From mboxrd@z Thu Jan  1 00:00:00 1970
From: Heiko Carstens <heiko.carstens-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
 running thread
Date: Thu, 28 Jan 2016 12:12:38 +0100
Message-ID: <20160128111238.GA5418@osiris>
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To: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
Cc: Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>, Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>, Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>, Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>, Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>, Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>, rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>, Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>, Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
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On Wed, Jan 27, 2016 at 10:47:37PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 5:11 PM, Josh Triplett josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org wrote:
> 
> > On Wed, Jan 27, 2016 at 09:34:35PM +0000, Mathieu Desnoyers wrote:
> >> ----- On Jan 27, 2016, at 12:37 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> >> 
> >> > On Wed, 27 Jan 2016, Thomas Gleixner wrote:
> >> > 
> >> >> On Wed, 27 Jan 2016, Mathieu Desnoyers wrote:
> >> >> > ----- On Jan 27, 2016, at 12:22 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> >> >> > Sounds fair. What is the recommended typing for "ptr" then ?
> >> >> > uint32_t ** or uint32_t * ?
> >> >> > 
> >> >> > It would be expected to pass a "uint32_t *" for the set
> >> >> > operation, but the "get" operation requires a "uint32_t **".
> >> >> 
> >> >> Well, you can't change the types depending on the opcode, so you need to stick
> >> >> with **.
> >> > 
> >> > Alternatively you make it:
> >> > 
> >> >  (opcode, *newptr, **oldptr, flags);
> >> 
> >> I'm tempted to stick to (opcode, **ptr, flags), because
> >> other syscalls that have "*newptr", "**oldptr"
> >> typically have them because they save the current state
> >> into oldptr, and set the new state, which is really
> >> not the case here. To eliminate any risk of confusion,
> >> I am tempted to keep a single "**ptr".
> >> 
> >> Unless someone has a better idea...
> > 
> > Either that or you could define it as "void *" and interpret it based on
> > flags, but that seems unfortunate; let's not imitate ioctl-style
> > typeless parameters.  I'd stick with the double pointer and the current
> > behavior.
> 
> Allright, will do! Thanks for the feedback :)

Please don't forget that you also need to implement compat handling since
the size of the pointer that is being pointed to is only four bytes for
compat tasks.