Re: [RFC PATCH-for-8.0 06/10] hw/virtio: Cache access_is_big_endian value in VirtIODevice state

[Thomas Huth <thuth@redhat.com>]

On 13/12/2022 08.30, Philippe Mathieu-Daudé wrote:
> On 13/12/22 01:14, Richard Henderson wrote:
>> On 12/12/22 17:05, Philippe Mathieu-Daudé wrote:
>>> The device endianness doesn't change during runtime.
>>
>> What are you talking about?  Of course it does.
> 
> The host CPU certainly does, but the virtio device doesn't... Does it?
> 
> This check only consider the device, not the CPU:
> 
>      bool virtio_access_is_big_endian(VirtIODevice *vdev)
>      {
>      #if defined(LEGACY_VIRTIO_IS_BIENDIAN)
>          return virtio_is_big_endian(vdev);
>      #elif TARGET_BIG_ENDIAN
>          if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>              /*Devices conforming to VIRTIO 1.0 or later are always LE.*/
>              return false;
>          }
>          return true;

Well, this part here means that the endianness can indeed change on the 
device side during runtime. Depending on whether VIRTIO_F_VERSION_1 is 
negotiated or not, the device is little or big endian. Happens on s390x for 
example - for legacy virtio, big endian is used, and for modern virtio, 
little endian is used instead.

  Thomas