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Date: Thu, 19 Sep 2019 15:10:18 -0400
From: Rich Felker <dalias@...c.org>
To: info@...ile-stream.com
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] mips: add single-instruction math functions

On Thu, Sep 19, 2019 at 09:26:18PM +0300, info@...ile-stream.com wrote:
> 
> >> 2) -mabs=2008 application will work correctly with -mabs=legacy musl
> >> on a -mabs=2008 system.
> 
> R> Are you sure? This seems to disagree with what you're saying below
> 
> Yes, assuming they all were built with the same -mnan= option.
> 
> The -mabs=2008 option has no ABI tag, it is just a compiler flag
> that controls how many instructions are used for |x| and -x.
> 
> I think there is some optimisation flag in gcc that does exactly the
> same without declaring __mips_abs2008.
> 
> Only -mnan=xxx consistency is enforced by the kernel basing on the
> corresponding bits in the ELF header and FCSR.
> 
> But yes, IIUC kernel assumes nan2008 binary is abs2008 too though
> this has no deep (or whatever) meaning on cores with hard-wired
> feature bits in FCSR.

I guess I was assuming that if the two generally (always?) match in
the hardware, you'd necessarily be using nan2008 when using abs2008.

> R> musl supports multiple ecosystems in the same filesystem regardless of
> R> whether a cpu does; that's the whole point of supporting even multiple
> R> unrelated archs like mips and arm or x86 and riscv and why they all
> R> have differing ldso names. For example you can be running the foreign
> R> ISA via qemu-user with binfmt_misc or explicitly.
> 
> OK. Though qemu-user has -L option for things like this.

Indeed you can use -L in the qemu-user case, but there are plenty of
other situations like shared rootfs across network or with qemu-system
guests. In any case, ability for different archs and subarchs (ABIs)
to coexist in the same filesystem regardless of any relationship or
non-relationship between them has always been part of musl.

> R> In that case, it seems like the kernel would refuse to load
> R> -mabs=legacy binaries on nan2008 hardware, gratuitously due to
> 
> No, no. The kernel only knows -mnan=xxx status for binary.
> 
> All this mess is because architecture spec defines these features as
> distinct, compilers have independent options for them (gcc even has
> --with-nan= in ./configure but no --with-abs=), kernel code is
> written like it can change FCSR bits etc.
> 
> It reality these bits are always either both zero (non-existant on
> <= R3) or both set in hardware and only NAN2008 FCSR bit has
> reflection in the ELF header flags.

In that case it seems like using abs2008 as you've proposed does not
make the situation any worse than it might already be.

Rich

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