Re: [PATCH v2] Xen: fix EFI stub wchar_t size warning of arm32 building

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.07.2022 09:18, Wei Chen wrote:
> Hi Jan,
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 2022年7月5日 14:35
>> To: Wei Chen <Wei.Chen@xxxxxxx>
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Julien Grall
>> <julien@xxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; Volodymyr
>> Babchuk <Volodymyr_Babchuk@xxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH v2] Xen: fix EFI stub wchar_t size warning of arm32
>> building
>>
>> On 05.07.2022 05:54, Wei Chen wrote:
>>> Xen uses "-fshort-wchar" in CFLAGS for EFI common code. Arm32
>>> is using stub.c of EFI common code for EFI stub functions. But
>>> "-fshort-wchar" CFLAG will cause a warning when build stub.c
>>> for Arm32:
>>> "arm-linux-gnueabihf-ld: warning: arch/arm/efi/built_in.o uses
>>> 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of
>>> wchar_t values across objects may fail"
>>>
>>> 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 Arm64 does not use wchar type directly in any code for
>>> parameters, variables and return values. And in EFI code, wchar
>>> has been replaced by CHAR16 (the UEFI "abstraction" of wchar_t).
>>> CHAR16 has been specified as unsigned short type in typedef, the
>>> "-fshort-wchar" flag will not affect CHAR16. So Arm64 object
>>> files are exactly the same with "-fshort-wchar" and without
>>> "-fshort-wchar".
>>>
>>> We are also not using wchar in Arm32 codes, but Arm32 will embed
>>> ABI information in ".ARM.attributes" section. This section stores
>>> some object file attributes, like ABI version, CPU arch and etc.
>>> And wchar size is described in this section by "Tag_ABI_PCS_wchar_t"
>>> too. Tag_ABI_PCS_wchar_t is 2 for object files with "-fshort-wchar",
>>> but for object files without "-fshort-wchar" is 4. Arm32 GCC
>>> ld will check this tag, and throw above warning when it finds
>>> the object files have different Tag_ABI_PCS_wchar_t values.
>>>
>>> As gnu-efi-3.0 use the GCC option "-fshort-wchar" to force wchar
>>> to use short integers (2 bytes) instead of integers (4 bytes).
>>> So keep "-fshort-wchar" for Xen EFI code is reasonable. In this
>>> patch, we add "-fno-short-wchar" to override "-fshort-wchar" for Arm
>>> architecutres without EFI enabled to remove above warning.
>>>
>>> Reported-and-Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
>>
>> Tested-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
> 
> Thanks for testing!
> 
>> As to the description: Why the reference to gnu-efi? I don't think
>> it matters how they build their code? All that's important is what
>> we do, I suppose.
>>
> 
> How about:
> "Xen need to keep "-fshort-wchar" in EFI code to force wchar to use
> short integers (2 bytes) instead of integers (4 bytes), but this is
> unnecessary for code out of EFI. So in this patch, we add
> "-fno-short-wchar" to override "-fshort-wchar" for Arm architectures
> without EFI enabled to remove above warning."

Reads better to me, thanks.

Jan