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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

Thanks, Roger.