Re: [PATCH v2 06/15] ppc/vof: Fix unaligned FDT property access

["Nicholas Piggin" <npiggin@gmail.com>]

On Sat Jun 29, 2024 at 1:16 PM AEST, David Gibson wrote:
> On Fri, Jun 28, 2024 at 04:20:02PM +0100, Peter Maydell wrote:
> > On Thu, 27 Jun 2024 at 14:39, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > >
> > > FDT properties are aligned by 4 bytes, not 8 bytes.
> > >
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > ---
> > >  hw/ppc/vof.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/hw/ppc/vof.c b/hw/ppc/vof.c
> > > index e3b430a81f4f..b5b6514d79fc 100644
> > > --- a/hw/ppc/vof.c
> > > +++ b/hw/ppc/vof.c
> > > @@ -646,7 +646,7 @@ static void vof_dt_memory_available(void *fdt, GArray *claimed, uint64_t base)
> > >      mem0_reg = fdt_getprop(fdt, offset, "reg", &proplen);
> > >      g_assert(mem0_reg && proplen == sizeof(uint32_t) * (ac + sc));
> > >      if (sc == 2) {
> > > -        mem0_end = be64_to_cpu(*(uint64_t *)(mem0_reg + sizeof(uint32_t) * ac));
> > > +        mem0_end = ldq_be_p(mem0_reg + sizeof(uint32_t) * ac);
> > >      } else {
> > >          mem0_end = be32_to_cpu(*(uint32_t *)(mem0_reg + sizeof(uint32_t) * ac));
> > >      }
> > 
> > I did wonder if there was a better way to do what this is doing,
> > but neither we (in system/device_tree.c) nor libfdt seem to
> > provide one.
>
> libfdt does provide unaligned access helpers (fdt32_ld() etc.), but
> not an automatic aligned-or-unaligned helper.   Maybe we should add that?

Runtime test if the pointer is aligned?

What about just fdt_prop32_ld() and fdt_prop64_ld() where you know it's
4 byte aligned. Then just do 2 x 4 byte loads for the 64-bit, I don't
think performance would matter so much to try get a single load.

Thanks,
Nick