hw/virtio: Simplify virtio_access_is_big_endian()

Thanks to previous refactoring, we can see more easily it is strictly
equivalent to always call virtio_vdev_is_big_endian.

static inline bool virtio_vdev_is_big_endian(VirtIODevice *vdev)
{
    if (virtio_vdev_is_legacy(vdev)) {
        assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
        return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
    }
    /* Devices conforming to VIRTIO 1.0 or later are always LE. */
    return false;
}

The key is to understand that vdev->device_endian is initialized as
expected. It always contains cpu endianness, and not
device endianness, ignoring if device is legacy or not.

By default, it's initialized to vdev->device_endian =
virtio_default_endian(), which matches target default endianness.
Then, on virtio_reset, it will be initialized with current_cpu
endianness (if there is one current_cpu).

void virtio_reset() {
    ...
    if (current_cpu) {
        /* Guest initiated reset */
        vdev->device_endian = virtio_current_cpu_endian();
    } else {
        /* System reset */
        vdev->device_endian = virtio_default_endian();
    }

Now, we can see how existing virtio_access_is_big_endian is equivalent
to virtio_vdev_is_big_endian. Let's break the existing function in its 3
variants, and compare that to virtio_vdev_is_big_endian.

static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
- #if defined(LEGACY_VIRTIO_IS_BIENDIAN)
    return virtio_vdev_is_big_endian(vdev);
  This is the exact replacement we did, so equivalent.
- #elif TARGET_BIG_ENDIAN
    if (!virtio_vdev_is_legacy(vdev)) {
        return false;
    }
    return true;

  we know target_is_big_endian(), so
  vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG.
  if (virtio_vdev_is_legacy(vdev)) {
      return VIRTIO_DEVICE_ENDIAN_BIG == VIRTIO_DEVICE_ENDIAN_BIG;
  }
  return false;
  It's written in opposite style compared to existing code (if legacy vs
  if modern), but it's strictly equivalent.
- #else
    return false;
  we know !target_is_big_endian(), so
  vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE.
  if virtio_vdev_is_legacy(vdev) {
    return VIRTIO_DEVICE_ENDIAN_LITTLE == VIRTIO_DEVICE_ENDIAN_BIG;
  }
  return false;
  So it always return false, as expected.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260225041948.52929-3-philmd@linaro.org
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

include/hw/virtio/virtio-access.h

@@ -21,23 +21,9 @@
 #include "hw/virtio/virtio.h"
 #include "hw/virtio/virtio-bus.h"
 
-#if defined(TARGET_PPC64) || defined(TARGET_ARM)
-#define LEGACY_VIRTIO_IS_BIENDIAN 1
-#endif
-
 static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
 {
-#if defined(LEGACY_VIRTIO_IS_BIENDIAN)
     return virtio_vdev_is_big_endian(vdev);
-#elif TARGET_BIG_ENDIAN
-    if (!virtio_vdev_is_legacy(vdev)) {
-        /* Devices conforming to VIRTIO 1.0 or later are always LE. */
-        return false;
-    }
-    return true;
-#else
-    return false;
-#endif
 }
 
 static inline void virtio_stw_p(VirtIODevice *vdev, void *ptr, uint16_t v)