Re: [Qemu-devel] [PATCH v2 1/2] cpu: Provide a proper prototype for target_words_bigendian() in a header

[Laszlo Ersek <lersek@redhat.com>]

On 10/05/18 15:01, Thomas Huth wrote:
> We've got three places already that provide a prototype for this
> function in a .c file - that's ugly. Let's provide a proper prototype
> in a header instead, with a proper description why this function should
> not be used in most cases.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  NB: I've tried to replace the "utterly broken" comment with something
>  better, as suggested by Laszlo... not sure whether I've really got the
>  message right here, so feel free to provide some better wording if you
>  like!
> 
>  exec.c             |  5 -----
>  hw/virtio/virtio.c |  1 -
>  include/qom/cpu.h  | 11 +++++++++++
>  qom/cpu.c          |  1 -
>  4 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index d0821e6..5d99ef5 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -3906,11 +3906,6 @@ int qemu_target_page_bits_min(void)
>  }
>  #endif
>  
> -/*
> - * A helper function for the _utterly broken_ virtio device model to find out if
> - * it's running on a big endian machine. Don't do this at home kids!
> - */
> -bool target_words_bigendian(void);
>  bool target_words_bigendian(void)
>  {
>  #if defined(TARGET_WORDS_BIGENDIAN)
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 94f5c8e..4e61944 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1169,7 +1169,6 @@ int virtio_set_status(VirtIODevice *vdev, uint8_t val)
>      return 0;
>  }
>  
> -bool target_words_bigendian(void);
>  static enum virtio_device_endian virtio_default_endian(void)
>  {
>      if (target_words_bigendian()) {
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index dc130cd..6d35c37 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -1085,6 +1085,17 @@ void cpu_exec_initfn(CPUState *cpu);
>  void cpu_exec_realizefn(CPUState *cpu, Error **errp);
>  void cpu_exec_unrealizefn(CPUState *cpu);
>  
> +/**
> + * target_words_bigendian:
> + * Returns true if the (default) endianness of the target is big endian,
> + * false otherwise. Note that in target-specifc code, you can use
> + * TARGET_WORDS_BIGENDIAN directly instead. On the other hand, common
> + * code should normally never need to know about the endianness of the
> + * target, so please do *not* use this function unless you know very well
> + * what you are doing!
> + */
> +bool target_words_bigendian(void);
> +
>  #ifdef NEED_CPU_H
>  
>  #ifdef CONFIG_SOFTMMU
> diff --git a/qom/cpu.c b/qom/cpu.c
> index 92599f3..f774654 100644
> --- a/qom/cpu.c
> +++ b/qom/cpu.c
> @@ -194,7 +194,6 @@ static bool cpu_common_debug_check_watchpoint(CPUState *cpu, CPUWatchpoint *wp)
>      return true;
>  }
>  
> -bool target_words_bigendian(void);
>  static bool cpu_common_virtio_is_big_endian(CPUState *cpu)
>  {
>      return target_words_bigendian();
> 

This makes sense to me. First I wasn't convinced, because I didn't
immediately get "target-specifc code". However, after looking at the 2nd
patch, I think I understand better (= the obj-XXX vs. common-obj-XXX
distinction). I also think that distinction is clear to any practicing
QEMU developers.

So, for what it's worth (which is admittedly not too much), for the series:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo