Re: [PATCH v5 04/10] vpci: Refactor REGISTER_VPCI_INIT

["Chen, Jiqian" <Jiqian.Chen@xxxxxxx>]

On 2025/6/9 18:40, Roger Pau Monné wrote:
> On Mon, Jun 09, 2025 at 10:18:42AM +0000, Chen, Jiqian wrote:
>> On 2025/6/9 17:29, Roger Pau Monné wrote:
>>> On Mon, Jun 09, 2025 at 07:50:21AM +0000, Chen, Jiqian wrote:
>>>> On 2025/6/6 17:14, Roger Pau Monné wrote:
>>>>> On Fri, Jun 06, 2025 at 09:05:48AM +0200, Jan Beulich wrote:
>>>>>> On 06.06.2025 08:29, Chen, Jiqian wrote:
>>>>>>> On 2025/6/5 20:50, Roger Pau Monné wrote:
>>>>>>>> On Mon, May 26, 2025 at 05:45:53PM +0800, Jiqian Chen wrote: 
>>>>>>>>> +  }; \
>>>>>>>>> +  static vpci_capability_t *const finit##_entry  \
>>>>>>>>> +               __used_section(".data.vpci") = &finit##_t
>>>>>>>>
>>>>>>>> IMO this should better use .rodata instead of .data. 
>>>>>>> Is below change correct?
>>>>>>>
>>>>>>> +    static const vpci_capability_t *const finit##_entry  \
>>>>>>> +        __used_section(".rodata") = &finit##_t
>>>>>>
>>>>>> No, specifically because ...
>>>>>>
>>>>>>>> Not that it matters much in practice, as we place it in .rodata 
>>>>>>>> anyway.  Note
>>>>>>>> however you will have to move the placement of the VPCI_ARRAY in the
>>>>>>>> linker script ahead of *(.rodata.*), otherwise that section match will
>>>>>>>> consume the vPCI data.
>>>>>>> I am sorry, how to move it ahead of *(.rodata.*) ?
>>>>>>> Is below change correct?
>>>>>>>
>>>>>>> diff --git a/xen/include/xen/xen.lds.h b/xen/include/xen/xen.lds.h
>>>>>>> index 793d0e11450c..3817642135aa 100644
>>>>>>> --- a/xen/include/xen/xen.lds.h
>>>>>>> +++ b/xen/include/xen/xen.lds.h
>>>>>>> @@ -188,7 +188,7 @@
>>>>>>>  #define VPCI_ARRAY               \
>>>>>>>         . = ALIGN(POINTER_ALIGN); \
>>>>>>>         __start_vpci_array = .;   \
>>>>>>> -       *(SORT(.data.vpci.*))     \
>>>>>>> +       *(.rodata)             \
>>>>>>
>>>>>> ... this isn't - you'd move _all_ of .rodata into here, which definitely
>>>>>> isn't what you want. What I understand Roger meant was a .rodata-like
>>>>>> section, e.g. .rodata.vpci.* (much like it was .data.vpci.* before).
>>>>>
>>>>> Indeed, my suggestion was merely to use .rodata instead of .data, as
>>>>> that's more accurate IMO.  I think it should be *(.rodata.vpci) (and
>>>>> same section change for the __used_section() attribute.
>>>>
>>>> If I understand correctly, the next version will be:
>>>>
>>>> +    static const vpci_capability_t *const finit##_entry  \
>>>> +        __used_section(".rodata.vpci") = &finit##_t
>>>> +
>>>>
>>>> and
>>>>
>>>>  #define VPCI_ARRAY               \
>>>>         . = ALIGN(POINTER_ALIGN); \
>>>>         __start_vpci_array = .;   \
>>>> -       *(SORT(.data.vpci.*))     \
>>>> +       *(.rodata.vpci)           \
>>>>         __end_vpci_array = .;
>>>
>>> Did you also move the instances of VPCI_ARRAY in the linker scripts so
>>> it's before the catch-all *(.rodata.*)?
>>>
>>>>
>>>> But, that encountered an warning when compiling.
>>>> " {standard input}: Assembler messages:
>>>> {standard input}:1160: Warning: setting incorrect section attributes for 
>>>> .rodata.vpci
>>>> {standard input}: Assembler messages:
>>>> {standard input}:3034: Warning: setting incorrect section attributes for 
>>>> .rodata.vpci
>>>> {standard input}: Assembler messages:
>>>> {standard input}:6686: Warning: setting incorrect section attributes for 
>>>> .rodata.vpci "
>>>
>>> What are the attributes for .rodata.vpci in the object files?  You can
>>> get those using objdump or readelf, for example:
>>>
>>> $ objdump -h xen/drivers/vpci/msi.o
>>> [...]
>>>  17 .data.vpci.9  00000008  0000000000000000  0000000000000000  00000a50  
>>> 2**3
>>>                   CONTENTS, ALLOC, LOAD, RELOC, DATA
>>>
>>> It should be READONLY, otherwise you will get those messages.
>>>
>>>> And, during booting Xen, all value of __start_vpci_array is incorrect.
>>>> Do I miss anything?
>>>
>>> I think that's likely because you haven't moved the instance of
>>> VPCI_ARRAY in the linker script?
>> Oh, right. Sorry, I forgot to move it.
>> After changing this, it works now.
>>
>> diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
>> index bf956b6c5fc0..c88fd62f4f0d 100644
>> --- a/xen/arch/x86/xen.lds.S
>> +++ b/xen/arch/x86/xen.lds.S
>> @@ -134,6 +134,7 @@ SECTIONS
>>         BUGFRAMES
>>
>>         *(.rodata)
>> +       VPCI_ARRAY
>>         *(.rodata.*)
>>         *(.data.rel.ro)
>>         *(.data.rel.ro.*)
>> @@ -148,7 +149,6 @@ SECTIONS
>>         *(.note.gnu.build-id)
>>         __note_gnu_build_id_end = .;
>>  #endif
>> -       VPCI_ARRAY
>>    } PHDR(text)
> 
> FWIW, I would put it ahead of *(.rodata).  Remember to also modify the
> linker script for all the other arches, not just x86.

Whether before *(.rodata.*) or before *(.rodata), there still is the warning " 
Warning: setting incorrect section attributes for .rodata.vpci "
And the objdump shows "rodata.vpci" has no "readonly" word.
But starting Xen dom0 is OK.
I attached the new this patch and the result of "objdump -h 
xen/drivers/vpci/msi.o"

> 
> Thanks, Roger.

-- 
Best regards,
Jiqian Chen.

Attachment: objdump_before_rodata.txt
Description: objdump_before_rodata.txt

Attachment: 0003-vpci-Refactor-REGISTER_VPCI_INIT.patch
Description: 0003-vpci-Refactor-REGISTER_VPCI_INIT.patch