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

On Fri, 15 Jun 2018 21:37:15 +1000
Michael Ellerman [off-list ref] wrote:

Nicholas Piggin [off-list ref] writes:

quoted

On Wed, 13 Jun 2018 23:24:14 +1000
Michael Ellerman [off-list ref] wrote:
 

quoted

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.

Yep this patch looks fine for a minimal fix.

quoted

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.

quoted

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?

Yeah I noticed that too a while back, was it an old OPAL API or maybe a
proposed new API that was never implemented? I would like to end up
doing most MCE decoding in firmware at some point, but I don't think
it's worth keeping this existing ABI thing around for it.

Thanks,
Nick