Re: [PATCH v3] powerpc: setup_64: set up PACA earlier to avoid kcov problems

Daniel Axtens's on March 11, 2020 9:03 am:

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So:
 - change the test when setting up a PACA to consider the actual value of
   the MSR rather than the CPU feature.

 - move the PACA setup to before the cpu feature parsing.

Hmm. Problem is that equally we want PACA to be sane before we call too
far into the rest of the kernel ("generic dt parsing code").

But currently we call into that code with no paca at all. Or rather,
with r13 pointing somewhere random that will be interpreted as being a
paca.

This took a while for Daniel to debug because depending on how you boot
r13 contains a different junk value. That junk value may not point to
memory at all, or if it does the memory it points to may or may not send
you down the wrong path, depending on which exact bit you're looking at
in some random location.

So this is really not about kcov from my POV, that's just how we
discovered it.

Ah, yes. I agree with mpe, and reading back over my commit message I
think I did a pretty poor job of explaining it. How about this for a
commit message:

Sorry yeah I'm not quite sure what I was thinking there, because
you actually are moving the paca setup earlier. Hmm. Anyway it seems
okay to me. I would suggest putting a comment in the mfmsr() & MSR_HV
test so it doesn't get used elsewhere. Maybe include CPU_FTR_HVMODE
in the comment so grep shows it up.

Thanks,
Nick