Re: [PATCH] hw/mips/malta: Fix the malta machine on big endian hosts

[Thomas Huth]

On 30/03/2023 17.33, Peter Maydell wrote:
> On Thu, 30 Mar 2023 at 16:27, Thomas Huth <thuth@redhat.com> wrote:
> > Booting a Linux kernel with the malta machine is currently broken
> > on big endian hosts. The cpu_to_gt32 macro wants to byteswap a value
> > for little endian targets only, but uses the wrong way to do this:
> > cpu_to_[lb]e32 works the other way round on big endian hosts! Fix
> > it by using the same ways on both, big and little endian hosts.
> >
> > Fixes: 0c8427baf0 ("hw/mips/malta: Use bootloader helper to set BAR registers")
> > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > ---
> >   I've checked that both, the kernel from
> >   https://landley.net/toybox/downloads/binaries/mkroot/0.8.9/mipsel.tgz
> >   and the kernel from
> >   https://landley.net/toybox/downloads/binaries/mkroot/0.8.9/mips.tgz
> >   now boot fine on both, a little endian (x86) and a big endian (s390x) host.
> >
> >   hw/mips/malta.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/hw/mips/malta.c b/hw/mips/malta.c
> > index af9021316d..b26ed1fc9a 100644
> > --- a/hw/mips/malta.c
> > +++ b/hw/mips/malta.c
> > @@ -629,9 +629,9 @@ static void bl_setup_gt64120_jump_kernel(void **p, uint64_t 
> > run_addr,
> >
> >       /* Bus endianess is always reversed */
> >   #if TARGET_BIG_ENDIAN
> > -#define cpu_to_gt32 cpu_to_le32
> > +#define cpu_to_gt32(x) (x)
> >   #else
> > -#define cpu_to_gt32 cpu_to_be32
> > +#define cpu_to_gt32(x) bswap32(x)
> >   #endif
>
> So if we:
>   * do nothing to the value on a BE host
>   * swap the value on an LE host
>
> isn't that the same as cpu_to_be32() in both cases?

No, it's about the *target*, not the host:

* do nothing to the value for a BE *target*
* swap the value for LE *targets*

It's quite weird and it also took me a while to understand this, but this 
seems to be the way it's working right with all combinations.

 Thomas