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

[Heiko Carstens]

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