Messages in this thread | | | Date | Fri, 12 Dec 2008 09:54:57 +0100 | From | Ingo Molnar <> | Subject | Re: [patch] Performance Counters for Linux, v3 |
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* 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
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