Subject: Re: [RFC][PATCH] powerpc/64s: Leave IRQs hard enabled over context switch

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 03 May 2017 18:24:29 +0200

On Wed, 2017-05-03 at 20:26 +1000, Michael Ellerman wrote:
> Couldn't we avoid the whole problem by just having two bolted slots for
> the stack, meaning we could have the current and next stack bolted at
> all times.
> 
> That would mean we'd be using 4 slots for bolted entries, which is one
> more than 3 - but it might be a good trade off.
> 
> Or we could make the SLB insertion algorithm smarter so that we could
> later free the slot that was used for previous kernel stack.

Changing the insertion algo is a bit nasty, it's nice and simple as-is,
an option would be to replace the bolted "threshold" by a bolted "map"
with a bit (or a byte) per entry.

Sadly, we have no way that I know of to search an slb entry that tells
you in which slot it is, do we ? So even if the "new" stack is already
somewhere in there, we can't know it and just "mark it bolted". We'd
still have to invalidate just in case before bolting it in.

Ben.