Re: [Qemu-devel] [PATCH 3/4] tests: fw_cfg: add reboot_timeout test case

[Laszlo Ersek <lersek@redhat.com>]

On 04/25/19 12:23, Philippe Mathieu-Daudé wrote:
> On 4/25/19 10:29 AM, Li Qiang wrote:
>>
>>
>> Gerd Hoffmann <kraxel@redhat.com <mailto:kraxel@redhat.com>> 于2019年4月
>> 25日周四 下午4:15写道：
>>
>>     On Wed, Apr 24, 2019 at 09:16:56AM +0800, Li Qiang wrote:
>>     > Thomas Huth <thuth@redhat.com <mailto:thuth@redhat.com>> 于2019年4
>>     月24日周三 上午12:29写道：
>>     >
>>     > > Is this endianess-safe? Or do you need to byteswap
>>     reboot_timeout if the
>>     > > host and guest endianess does not match?
>>     >
>>     > Good question!
>>     >
>>     > IIUC, the qemu fw_cfg store the 'file' entry data just in byte stream.
>>
>>     No.  Integers are defined to be little endian.  See fw_cfg_add_i64() for
>>     example, there is an explicit cpu_to_le64() call for that.
>>
>>
>>
>> Yes, for the fw_cfg 'integer' entry it is stored as little endian.
>> But for the fw_cfg 'file' entry interpred as an integer, there is no
>> specify the endianess.
> 
> I agree with Li, the endianess of 'reboot-timeout' is not clear.
> 
> From docs/specs/fw_cfg.txt:
> 
>   === All Other Data Items ===
> 
>   Please consult the QEMU source for the most up-to-date
>   and authoritative list of selector keys and their respective
>   items' purpose, format and writeability.
> 
> So checking the git history, this code was introduced in commit
> ac05f3492421, very similar to commit 3d3b8303c6f8 for the
> 'boot-menu-wait' entry, which explicitely use little-endian, so I think
> it is safe to consider it little-endian and add a comment about its
> endianess (referring the previous commits in the patch description).

OVMF consumes "boot-menu-wait", so that one must remain LE.

OVMF doesn't consume "reboot-timeout", so I can't really comment on it.
I guess, if a named fw_cfg file that already exists doesn't explicitly
define the endiannesses of its integer fields, err for safety and opt
for LE.

For new fw_cfg files with integers in them, choose the endianness
explicitly and document it.

Thanks
Laszlo