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Date: Sat, 28 Jun 2014 00:40:17 +0200
From: Szabolcs Nagy <>
To: Russell King - ARM Linux <>
Cc: Rich Felker <>,,
	Kees Cook <>,,
	Andy Lutomirski <>
Subject: Re: Re: Thread pointer changes

* Russell King - ARM Linux <> [2014-06-27 23:17:44 +0100]:
> I think you're are missing one obvious solution to this which you can do:
> you are passed the HWCAP fields in the ELF auxinfo.  This will tell you
> if the CPU has TLS support or not.  If it has TLS support, then you don't
> need to use the kuser helpers, and you know that it is a CPU which is ARM
> architecture v6k or later, and it has things like the CP15 barrier
> instructions.  If you want to know that the CPU supports the DMB
> instruction rather than the CP15 barrier instruction, then you have to
> check the uname details, or read /proc/cpuinfo (but I'd rather you
> didn't.)

but cp15 barrier is deprecated on armv7+

and i think the kernel can avoid the barriers on non-smp systems making
kuser helpers possibly preferable even if TLS HWCAP flag is set

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