Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2
Date	2016-03-21 23:20 +0100
Message-ID	<rfgl4-3uK-11@gated-at.bofh.it> (permalink)
References	(1 earlier) <rfaIG-8aj-23@gated-at.bofh.it> <rfdQe-1Ku-15@gated-at.bofh.it> <rfdQe-1Ku-17@gated-at.bofh.it> <rfdQe-1Ku-13@gated-at.bofh.it> <rfdZV-1O7-23@gated-at.bofh.it>
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On Mar 21, 2016 12:43 PM, "Andi Kleen" <andi@firstfloor.org> wrote:
>
> > It would be less complicated actually, as normal userspace would just
> > continue to call arch_prctl() as it does today.  Glibc would implement
>
> We already have that through the system call, no advantage of
> putting it into a vsyscall.
>
> Also the experience with getcpu and similar so far is that
> vsyscalls are too slow for the users who want really fast paths.
> So they're prefer to use the direct instructions anyways.

Getcpu is mainly slow because the overcomplicated API requires
branches.  I've been tempted to add __vdso_get_cpu as an alternative
that simply returns the CPU number.

>
> > arch_prctl() just like it does with gettimeofday() -- with an ifunc
> > selector [1] that calls the VDSO function if it is available, or the
> > syscall if not.  No custom assembly needed.
>
> vdso always needs custom assembler, please see how glibc implements it.

This is simply not true.

I haven't checked the glibc implementation, and I wouldn't be remotely
surprised if it is maliciously incomprehensible, but there is no
reason whatsoever that using any vdso mechanism other than AT_SYSINFO
itself requires assembler.  AT_SYSINFO, of course, requires assembler
because the calling convention is weird.

--Andy

Thread

[PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Andi Kleen <andi@firstfloor.org> - 2016-03-21 17:20 +0100
  Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Brian Gerst <brgerst@gmail.com> - 2016-03-21 20:40 +0100
    Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Andi Kleen <andi@firstfloor.org> - 2016-03-21 20:50 +0100
      Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Andy Lutomirski <luto@amacapital.net> - 2016-03-21 23:20 +0100
  Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Andi Kleen <andi@firstfloor.org> - 2016-03-21 20:40 +0100
  Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 Brian Gerst <brgerst@gmail.com> - 2016-03-21 20:50 +0100

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