Re: [PATCH v6 1/8] xen: reuse x86 EFI stub functions for Arm

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.07.2022 08:10, Wei Chen wrote:
> Hi Jan,
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 2022年7月1日 13:54
>> To: Wei Chen <Wei.Chen@xxxxxxx>
>> Cc: nd <nd@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand
>> Marquis <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
>> <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>;
>> Roger Pau Monné <roger.pau@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Jiamei Xie
>> <Jiamei.Xie@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx; Julien Grall
>> <julien@xxxxxxx>
>> Subject: Re: [PATCH v6 1/8] xen: reuse x86 EFI stub functions for Arm
>>
>>>>>>> this way.
>>>>>
>>>>> I have taken a look into this warning:
>>>>> This is because the "-fshort-wchar" flag causes GCC to generate
>>>>> code that is not binary compatible with code generated without
>>>>> that flag. Why this warning hasn't been triggered in Arm64 is
>>>>> because we don't use any wchar in Arm64 codes.
>>>>
>>>> I don't think that's quite right - you actually say below that we
>>>> do use it there when interacting with UEFI. There's no warning
>>>> there solely because the information isn't embedded in the object
>>>> files there, from all I can tell.
>>>>
>>>
>>> Maybe I should describe it this way: Arm64 does not use wchar type
>>> directly in any code for parameters, variables and return values.
>>> So Arm64 object files are exactly the same with "-fshort-wchar" and
>>> without "-fshort-wchar".
>>>
>>> Although Xen's EFI code interacts with UEFI firmware, similar to RPC
>>> function calls, these code also do not explicitly use wchar.
>>
>> How does it not? There are ample string literals as well as enough
>> uses of CHAR16 (the UEFI "abstraction" of wchar_t).
>>
> 
> But I don't think CHAR16 will be affected by -fshort-wchar, because we
> have specified CHAR16 as unsigned short type in typedef.

Oh, I didn't recall that anomaly. Yes, then only string literals are
affected. Albeit I'd hope gnu-efi would at some point switch to using
wchar_t as the underlying type, as long as the compiler provides it.

Jan