Subject: Re: [PATCH 4/4] powerpc/64s: Support shrinking the SLB for debugging

From: Michael Ellerman <[email-masked]>
Date: Sat, 19 Jan 2019 21:27:21 +1100

Michal Suchánek <[email-masked]> writes:

> On Thu, 17 Jan 2019 23:13:28 +1100
> Michael Ellerman <[email-masked]> wrote:
>
>> On machines with 1TB segments and a 32-entry SLB it's quite hard to
>> cause sufficient SLB pressure to trigger bugs caused due to badly
>> timed SLB faults.
>> 
>> We have seen this in the past and a few years ago added the
>> disable_1tb_segments command line option to force the use of 256MB
>> segments. However even this allows some bugs to slip through testing
>> if the SLB entry in question was recently accessed.
>> 
>> So add a new command line parameter for debugging which shrinks the
>> SLB to the minimum size we can support. Currently that size is 3, two
>> bolted SLBs and 1 for dynamic use. This creates the maximal SLB
>
> Doesn't this violate the power of 2 requirement stated in 2/4?

Yes. Good point. This was originally a hack patch in my tree, back when
SLB_NUM_BOLTED was 3 and before Nick even added the slb_used_bitmap, so
back then it was a power of 2 but it also didn't matter :)

I think I'll rework the slb_full_bitmap patch anyway and remove the
power of 2 requirement, so then this patch will be OK.

Thanks for the review!

cheers