Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 21 Mar 2016 15:10:58 -0700 | Subject | Re: [PATCH 4/9] x86: Enumerate kernel FSGS capability in AT_HWCAP2 |
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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
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