Re: [patch] Performance Counters for Linux, v3

Ingo Molnar
2008-12-12T08:55:33Z

* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Thu, 2008-12-11 at 20:34 +0100, Ingo Molnar wrote:
>
> > struct perf_counter_hw_event {
> > s64 type;
> >
> > u64 irq_period;
> > u32 record_type;
> >
> > u32 disabled : 1, /* off by default */
> > nmi : 1, /* NMI sampling */
> > raw : 1, /* raw event type */
> > __reserved_1 : 29;
> >
> > u64 __reserved_2;
> > };
> >
> > if the hw_event.raw bit is set to 1, then the hw_event.type is fully
> > 'raw'. The default is for raw to be 0. So negative numbers can be used
> > for sw events, positive numbers for hw events. Both can be extended
> > gradually, without arbitrarily limits introduced.
>
> On that, I still don't think its a good idea to use bitfields in an
> ABI. The C std is just not strict enough on them, and I guess that is
> the reason this would be the first such usage.

I dont feel strongly about this, we could certainly change it.

But these are system calls which have per platform bit order anyway - is
it really an issue? I'd agree that it would be bad for any sort of
persistent or otherwise cross-platform data such as filesystems, network
protocol bits, etc.

We use bitfields in a couple of system calls ABIs already, for example in
PPP:

if_ppp.h-/* For PPPIOCGL2TPSTATS */
if_ppp.h-struct pppol2tp_ioc_stats {
if_ppp.h- __u16 tunnel_id; /* redundant */
if_ppp.h- __u16 session_id; /* if zero, get tunnel stats */
if_ppp.h: __u32 using_ipsec:1; /* valid only for session_id ==

Ingo