On Mar 21, 2016 12:43 PM, "Andi Kleen" <a...@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
