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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