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

[Jan Beulich <jbeulich@xxxxxxxx>]

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; 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.

Jan