Re: powerpc: Incorrect stw operand modifier in __set_pte_at
On Wed, Jul 08, 2020 at 06:16:54PM +0200, Christophe Leroy wrote: > Le 08/07/2020 à 16:45, Mathieu Desnoyers a écrit : > >Reviewing use of the patterns "Un%Xn" with lwz and stw instructions > >(where n should be the operand number) within the Linux kernel led > >me to spot those 2 weird cases: > > > >arch/powerpc/include/asm/nohash/pgtable.h:__set_pte_at() > > > > __asm__ __volatile__("\ > > stw%U0%X0 %2,%0\n\ > > eieio\n\ > > stw%U0%X0 %L2,%1" > > : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) > > : "r" (pte) : "memory"); > > > >I would have expected the stw to be: > > > > stw%U1%X1 %L2,%1" > > > >and: > >arch/powerpc/include/asm/book3s/32/pgtable.h:__set_pte_at() > > > > __asm__ __volatile__("\ > > stw%U0%X0 %2,%0\n\ > > eieio\n\ > > stw%U0%X0 %L2,%1" > > : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) > > : "r" (pte) : "memory"); > > > >where I would have expected: > > > > stw%U1%X1 %L2,%1" > > > >Is it a bug or am I missing something ? > > Well spotted. I guess it's definitly a bug. Yes :-) > Introduced 12 years ago by commit > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9bf2b5cd > > ("powerpc: Fixes for CONFIG_PTE_64BIT for SMP support"). > > It's gone unnoticed until now it seems. Apparently it always could use offset form memory accesses? Or even when not, %0 and %1 are likely to use the same base register for addressing :-) Segher
Re: powerpc: Incorrect stw operand modifier in __set_pte_at
Le 08/07/2020 à 16:45, Mathieu Desnoyers a écrit : Hi, Reviewing use of the patterns "Un%Xn" with lwz and stw instructions (where n should be the operand number) within the Linux kernel led me to spot those 2 weird cases: arch/powerpc/include/asm/nohash/pgtable.h:__set_pte_at() __asm__ __volatile__("\ stw%U0%X0 %2,%0\n\ eieio\n\ stw%U0%X0 %L2,%1" : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) : "r" (pte) : "memory"); I would have expected the stw to be: stw%U1%X1 %L2,%1" and: arch/powerpc/include/asm/book3s/32/pgtable.h:__set_pte_at() __asm__ __volatile__("\ stw%U0%X0 %2,%0\n\ eieio\n\ stw%U0%X0 %L2,%1" : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) : "r" (pte) : "memory"); where I would have expected: stw%U1%X1 %L2,%1" Is it a bug or am I missing something ? Well spotted. I guess it's definitly a bug. Introduced 12 years ago by commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9bf2b5cd ("powerpc: Fixes for CONFIG_PTE_64BIT for SMP support"). It's gone unnoticed until now it seems. Can you submit a patch for it ? Christophe
powerpc: Incorrect stw operand modifier in __set_pte_at
Hi, Reviewing use of the patterns "Un%Xn" with lwz and stw instructions (where n should be the operand number) within the Linux kernel led me to spot those 2 weird cases: arch/powerpc/include/asm/nohash/pgtable.h:__set_pte_at() __asm__ __volatile__("\ stw%U0%X0 %2,%0\n\ eieio\n\ stw%U0%X0 %L2,%1" : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) : "r" (pte) : "memory"); I would have expected the stw to be: stw%U1%X1 %L2,%1" and: arch/powerpc/include/asm/book3s/32/pgtable.h:__set_pte_at() __asm__ __volatile__("\ stw%U0%X0 %2,%0\n\ eieio\n\ stw%U0%X0 %L2,%1" : "=m" (*ptep), "=m" (*((unsigned char *)ptep+4)) : "r" (pte) : "memory"); where I would have expected: stw%U1%X1 %L2,%1" Is it a bug or am I missing something ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com