Messages in this thread | | | Date | Tue, 16 Dec 2008 01:26:42 -0800 | From | Jeremy Fitzhardinge <> | Subject | Re: [patch] x86: convert rdtscll() to use __native_read_tsc |
| |
Ingo Molnar wrote: > The reason for the __native_read_tsc() / native_read_tsc() distinction is > and obscure problem with paravirt function pointers. Such constructs: > > ./xen/enlighten.c: .read_tsc = native_read_tsc, > > do not always work fine with all versions of gcc, if native_read_tsc() is > a simple static inline (as it should be) - the build would fail with > certain gcc flags.
I don't think that's true. We rely on taking function pointers of static inlines pretty extensively; native_read_tsc is hardly unique in this respect. I don't remember seeing any problems of the sort you describe. (I can well believe this may have been a problem at some point, but not during the pv-ops development timeframe.)
> Perhaps the real fix is to do this rename as well: > > native_read_tsc => native_read_tsc_paravirt > __native_read_tsc => native_read_tsc > > as this makes the native_read_tsc_paravirt() a pure technical variant, to > be used in paravirt_ops function pointer assignments. People would thus > just use the obvious native_read_tsc() inline function most of the time > and could forget about native_read_tsc_paravirt(). > > Jeremy? >
I'm trying to remember the real reason for __native_read_tsc/native_read_tsc. At least part of it is that __native_read_tsc is used in a vdso, and so *must* be inlined to avoid a bogus call from user to kernel space. But I don't know why you wouldn't want to inline native_read_tsc everywhere. I have a feeling it may be a relic from unification - possibly because x86-64 was late to the clocksource party - but I don't remember anything specific.
I think we can probably make do with a single native_read_tsc, so long as its always inlined.
J
| |