[AArch64 ELF ABI] Vector calls and lazy binding on AArch64

[Florian Weimer]

* Szabolcs Nagy:

> On 22/05/2019 16:06, Florian Weimer wrote:
>> * Szabolcs Nagy:
>> 
>>> AAELF64: in the Symbol Table section add
>>>
>>>  st_other Values
>>>      The  st_other  member  of  a symbol table entry specifies the symbol's
>>>      visibility in the lowest 2 bits.  The top 6 bits  are  unused  in  the
>>>      generic  ELF ABI [SCO-ELF], and while there are no values reserved for
>>>      processor-specific semantics, many other architectures have used these
>>>      bits.
>>>
>>>      The  defined  processor-specific  st_other  flag  values are listed in
>>>      Table 4-5-1.
>>>
>>>  Table 4-5-1, Processor specific st_other flags
>>>              +------------------------+------+---------------------+
>>>              |Name                    | Mask | Comment             |
>>>              +------------------------+------+---------------------+
>>>              |STO_AARCH64_VARIANT_PCS | 0x80 | The        function |
>>>              |                        |      | associated with the |
>>>              |                        |      | symbol may follow a |
>>>              |                        |      | variant   procedure |
>>>              |                        |      | call  standard with |
>>>              |                        |      | different  register |
>>>              |                        |      | usage convention.   |
>>>              +------------------------+------+---------------------+
>>>
>>>      A  symbol  table entry that is marked with the STO_AARCH64_VARIANT_PCS
>>>      flag set in its st_other field may be associated with a function  that
>>>      follows  a  variant  procedure  call  standard with different register
>>>      usage convention from the one  defined  in  the  base  procedure  call
>>>      standard  for  the  list  of  argument,  caller-saved and callee-saved
>>>      registers [AAPCS64].  The rules  in  the  Call  and  Jump  relocations
>>>      section  still  apply to such functions, and if a subroutine is called
>>>      via a symbol reference that  is  marked  with  STO_AARCH64_VARIANT_PCS
>>>      then  code that runs between the calling routine and called subroutine
>>>      must preserve the contents of all registers except IP0,  IP1  and  the
>>>      condition code flags [AAPCS64].
>> 
>> Can you clarify if there has to be a valid stack at this point which can
>> be used during the call transfer?  What about the stack alignment
>> requirement?
>
> the intention is to only allow 'register usage convention' to be
> relaxed compared to the base PCS (which has rules for stack etc),
> and even the register usage convention has to be compatible with
> the 'Call and Jump relocations section' which essentially says that
> veneers inserted by the linker between calls can clobber IP0, IP1
> and the condition flags.
>
> i.e. a variant pcs function follows the same rules as base pcs, but
> it may use different caller-/callee-saved/argument regiseters.
>
> when SVE pcs is merged into the current AAPCS document, then i hope
> the 'variant pcs' term used here will be properly specified so the
> ELF ABI will just refer back to that.

My concern is that with the current language, it's not clear whether
it's possible to use the stack as a scratch area during the call
transition, or rely on a valid TCB.  I think this is rather
underspecified.

Thanks,
Florian