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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.06.2022 11:49, Julien Grall wrote:
> Hi Jan,
> 
> On 24/06/2022 10:33, Jan Beulich wrote:
>> On 24.06.2022 10:35, Julien Grall wrote:
>>> On 24/06/2022 08:18, Wei Chen wrote:
>>>>> -----Original Message-----
>>>>> From: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> Sent: 2022年6月23日 20:54
>>>>> To: Wei Chen <Wei.Chen@xxxxxxx>
>>>>> Cc: nd <nd@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Julien
>>>>> Grall <julien@xxxxxxx>; 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
>>>>> Subject: Re: [PATCH v6 1/8] xen: reuse x86 EFI stub functions for Arm
>>>>>
>>>>> On 10.06.2022 07:53, Wei Chen wrote:
>>>>>> --- a/xen/arch/arm/Makefile
>>>>>> +++ b/xen/arch/arm/Makefile
>>>>>> @@ -1,6 +1,5 @@
>>>>>>    obj-$(CONFIG_ARM_32) += arm32/
>>>>>>    obj-$(CONFIG_ARM_64) += arm64/
>>>>>> -obj-$(CONFIG_ARM_64) += efi/
>>>>>>    obj-$(CONFIG_ACPI) += acpi/
>>>>>>    obj-$(CONFIG_HAS_PCI) += pci/
>>>>>>    ifneq ($(CONFIG_NO_PLAT),y)
>>>>>> @@ -20,6 +19,7 @@ obj-y += domain.o
>>>>>>    obj-y += domain_build.init.o
>>>>>>    obj-y += domctl.o
>>>>>>    obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
>>>>>> +obj-y += efi/
>>>>>>    obj-y += gic.o
>>>>>>    obj-y += gic-v2.o
>>>>>>    obj-$(CONFIG_GICV3) += gic-v3.o
>>>>>> --- a/xen/arch/arm/efi/Makefile
>>>>>> +++ b/xen/arch/arm/efi/Makefile
>>>>>> @@ -1,4 +1,12 @@
>>>>>>    include $(srctree)/common/efi/efi-common.mk
>>>>>>
>>>>>> +ifeq ($(CONFIG_ARM_EFI),y)
>>>>>>    obj-y += $(EFIOBJ-y)
>>>>>>    obj-$(CONFIG_ACPI) +=  efi-dom0.init.o
>>>>>> +else
>>>>>> +# Add stub.o to EFIOBJ-y to re-use the clean-files in
>>>>>> +# efi-common.mk. Otherwise the link of stub.c in arm/efi
>>>>>> +# will not be cleaned in "make clean".
>>>>>> +EFIOBJ-y += stub.o
>>>>>> +obj-y += stub.o
>>>>>> +endif
>>>>>
>>>>> This has caused
>>>>>
>>>>> 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
>>>>>
>>>>> for the 32-bit Arm build that I keep doing every once in a while, with
>>>>> (if it matters) GNU ld 2.38. I guess you will want to consider building
>>>>> all of Xen with -fshort-wchar, or to avoid building stub.c with that
>>>>> option.
>>>>>
>>>>
>>>> Thanks for pointing this out. I will try to use -fshort-wchar for Arm32,
>>>> if Arm maintainers agree.
>>>
>>> Looking at the code we don't seem to build Xen arm64 with -fshort-wchar
>>> (aside the EFI files). So it is not entirely clear why we would want to
>>> use -fshort-wchar for arm32.
>>
>> We don't use wchar_t outside of EFI code afaict. Hence to all other code
>> it should be benign whether -fshort-wchar is in use. So the suggestion
>> to use the flag unilaterally on Arm32 is really just to silence the ld
>> warning;
> 
> Ok. This is odd. Why would ld warn on arm32 but not other arch?

Arm32 embeds ABI information in a note section in each object file.
The mismatch of the wchar_t part of this information is what causes
ld to emit the warning.

>> off the top of my head I can't see anything wrong with using
>> the option also for Arm64 or even globally. Yet otoh we typically try to
>> not make changes for environments where they aren't really needed.
> 
> I agree. If we need a workaround, then my preference would be to not 
> build stub.c with -fshort-wchar.

This would need to be an Arm-special then, as on x86 it needs to be built
this way.

Jan