Re: [PATCH v8 11/30] powerpc: Use a datatype for instructions

From: Jordan Niethe <jniethe5@gmail.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Alistair Popple <alistair@popple.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Balamuruhan S <bala24@linux.ibm.com>,
	naveen.n.rao@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org,
	Daniel Axtens <dja@axtens.net>
Subject: Re: [PATCH v8 11/30] powerpc: Use a datatype for instructions
Date: Mon, 11 May 2020 11:19:21 +1000	[thread overview]
Message-ID: <CACzsE9r+BKgEiiz5SztLESZKwYrO-=Y+-pQ+3THJcKzd71udhA@mail.gmail.com> (raw)
In-Reply-To: <7494f275-da54-d6d8-92a8-aede820bb2a7@csgroup.eu>

On Fri, May 8, 2020 at 5:17 PM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 08/05/2020 à 03:51, Jordan Niethe a écrit :
> > On Wed, May 6, 2020 at 1:45 PM Jordan Niethe <jniethe5@gmail.com> wrote:
> >>
> >> Currently unsigned ints are used to represent instructions on powerpc.
> >> This has worked well as instructions have always been 4 byte words.
> >> However, a future ISA version will introduce some changes to
> >> instructions that mean this scheme will no longer work as well. This
> >> change is Prefixed Instructions. A prefixed instruction is made up of a
> >> word prefix followed by a word suffix to make an 8 byte double word
> >> instruction. No matter the endianness of the system the prefix always
> >> comes first. Prefixed instructions are only planned for powerpc64.
> >>
> >> Introduce a ppc_inst type to represent both prefixed and word
> >> instructions on powerpc64 while keeping it possible to exclusively have
> >> word instructions on powerpc32.
> >>
> >> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
> >> ---
> >> v4: New to series
> >> v5: Add to epapr_paravirt.c, kgdb.c
> >> v6: - setup_32.c: machine_init(): Use type
> >>      - feature-fixups.c: do_final_fixups(): Use type
> >>      - optprobes.c: arch_prepare_optimized_kprobe(): change a void * to
> >>        struct ppc_inst *
> >>      - fault.c: store_updates_sp(): Use type
> >>      - Change ppc_inst_equal() implementation from memcpy()
> >> v7: - Fix compilation issue in early_init_dt_scan_epapr() and
> >>        do_patch_instruction() with CONFIG_STRICT_KERNEL_RWX
> >> v8: - style
> >>      - Use in crash_dump.c, mpc86xx_smp.c, smp.c
> >> ---
>
> [...]
>
> >>
> >
> > Hi mpe,
> > Could you add this fixup.
> > --- a/arch/powerpc/lib/feature-fixups.c
> > +++ b/arch/powerpc/lib/feature-fixups.c
> > @@ -356,7 +356,7 @@ static void patch_btb_flush_section(long *curr)
> >          end = (void *)curr + *(curr + 1);
> >          for (; start < end; start++) {
> >                  pr_devel("patching dest %lx\n", (unsigned long)start);
> > -               patch_instruction(start, ppc_inst(PPC_INST_NOP));
> > +               patch_instruction((struct ppc_inst *)start,
> > ppc_inst(PPC_INST_NOP));
> >          }
> >   }
> >
>
> Why not declare stard and end as struct ppc_inst ? Wouldn't it be
> cleaner than a cast ?
What I was thinking was if struct ppc_inst was the prefixed version,
then start++ would increase it by 8 bytes, but the NOPs that are being
patched in are only 4 bytes.
>
> Christophe