Re: [PATCH 1/2] x86: move arch_generic_hweightl() to arch-specific library

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.11.2025 15:05, Andrew Cooper wrote:
> On 26/11/2025 1:58 pm, Jan Beulich wrote:
>> On 26.11.2025 14:51, Andrew Cooper wrote:
>>> On 26/11/2025 1:24 pm, Jan Beulich wrote:
>>>> diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
>>>> index 16368a498bb7..a0ee050c931b 100644
>>>> --- a/xen/arch/x86/arch.mk
>>>> +++ b/xen/arch/x86/arch.mk
>>>> @@ -3,6 +3,8 @@
>>>>  
>>>>  export XEN_IMG_OFFSET := 0x200000
>>>>  
>>>> +ALL_LIBS-y += arch/x86/lib/lib.a
>>>> +
>>> Oh, I'd realised it was this easy, I'd have done so straight away when
>>> adding x86's custom arch_generic_hweightl().
>>>
>>> I assumed it was going to be more complicated getting the order of the
>>> arch specific lib correct with the generic lib.
>>>
>>> More concretely.  Given an x86 lib, we should move things like
>>> arch/x86/memcpy.S to it.
>>>
>>> Therefore, when we have common/lib.a and arch/lib.a, do we guarantee to
>>> have arch/lib.a with higher precedence so for matching functions the
>>> arch specific one guarantees to be taken?
>> Not with the change above, it would need to become
>>
>> ALL_LIBS-y := arch/x86/lib/lib.a $(ALL_LIBS-y)
>>
>> to achieve that, requiring that ALL_LIBS-y won't change into a lazy-expansion
>> variable. If that's okay (please confirm), I can adjust the patch.
>>
>> Things would be yet easier if every arch had a lib/lib.a, as then in
>> xen/Makefile we could simply have
>>
>> ALL_LIBS-y                := arch/$(SRCARCH)/lib/lib.a
>> ALL_LIBS-y                += lib/lib.a
>>
>> Alternatively we could move the setting of ALL_LIBS-y in xen/Makefile to
>> after the arch/$(SRCARCH)/arch.mk inclusion. I'd be a little wary of that,
>> though, as it would then be different from ALL_OBJS-y.
> 
> I think this would be better handled by common code.
> 
> Arches are going to want a lib.a eventually.  ARM even has
> arch/arm/arm{32,64}/lib/ but like x86 they're just simple obj-y += at
> the moment.
> 
> However, arches shouldn't be forced to make an empty one simply to build.
> 
> Does this work:
> 
> ALL_LIBS-y                := $(wildcard arch/$(SRCARCH)/lib/lib.a)
> ALL_LIBS-y                += lib/lib.a
> 
> ?  If so, I think it's the nicest option.

I had thought of doing it this way initially, but on a fresh build
arch/$(SRCARCH)/lib/lib.a wouldn't be there when the Makefile is read in.
Whether switching ALL_LIBS-y to be a lazy-expansion variable would work
I didn't try; I'd prefer not to change the kind of variable that it is.

Jan