linux-kernel - Re: [patch] x86: convert rdtscll() to use __native_read

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Message-ID: <20081216091509.GA29872@elte.hu>
Date:	Tue, 16 Dec 2008 10:15:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Ken Chen <kenchen@...gle.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] x86: convert rdtscll() to use __native_read_tsc

* Ken Chen <kenchen@...gle.com> wrote:

> Is there any reason why x86 rdtscll have to use the out of line function 
> instead of inline __native_read_tsc()?  native_read_tsc and 
> __native_read_tsc is essentially the same functions.

Your patch is correct.

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. (and i remember runtime problems too) The C semantics 
of taking the address of an inline function seem pretty clear: the inlined 
function should be instantiated in that .o and a pointer should be 
generated out of that local instantiation.

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?

	Ingo
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