[PATCH] powerpc/64s: Report SLB multi-hit rather than parity error

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:
> On Wed, 13 Jun 2018 23:24:14 +1000
> Michael Ellerman <mpe at ellerman.id.au> wrote:
>
>> When we take an SLB multi-hit on bare metal, we see both the multi-hit
>> and parity error bits set in DSISR. The user manuals indicates this is
>> expected to always happen on Power8, whereas on Power9 it says a
>> multi-hit will "usually" also cause a parity error.
>> 
>> We decide what to do based on the various error tables in mce_power.c,
>> and because we process them in order and only report the first, we
>> currently always report a parity error but not the multi-hit, eg:
>> 
>>   Severe Machine check interrupt [Recovered]
>>     Initiator: CPU
>>     Error type: SLB [Parity]
>>       Effective address: c000000ffffd4300
>> 
>> Although this is correct, it leaves the user wondering why they got a
>> parity error. It would be clearer instead if we reported the
>> multi-hit because that is more likely to be simply a software bug,
>> whereas a true parity error is possibly an indication of a bad core.
>> 
>> We can do that simply by reordering the error tables so that multi-hit
>> appears before parity. That doesn't affect the error recovery at all,
>> because we flush the SLB either way.
>
> Yeah this is a good idea. I wonder if there are any other conditions
> like this that should be reordered.

Yeah good point, this one just caught my eye because I was testing it.
Ideally it wouldn't matter and we could actually report multiple, but
that would be a bit of a bigger change.

> I think the i-side should not have to be changed here because it
> matches the value not bits, so that shouldn't matter.

Ah OK, will check.

> A bit of a shame we don't report i/d side, and ideally we'd be able
> to report multiple conditions. The reporting APIs really want to be
> massaged a bit, but for now this is a good step.

Ah snap, yep, more detail & multiple conditions would be nice.

I don't really understand the way we do the reporting now. The
struct machine_check_event is all carefully laid out with reserved
fields and a version number and everything as if it's an ABI. But AFAICS
it's purely internal to the kernel.

And then we have struct mce_error_info, but that's a separate thing and
struct machine_check_event doesn't contain one of them?

cheers