Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

YiFei Zhu
2020-09-25T16:39:35Z

On Thu, Sep 24, 2020 at 10:28 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote:
> Ah. Makes sense.
>
> > Ironicailly, that's the only place I actually know for sure where people
> > using x32 because it shows measurable (10%) speed-up for builders:
> > https://lore.kernel.org/lkml/CAOesGMgu1i3p7XMZuCEtj63T-ST_jh+BfaHy-K6LhgqNriKHAA@mail.gmail.com
>
> Wow. 10% is significant. Makes you wonder why x32 hasn't conquered the world.
>
> > So, yes, as you and Jann both point out, it wouldn't be terrible to just
> > ignore x32, it seems a shame to penalize it. That said, if the masking
> > step from my v1 is actually noticable on a native workload, then yeah,
> > probably x32 should be ignored. My instinct (not measured) is that it's
> > faster than walking a small array.[citation needed]
>
> You convince me that penalizing supporting x32 would be a pity :( The
> 10% is so nice I want it.

I'm rethinking this -- the majority of our users will not use x32. I
don't think it's that useful for the majority to run all the
simulations and have the memory footprint if only a small minority
will use it.

I also just checked Debian, and it has boot-time disabling of the x32
arch downstream [1]:
CONFIG_X86_X32=y
CONFIG_X86_X32_DISABLED=y

Which means we will still generate all the code for x32 in seccomp
even though people probably won't be using it...

I also talked to some of my peers and they had a point regarding how
x32 limiting address space to 4GiB is very harsh on many modern
language runtimes, so even though it provides a 10% speed boost, its
adoption is hard -- one has to compile all the C libraries in x32 in
addition to x86_64, since one would have programs needing > 4GiB
address space needing x86_64 version of the libraries.

[1] https://wiki.debian.org/X32Port

YiFei Zhu