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From: Nicholas Piggin <npiggin@gmail.com>
To: Daniel Axtens <dja@axtens.net>,
	linuxppc-dev@lists.ozlabs.org,
	Michael Ellerman <mpe@ellerman.id.au>
Cc: ajd@linux.ibm.com
Subject: Re: [PATCH v3] powerpc: setup_64: set up PACA earlier to avoid kcov problems
Date: Sat, 14 Mar 2020 13:37:15 +1000	[thread overview]
Message-ID: <1584156888.f924kplz26.astroid@bobo.none> (raw)
In-Reply-To: <87lfo7q0cb.fsf@dja-thinkpad.axtens.net>

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

      reply	other threads:[~2020-03-14  3:39 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-06  7:30 [PATCH v3] powerpc: setup_64: set up PACA earlier to avoid kcov problems Daniel Axtens
2020-03-06  7:59 ` Andrew Donnellan
2020-03-06 13:30   ` Daniel Axtens
2020-03-07  1:32 ` Nicholas Piggin
2020-03-10 11:41   ` Michael Ellerman
2020-03-10 23:03     ` Daniel Axtens
2020-03-14  3:37       ` Nicholas Piggin [this message]

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