linux-kernel - Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

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Date:	Thu, 28 Jan 2016 12:12:38 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
 running thread

On Wed, Jan 27, 2016 at 10:47:37PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 5:11 PM, Josh Triplett josh@...htriplett.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@...utronix.de 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@...utronix.de 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.