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

[Pali Rohár]

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 kernel had
> 
> #define PPC_RAW_LWARX(t, a, b, eh)       (0x7c000028 | ___PPC_RT(t) | ___PPC_RA(a) | ___PPC_RB(b) | __PPC_EH(eh))
> 
> and
> 
> #define PPC_LWARX(t, a, b, eh) stringify_in_c(.long PPC_RAW_LWARX(t, a, b, eh))
> 
> and
> 
> #ifdef CONFIG_PPC64
> #define __PPC_EH(eh)    (((eh) & 0x1) << 0)
> #else
> #define __PPC_EH(eh)    0
> #endif
> 
> but Christophe's 9401f4e46cf6 changed
> 
> -"1:    " PPC_LWARX(%0,0,%2,1) "\n\
> +"1:    lwarx           %0,0,%2,1\n\
> 
> no longer checking CONFIG_PPC64.  That appears to be the bug.

Nice catch!

Now I have tried to apply following change on master (without reverting
anything)

diff --git a/arch/powerpc/include/asm/simple_spinlock.h b/arch/powerpc/include/asm/simple_spinlock.h
index 7ae6aeef8464..72d3657fd2f7 100644
--- a/arch/powerpc/include/asm/simple_spinlock.h
+++ b/arch/powerpc/include/asm/simple_spinlock.h
@@ -51,7 +51,7 @@ static inline unsigned long __arch_spin_trylock(arch_spinlock_t *lock)
 
 	token = LOCK_TOKEN;
 	__asm__ __volatile__(
-"1:	lwarx		%0,0,%2,1\n\
+"1:	lwarx		%0,0,%2,0\n\
 	cmpwi		0,%0,0\n\
 	bne-		2f\n\
 	stwcx.		%1,0,%2\n\
@@ -158,7 +158,7 @@ static inline long __arch_read_trylock(arch_rwlock_t *rw)
 	long tmp;
 
 	__asm__ __volatile__(
-"1:	lwarx		%0,0,%1,1\n"
+"1:	lwarx		%0,0,%1,0\n"
 	__DO_SIGN_EXTEND
 "	addic.		%0,%0,1\n\
 	ble-		2f\n"
@@ -182,7 +182,7 @@ static inline long __arch_write_trylock(arch_rwlock_t *rw)
 
 	token = WRLOCK_TOKEN;
 	__asm__ __volatile__(
-"1:	lwarx		%0,0,%2,1\n\
+"1:	lwarx		%0,0,%2,0\n\
 	cmpwi		0,%0,0\n\
 	bne-		2f\n"
 "	stwcx.		%1,0,%2\n\

and with this change, objdump showed exactly same result as if I revert
that problematic commit on top of master branch.

I guess that simple_spinlock.h should be fixed to pass 1 to lwarx for
CONFIG_PPC64 and 0 otherwise.

Christophe, are you going to look at it?

> 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.
> 
> 
> Segher

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.