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

[Richard Henderson <rth at twiddle dot net>]

On 08/02/2018 11:24 AM, Carlos O'Donell 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.

Yes.

> 
> (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.

Yes.

> 
> (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.

Yes.  This is the biggie, really.

> (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.

Yes.

> (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.

Yes.  Indeed, the normal AdvSIMD restore within _dl_runtime_resolve is exactly
what clobbers the SVE state.

> 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.

This would be the ideal solution, yes.

> I don't expect you signed up for this, but that's my analysis.

The right people are now talking about the problem, which is the main thing.

>> 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.

Yes.  We do need to talk about the design for this though; it's not as simple
as for x86_64.  I did comment on this more in patch 3:

+	   ??? Extending the profiling hook for full SVE register export
+	   is tricky given the variable register size.  Perhaps the new
+	   La_aarch64_regs should contain pointers to Z0 and P0, and
+	   the current VL, and one infers the addresses from there.
+
+	   This one new form could be used for all, with AdvSIMD
+	   devolving into VL=16 with no predicate registers.



r~