Re: [PATCH v6 2/8] vpci: Refactor REGISTER_VPCI_INIT

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.06.2025 11:27, Chen, Jiqian wrote:
> On 2025/6/25 16:36, Jan Beulich wrote:
>> On 25.06.2025 08:51, Chen, Jiqian wrote:
>>> On 2025/6/24 18:08, Jan Beulich wrote:
>>>> On 24.06.2025 11:29, Chen, Jiqian wrote:
>>>>> On 2025/6/24 16:05, Jan Beulich wrote:
>>>>>> On 24.06.2025 10:02, Chen, Jiqian wrote:
>>>>>>> On 2025/6/20 14:38, Jan Beulich wrote:
>>>>>>>> On 19.06.2025 08:39, Chen, Jiqian wrote:
>>>>>>>>> On 2025/6/18 22:05, Jan Beulich wrote:
>>>>>>>>>> On 12.06.2025 11:29, Jiqian Chen wrote:
>>>>>>>>>>> @@ -29,9 +30,22 @@ typedef int vpci_register_init_t(struct pci_dev 
>>>>>>>>>>> *dev);
>>>>>>>>>>>   */
>>>>>>>>>>>  #define VPCI_MAX_VIRT_DEV       (PCI_SLOT(~0) + 1)
>>>>>>>>>>>  
>>>>>>>>>>> -#define REGISTER_VPCI_INIT(x, p)                \
>>>>>>>>>>> -  static vpci_register_init_t *const x##_entry  \
>>>>>>>>>>> -               __used_section(".data.vpci." p) = (x)
>>>>>>>>>>> +#define REGISTER_VPCI_CAPABILITY(cap, finit, fclean, ext) \
>>>>>>>>>>> +    static const vpci_capability_t finit##_t = { \
>>>>>>>>>>> +        .id = (cap), \
>>>>>>>>>>> +        .init = (finit), \
>>>>>>>>>>> +        .cleanup = (fclean), \
>>>>>>>>>>> +        .is_ext = (ext), \
>>>>>>>>>>> +    }; \
>>>>>>>>>>> +    static const vpci_capability_t *const finit##_entry  \
>>>>>>>>>>> +        __used_section(".data.rel.ro.vpci") = &finit##_t
>>>>>>>>>>
>>>>>>>>>> Could you remind me why the extra level of indirection is necessary 
>>>>>>>>>> here?
>>>>>>>>>> That is, why can't .data.rel.ro.vpci be an array of 
>>>>>>>>>> vpci_capability_t?
>>>>>>>>> You mean I should change to be:
>>>>>>>>> #define REGISTER_VPCI_CAPABILITY(cap, finit, fclean, ext) \
>>>>>>>>>     static const vpci_capability_t finit##_t \
>>>>>>>>>         __used_section(".data.rel.ro.vpci") = { \
>>>>>>>>>         .id = (cap), \
>>>>>>>>>         .init = (finit), \
>>>>>>>>>         .cleanup = (fclean), \
>>>>>>>>>         .is_ext = (ext), \
>>>>>>>>>     }
>>>>>>>>>
>>>>>>>>> Right?
>>>>>>>>
>>>>>>>> Yes, subject to the earlier comments on the identifier choice.
>>>>>>> Got it.
>>>>>>> One more question, if change to be that, then how should I modify the 
>>>>>>> definition of VPCI_ARRAY?
>>>>>>> Is POINTER_ALIGN still right?
>>>>>>
>>>>>> Yes. The struct doesn't require bigger alignment afaics. (In fact in 
>>>>>> principle
>>>>>> no alignment should need specifying there, except that this would require
>>>>>> keeping the section separate in the final image. Which I don't think we 
>>>>>> want.)
>>>>>>
>>>>>>> Since I encountered errors that the values of __start_vpci_array are 
>>>>>>> not right when I use them in vpci_init_capabilities().
>>>>>>
>>>>>> Details please.
>>>>> After changing __start_vpci_array to be vpci_capability_t array, codes 
>>>>> will be (maybe I modified wrong somewhere):
>>>>>
>>>>> diff --git a/xen/drivers/vpci/vpci.c b/xen/drivers/vpci/vpci.c
>>>>> index c51bbb8abb19..9f2f438b4fdd 100644
>>>>> --- a/xen/drivers/vpci/vpci.c
>>>>> +++ b/xen/drivers/vpci/vpci.c
>>>>> @@ -36,8 +36,8 @@ struct vpci_register {
>>>>>  };
>>>>>
>>>>>  #ifdef __XEN__
>>>>> -extern const vpci_capability_t *const __start_vpci_array[];
>>>>> -extern const vpci_capability_t *const __end_vpci_array[];
>>>>> +extern vpci_capability_t __start_vpci_array[];
>>>>> +extern vpci_capability_t __end_vpci_array[];
>>>>
>>>> Just fyi: You lost const here.
