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

[Nicholas Piggin]

On Wed, 03 May 2017 10:28:27 +0200
Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:

> On Wed, 2017-05-03 at 17:34 +1000, Nicholas Piggin wrote:
> > Extending the soft IRQ disable to cover PMU interrupts will allow this
> > hard disable to be removed from hash based kernels too, but they will
> > still have to soft-disable PMU interrupts.
> > 
> > - Q1: Can we do this? It gives nice profiles of context switch code
> >   rather than assigning it all to local_irq_enable.  
> 
> Probably ok with radix yes.

Cool.

> > - Q2: What is the unrecoverable SLB miss on exception entry? Is there
> >   anywhere we access the kernel stack with RI disabled? Something else?  
> 
> Not sure what you mean by Q2, but the original problem is an occurrence
> of what we call the 'megabug' which hit us in different forms over the
> years, and happens when we get a kernel stack SLB entry wrong.
> 
> Normally, the segment containing the current kernel stack is always
> bolted in the SLB in a specific slot. If we accidentally lose that
> "bolt", we can end up faulting it into the wrong slot, thus making it
> possible for it to get evicted later on etc...
> 
> That in turns hits the exception return path which accesses the kernel
> stack after clearing RI and setting SRR0/1 to the return values.

This is exactly my question. The original patch said there was an
unrecoverable SLB miss at exception entry. Either I missed that or
it has since been removed. The stack access at exit would do it. I
didn't want to change anything for hash, just wondering where the
bug was (subject line got truncated but is supposed to say "for radix").

Thanks,
Nick