.com,
	mpe@...erman.id.au
Subject: Re: [PATCH] powerpc/powernv: Initialise nest mmu

Balbir,

<snip>

> > +	/* Update partition table control register on all Nest MMUs */
> > +	opal_nmmu_set_ptcr(-1UL, __pa(partition_tb) | (PATB_SIZE_SHIFT - 12));
> > +
> 
> Just wondering if
> 
> 1. Instead of using -1 for all cpus, we should do
> 	for_each_online_cpu() {
> 		opal_numm_set_ptcr(...)
> 	}

Good question, but I don't think it makes sense to do that. The NMMU is
per-chip/socket rather than per-cpu so it shouldn't be tied to
onlining/offlining of individual CPUs.

> 2. In cpu hotplug path do the same when onlining and set to NULL on
> offlining?

Again, the nmmu isn't tied to a specific CPU but rather a chip/socket. So in
theory at least it's possible that all CPUs in a chip could be offline but
other units on the chip could still be using the nmmu so we wouldn't want to
disable the nmmu at that point.

> I think we should definitely try and become hotplug aware from the OS
> view point and drive it from the OS
> 
> What do you think?
>
> Balbir Singh.