>>>>
>>>>> @@ -255,7 +255,7 @@ static int vpci_init_capabilities(struct pci_dev 
>>>>> *pdev)
>>>>>  {
>>>>>      for ( unsigned int i = 0; i < NUM_VPCI_INIT; i++ )
>>>>>      {
>>>>> -        const vpci_capability_t *capability = __start_vpci_array[i];
>>>>> +        const vpci_capability_t *capability = &__start_vpci_array[i];
>>>>>          const unsigned int cap = capability->id;
>>>>>          const bool is_ext = capability->is_ext;
>>>>>          int rc;
>>>>> diff --git a/xen/include/xen/vpci.h b/xen/include/xen/vpci.h
>>>>> index f4ec1c25922d..77750dd4131a 100644
>>>>> --- a/xen/include/xen/vpci.h
>>>>> +++ b/xen/include/xen/vpci.h
>>>>> @@ -31,14 +31,13 @@ typedef struct {
>>>>>  #define VPCI_MAX_VIRT_DEV       (PCI_SLOT(~0) + 1)
>>>>>
>>>>>  #define REGISTER_VPCI_CAPABILITY(cap, finit, fclean, ext) \
>>>>> -    static const vpci_capability_t finit##_t = { \
>>>>> +    static vpci_capability_t finit##_entry \
>>>>> +        __used_section(".data.rel.ro.vpci") = { \
>>>>>          .id = (cap), \
>>>>>          .init = (finit), \
>>>>>          .cleanup = (fclean), \
>>>>>          .is_ext = (ext), \
>>>>> -    }; \
>>>>> -    static const vpci_capability_t *const finit##_entry  \
>>>>> -        __used_section(".data.rel.ro.vpci") = &finit##_t
>>>>> +    }
>>>>>
>>>>>  #define REGISTER_VPCI_CAP(cap, finit, fclean) \
>>>>>      REGISTER_VPCI_CAPABILITY(cap, finit, fclean, false)
>>>>>
>>>>> I print the value of NUM_VPCI_INIT, it is a strange number 
>>>>> (6148914691236517209).
>>>>
>>>> What are the addresses of the two symbols __start_vpci_array and 
>>>> __end_vpci_array?
>>> __end_vpci_array is 0xffff82d0404251b8
>>> __start_vpci_array is 0xffff82d040425160
>>> NUM_VPCI_INIT is 0x5555555555555559
>>> sizeof(vpci_capability_t) is 0x18
>>
>> Oh, of course - there's a psABI peculiarity that you run into here: 
>> Aggregates
>> larger than 8 bytes are required to have 16-byte alignment. Hence while
>> sizeof() == 0x18 and __alignof() == 8, the section contributions still are
>> accompanied by ".align 16", and thus respective padding is inserted by
>> assembler and linker. IOW you end up with two 32-byte entries and a trailing
>> 24-byte one. The easiest (and least problematic going forward) approach to
>> deal with that is probably going to be to add __aligned(16) to the struct
>> decl. (Whether to limit this to just x86 I'm not sure: While other psABI-s 
>> may
>> be different in this regard, we may want to be on the safe side.)
> Thanks for you detailed explanation.
> If I understand correctly, I need to change the definition of 
> vpci_capability_t to be:
> 
> typedef struct {
>     unsigned int id;
>     bool is_ext;
>     int (* init)(const struct pci_dev *pdev);
>     int (* cleanup)(const struct pci_dev *pdev);
> }
> #ifdef CONFIG_X86
> __aligned(16)
> #endif
> vpci_capability_t;

You'll need to check whether this has the intended effect. There are yet more
peculiarities when it comes to attributes on structs, typedefs, and the
combination of the two. I wonder though: Do we really need a typedef here?
Going with just struct vcpi_capability would eliminate concerns over those
peculiarities.

Also, as said - you will need to check whether other architectures are
different from x86-64 in this regard. We better wouldn't leave a trap here,
for them to fall into when they enable vPCI support. I.e. my recommendation
would be that if in doubt, we put the __aligned() there unconditionally.

Jan