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Date:   Thu, 5 Oct 2017 21:04:20 +0200
From:   Fredrik Noring <noring@nocrew.org>
To:     "Maciej W. Rozycki" <macro@imgtec.com>
Cc:     linux-mips@linux-mips.org
Subject: Re: [PATCH v2] MIPS: Add basic R5900 support
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Hi Maciej,

> >  Anyway, as noted above that shouldn't cause a problem with user software
> > and I think that any corruption you can see comes from elsewhere.  You'll
> > have to paper this $ra non-sign-extension issue over somehow to proceed
> > though.
> 
> During early kernel initialisation at least one other register (besides
> $ra) appears to fail the sign-extension test, and the error cannot be
> ignored. I will now try figure this out.

Trap 12 is causing kernel inconvenience resulting in "kernel bug" crashes
when triggering sign-extension failures for the remaining registers. As a
less intrusive measure I've collected some statistics:

Failed registers are marked by bit position in 0x8000FF74, hence 
registers $v0, $a0-$a2, $t0-$t7 and $ra fail the sign-extension test.

Problems are present during kernel initialisation, so either 1) the R5900
is not really 32-bit compatible, 2) the compiler is somehow generating
64-bit code, 3) assembler directives in the R5900 patch produce 64-bit code,
or 4) the sign-extension tests are wrong (or a combination of the above).

In principle, I suppose one could carefully write assembly code to save
all registers and stack traces in a reserved memory area for later
examination, avoiding "kernel bug" crashes when immediately triggering
trap 12 in a sensitive kernel context.

What are your thoughts on how to proceed?

Fredrik