linux-kernel - Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT

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Date:	Mon, 21 Mar 2016 20:43:36 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2

> 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.

> 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.

-Andi