Re: [patch] x86: convert rdtscll() to use __native_read_tsc
From: Jeremy Fitzhardinge
Date: Tue Dec 16 2008 - 04:26:55 EST
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
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