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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 21 Mar 2016 15:10:58 -0700
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Subject: Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2
To: Andi Kleen <andi@firstfloor.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Andi Kleen <ak@linux.intel.com>
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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