Regression: Linux v5.15+ does not boot on Freescale P2020

[Arnd Bergmann]

On Tue, Jul 26, 2022 at 10:34 AM Pali Rohár <pali at kernel.org> wrote:
> On Monday 25 July 2022 16:54:16 Segher Boessenkool wrote:
> > On Mon, Jul 25, 2022 at 10:10:09PM +0200, Pali Rohár wrote:
> > > On Monday 25 July 2022 16:20:49 Christophe Leroy wrote:
> > > Now I did again clean test with same Debian 10 cross compiler.
> > >
> > > $ git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git && cd linux
> > > $ git checkout v5.15
> > > $ make mpc85xx_smp_defconfig ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnuspe-
> > > $ make vmlinux ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnuspe-
> > > $ cp -a vmlinux vmlinux.v5.15
> > > $ git revert 9401f4e46cf6965e23738f70e149172344a01eef
> > > $ make vmlinux ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnuspe-
> > > $ cp -a vmlinux vmlinux.revert
> > > $ powerpc-linux-gnuspe-objdump -d vmlinux.revert > vmlinux.revert.dump
> > > $ powerpc-linux-gnuspe-objdump -d vmlinux.v5.15 > vmlinux.v5.15.dump
> > > $ diff -Naurp vmlinux.v5.15.dump vmlinux.revert.dump
> > >
> > > And there are:
> > >
> > > -c000c304:      7d 20 f8 29     lwarx   r9,0,r31,1
> > > +c000c304:      7d 20 f8 28     lwarx   r9,0,r31
> > >
> > > I guess it must be reproducible this issue as I'm using regular
> > > toolchain from distribution.
> >
>
> > The EH field in larx insns is new since ISA 2.05, and some ISA 1.x cpu
> > implementations actually raise an illegal insn exception on EH=1.  It
> > appears P2020 is one of those.
> >
>
> P2020 has e500 cores. e500 cores uses ISA 2.03. So this may be reason.
> But in official Freescale/NXP documentation for e500 is documented that
> lwarx supports also eh=1. Maybe it is not really supported.
> https://www.nxp.com/files-static/32bit/doc/ref_manual/EREF_RM.pdf (page 562)
> At least there is NOTE:
> Some older processors may treat EH=1 as an illegal instruction.

In commit d6ccb1f55ddf ("powerpc/85xx: Make sure lwarx hint isn't set on ppc32")
this was clarified to affect (all?) e500v1/v2, this one apparently
fixed it before,
but Christophe's commit effectively reverted that change.

I think only the simple_spinlock.h file actually uses EH=1 and this is not
included in non-SMP kernels, so presumably the only affected machines were
the rare dual-core e500v2 ones (p2020, MPC8572, bsc9132), which would
explain why nobody noticed for the past 9 months.

          Arnd