Re: [perfmon2] I.1 - System calls - ioctl

Peter Zijlstra
2009-07-13T17:34:14Z

On Mon, 2009-07-13 at 19:30 +0200, Arnd Bergmann wrote:
> On Monday 13 July 2009, Peter Zijlstra wrote:
> > On Mon, 2009-06-22 at 08:58 -0400, Christoph Hellwig wrote:
> > > But talking about syscalls the sys_perf_counter_open prototype is
> > > really ugly - it uses either the pid or cpu argument which is a pretty
> > > clear indicator it should actually be two sys calls.
> >
> > Would something like the below be any better?
> >
> > It would allow us to later add something like PERF_TARGET_SOCKET and
> > things like that.
>
> I don't think it helps on the ugliness side. You basically make the
> two arguments a union, but instead of adding another flag and directly
> passing a union, you also add interface complexity.
>
> A strong indication for the complexity is that you got it wrong ;-) :
>
> > +struct perf_counter_target {
> > + __u32 id;
> > + __u64 val;
> > +};
>
> This structure is not compatible between 32 and 64 bit user space on x86,
> because everything except i386 adds implicit padding between id and val.

Humm, __u64 doesn't have natural alignment? That would break more than
just this I think -- it sure surprises me.

> Other than that, making it extensible sounds reasonable. How about just
> using a '__u64 *target' and a bit in the 'flags' argument?

Would there still be a point in having it a pointer in that case?, but
yeah, that might work too?