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

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Jul 04, 2024 at 01:15:57PM +0100, Peter Maydell wrote:
> On Sat, 29 Jun 2024 at 04:17, David Gibson <david@gibson.dropbear.id.au> 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?
> 
> fdt32_ld() and friends only do the "load from this bit of memory"
> part, which we already have QEMU utility functions for (and which
> are this patch uses).
> 
> This particular bit of code is dealing with an fdt property ("memory")
> that is an array of (address, size) tuples where address and size
> can independently be either 32 or 64 bits, and it wants the
> size value of tuple 0. So the missing functionality is something at
> a higher level than fdt32_ld() which would let you say "give me
> tuple N field X" with some way to specify the tuple layout. (Which
> is an awkward kind of API to write in C.)

Ah, right.  Yeah.. that's a pretty awkward API in C.

> Slightly less general, but for this case we could perhaps have
> something like the getprop equivalent of qemu_fdt_setprop_sized_cells():
> 
>   uint64_t value_array[2];
>   qemu_fdt_getprop_sized_cells(fdt, nodename, "memory", &value_array,
>                                ac, sc);
>   /*
>    * fills in value_array[0] with address, value_array[1] with size,
>    * probably barfs if the varargs-list of cell-sizes doesn't
>    * cover the whole property, similar to the current assert on
>    * proplen.
>    */
>   mem0_end = value_array[0];

Seems reasonable to me.  The only other thought I had was something
like Python's struct.unpack() [0].  But your suggestion is probably
more natural in C.

[0] https://docs.python.org/3/library/struct.html#struct.unpack

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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