Re: [RFC v1 0/8] x86/init: Linux linker tables

["H. Peter Anvin" <hpa@zytor.com>]

On 12/17/15 15:46, Luis R. Rodriguez wrote:
> 
> I explain why I do that there but the gist of it is that on Linux we may also
> want stronger semantics for specific linker table solutions, and solutions such
> as those devised on the IOMMU init stuff do memmove() for sorting depending on
> semantics defined (in the simplest case here so far dependency between init
> sequences), this makes each set of sequences very subsystem specific. An issue
> with *one* subsystem could make things really bad for others. I thought about
> this quite a bit and figured its best left to the subsystem maintainers to
> decide.
> 

A table that needs sorting or other runtime handling is just a
read-write table for the purpose of the linker table construct.  It
presents to C as an array of initialized data.

> Perhaps a new sections.h file (you tell me) which documents the different
> section components:
> 
> /* document this *really* well */
> #define SECTION_RODATA		".rodata"
> #define SECTION_INIT		".init"
> #define SECTION_INIT_RODATA	".init_rodata"
> #define SECTION_READ_MOSTLY	".read_mostly"
> 
> Then on tables.h we add the section components support:

Yes, something like that.  How to macroize it cleanly is another matter;
we may want to use slightly different conventions that iPXE to match our
own codebase.

> #define __table(component, type, name) (component, type, name) 
> 
> #define __table_component(table) __table_extract_component table                          
> #define __table_extract_component(component, type, name) component
> 
> #define __table_type(table) __table_extract_type table                          
> #define __table_extract_type(component, type, name) type
> 
> #define __table_name(table) __table_extract_name table                          
> #define __table_extract_name(component, type, name) name 
> 
> #define __table_str(x) #x 
> 
> #define __table_section(table, idx) \                                           
>         "." __table_component (table) ".tbl." __table_name (table) "." __table_str (idx)                      
> 
> #define __table_entry(table, idx)                                       \       
>         __attribute__ ((__section__(__table_section(table, idx)),       \       
>                         __aligned__(__table_alignment(table))))
> 
> A user could then be something as follows:
> 
> #define X86_INIT_FNS __table(SECTION_INIT, struct x86_init_fn, "x86_init_fns") 
> #define __x86_init_fn(order_level) __table_entry(X86_INIT_FNS, order_level)

Yes, but in particular the common case of function initialization tables
should be generic.

I'm kind of thinking a syntax like this:

DECLARE_LINKTABLE_RO(struct foo, tablename);
DEFINE_LINKTABLE_RO(struct foo, tablename);
LINKTABLE_RO(tablename,level) = /* contents */;
LINKTABLE_SIZE(tablename)

... which would turn into something like this once it goes through all
the preprocessing phases

/* DECLARE_LINKTABLE_RO */
extern const struct foo tablename[], tablename__end[];

/* DEFINE_LINKTABLE_RO */
DECLARE_LINKTABLE_RO(struct foo, tablename);

const struct
foo__attribute__((used,section(".rodata.tbl.tablename.0"))) tablename[0];

const struct
foo__attribute__((used,section(".rodata.tbl.tablename.999")))
tablename__end[0];

/* LINKTABLE_RO */
static const __typeof__(tablename)
__attribute__((used,section(".rodata.tbl.tablename.50")))
__tbl_tablename_12345

/* LINKTABLE_SIZE */
((tablename__end) - (tablename))

... and so on for all the possible sections where we may want tables.

Note: I used 0 and 999 above since they sort before and after all
possible 2-digit decimal numbers, but that's just cosmetic.

> If that's what you mean?
> 
> I'm a bit wary about having the linker sort any of the above SECTION_*'s, but
> if we're happy to do that perhaps a simple first step might be to see if 0-day
> but would be happy with just the sort without any consequences to any
> architecture. Thoughts?

I don't see what is dangerous about it.  The section names are such that
a lexographical sort will do the right thing, and we can simply use
SORT(.rodata.tbl.*) in the linker script, for example.

>> The other thing is to take a
>> clue from the implementation in iPXE, which uses priority levels 00 and
>> 99 (or we could use non-integers which sort appropriately instead of
>> using "real" levels) to contain the start and end symbols, which
>> eliminates any need for linker script modifications to add new tables.
> 
> This solution uses that as well. The only need for adding custom sections
> is when they have a requirement for a custom run time sort, and also to
> ensure they don't cause regressions on other subsystems if they have a buggy
> sort. The run time sorting is all subsystem specific and up to their own
> semantics.

Again, from a linker table POV this is nothing other than a read-write
table; there is a runtime function that then operates on that read-write
table.

	-hpa