Re: [Qemu-devel] [PATCH v4 1/8] fw_cfg: max access size and region size are the same for MMIO data reg

[Paolo Bonzini]

On 16/12/2014 20:00, Laszlo Ersek wrote:
> Yes.
> 
> The root of this question is what each of
> 
> enum device_endian {
>     DEVICE_NATIVE_ENDIAN,
>     DEVICE_BIG_ENDIAN,
>     DEVICE_LITTLE_ENDIAN,
> };

Actually, I think the root of the answer :) is that fw_cfg_read (and
thus fw_cfg_data_mem_read) is not idempotent.  The split/compose stuff
accesses the bytes at offsets 8,9,10,11,12,13,14,15 and composes them
according to the endianness.

In the case of fw_cfg it just retrieves 8 bytes, but in the case of host
big endian it reads them in the "wrong" order for some reason (sorry, I
haven't looked at this thoroughly).


So the solution is:

1) make fw_cfg_data_mem_ops DEVICE_LITTLE_ENDIAN

2) make fw_cfg_data_mem_read and fw_cfg_data_mem_write call fw_cfg_read
and fw_cfg_write SIZE times and build up a value from the lowest byte up.

Paolo