Re: [PATCH v2 2/2] xen: Populate xen.lds.h and make use of its macros

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 29/03/2022 11:37, Jan Beulich wrote:
> On 29.03.2022 11:54, Julien Grall wrote:
> > On 22/03/2022 08:02, Michal Orzel wrote:
> > > --- a/xen/include/xen/xen.lds.h
> > > +++ b/xen/include/xen/xen.lds.h
> > > @@ -5,4 +5,104 @@
> > >     * Common macros to be used in architecture specific linker scripts.
> > >     */
> > >    
> > > +/* Macros to declare debug sections. */
> > > +#ifdef EFI
> > AFAIK, we don't define EFI on Arm (just CONFIG_EFI). Yet we do support
> > EFI on arm64.
> >
> > As this #ifdef is now in generic code, can you explain how this is meant
> > to be used?
> The identifier may now be somewhat misleading, yes - it has always meant
> "linking a native EFI (i.e. PE/COFF) binary". The equivalence "EFI binary"
> == "EFI support" has long been lost.
On Arm, we will be generating a EFI binary (or better a Image/EFI). So 
IIUC the description, we should in theory set EFI.
But I think it would do the wrong thing on Arm. Would you be able to 
explain why you need to differentiate it on x86?
> > > +/*
> > > + * Use the NOLOAD directive, despite currently ignored by (at least) GNU ld
> > > + * for PE output, in order to record that we'd prefer these sections to not
> > > + * be loaded into memory.
> > > + */
> > > +#define DECL_DEBUG(x, a) #x ALIGN(a) (NOLOAD) : { *(x) }
> > > +#define DECL_DEBUG2(x, y, a) #x ALIGN(a) (NOLOAD) : { *(x) *(y) }
> > > +#else
> > > +#define DECL_DEBUG(x, a) #x 0 : { *(x) }
> > > +#define DECL_DEBUG2(x, y, a) #x 0 : { *(x) *(y) }
> > > +#endif
> > > +
> > > +/* DWARF debug sections. */
> > > +#define DWARF_DEBUG_SECTIONS                      \
> > > +  DECL_DEBUG(.debug_abbrev, 1)                    \
> > > +  DECL_DEBUG2(.debug_info, .gnu.linkonce.wi.*, 1) \
> > > +  DECL_DEBUG(.debug_types, 1)                     \
> > > +  DECL_DEBUG(.debug_str, 1)                       \
> > > +  DECL_DEBUG2(.debug_line, .debug_line.*, 1)      \
> > > +  DECL_DEBUG(.debug_line_str, 1)                  \
> > > +  DECL_DEBUG(.debug_names, 4)                     \
> > > +  DECL_DEBUG(.debug_frame, 4)                     \
> > > +  DECL_DEBUG(.debug_loc, 1)                       \
> > > +  DECL_DEBUG(.debug_loclists, 4)                  \
> > > +  DECL_DEBUG(.debug_macinfo, 1)                   \
> > > +  DECL_DEBUG(.debug_macro, 1)                     \
> > > +  DECL_DEBUG(.debug_ranges, 8)                    \
> > > +  DECL_DEBUG(.debug_rnglists, 4)                  \
> > > +  DECL_DEBUG(.debug_addr, 8)                      \
> > > +  DECL_DEBUG(.debug_aranges, 1)                   \
> > > +  DECL_DEBUG(.debug_pubnames, 1)                  \
> > > +  DECL_DEBUG(.debug_pubtypes, 1)
> > > +
> > > +/* Stabs debug sections. */
> > > +#define STABS_DEBUG_SECTIONS                 \
> > > +  .stab 0 : { *(.stab) }                     \
> > > +  .stabstr 0 : { *(.stabstr) }               \
> > > +  .stab.excl 0 : { *(.stab.excl) }           \
> > > +  .stab.exclstr 0 : { *(.stab.exclstr) }     \
> > > +  .stab.index 0 : { *(.stab.index) }         \
> > > +  .stab.indexstr 0 : { *(.stab.indexstr) }
> > > +
> > > +/*
> > > + * Required sections not related to debugging.
> > > + *
> > > + * LLVM ld also wants .symtab, .strtab, and .shstrtab placed. These look to
> > > + * be benign to GNU ld, so we can have them here unconditionally.
> > > + */
> > > +#define ELF_DETAILS_SECTIONS     \
> > > +  .comment 0 : { *(.comment) }   \
> > This is a bit confusing. Here you seem to use the section .comment. But...
> >
> > > +  .symtab 0 : { *(.symtab) }     \
> > > +  .strtab 0 : { *(.strtab) }     \
> > > +  .shstrtab 0 : { *(.shstrtab) }
> > > +
> > > +#ifdef EFI
> > > +#define DISCARD_EFI_SECTIONS \
> > > +       *(.comment)   \
> > ... here you will discard it if EFI is set. Which one take precedence if
> > the caller use both ELF_DETAILS_SECTIONS and DISCARD_EFI_SECTION?
> Given the above explanation I think it's clear that only one of the
> two may be used at a time: ELF_DETAILS_SECTIONS when linking an ELF
> binary and DISCARD_EFI_SECTIONS when linking a PE/COFF binary.
I guess this may be obvious on x86. But for Arm, we are generating the 
ELF first and then extracting the information to generate the binary. 
The end result will be a binary that is PE/COFF compatible.
So to me, it would make sense to include DISCARD_EFI_SECTIONS because we 
going to create an EFI binary and also include EFI_DETAILS_SECTIONS 
because we are building an ELF.
Overall it sounds like to me that it is too premature to move the 
#if{,n}def EFI bits in the generic header.
Cheers,

--
Julien Grall