Re: [PATCH 0/3] aarch64: Update ld.so for vector abi

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Thu, Aug 2, 2018 at 4:24 PM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 08/01/2018 06:23 PM, rth@twiddle.net wrote:
>> From: Richard Henderson <richard.henderson@linaro.org>
>>
>> There is a new calling convention defined for vectorized functions [1].
>
> Correct me if I'm wrong.
>
> (a) We have a lot of stuff to save/restore in SVE.
>
> (b) It appears Szabolcs really wants to avoid the PLT at all with the
>     new vector procedure call stanadard, since this avoids ever
>     having to save/restore the large amounts of register data.
>
> (b.1) Assumes that save and restore of SVE has serious negative performance
>       consequences, both in userspace and in kernel save/restore for
>       context switches.
>
> (c) A PLT generally has only one kind of save/restore ABI that it follows
>     and it follows pessimistically the worse case to support all possible
>     calling conventions.
>
> (d) The compiler you are using is generating calls using the new ABI and
>     those are going through the PLT, something in the dynamic loader is
>     also using these registers and corrupting call results, otherwise
>     you would never have made this patch to fix the problem.
>
> If all this is true, I think this is the wrong solution.
>
> The better solution for aarch64 is:
>
> (1) All new-style SVE calls do *not* go through the PLT by default, but
>     indirect through the GOT and are always bind-now.
>
> (2) By default ld.so does not save/restore the SVE registers during
>     lazy binding.
>
> (3) If ld.so detects LD_AUDIT in use, or BIND_NOW=0, or lazy binding
>     is being forced, then it flips to PLT save/restore sequences that
>     do save all the required SVE registers, and routes the GOT entries
>     to the PLT entries, and we get *slow* lazy binding semantics that
>     work.
>
> I don't expect you signed up for this, but that's my analysis.

This is a good summary. Thank you Carlos.

We would also need to discuss this internally with some ABI people before
taking a direction here.

regards
Ramana
>
>> I have *not* attempted to extend the <bits/link.h> interface for
>> the new ABI.  This should be done with more discussion on list.
>> I have instead simply saved and restored registers as the abi
>> requires, so that the actual callee gets the correct data.
>
> We *should* adjust bits/link.h at the same time and extend it like
> we did for x86_64. LD_AUDIT should work.
>
> --
> Cheers,
> Carlos.