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Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
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To: Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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From: "H. Peter Anvin" <hpa@zytor.com>
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Date: Tue, 1 Mar 2016 13:47:38 -0800
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On 03/01/16 13:32, Peter Zijlstra wrote:
> On Tue, Mar 01, 2016 at 08:23:12PM +0000, Mathieu Desnoyers wrote:
>> I think it's important that user-space fast-paths can quickly
>> detect whether the feature is enabled without having to rely on
>> always reading a separate cache-line. I've put together an ABI
>> proposal that take into account the feedback received so far.
> 
> Nah, adding detectoring code to fast paths is silly, makes them less
> fast. Doesn't userspace have self modifying code? I know that at least
> glibc does linker trickery to call different functions depending on
> runtime context.
> 

No, userspace does not have self-modifying code.  The glibc indirect
function is done at dynamic link time; it is also worth noting that
resolving global symbols through dynamic linking often requires an
indirect call.

	-hpa