linux-kernel - Re: [PATCH] ftrace/x86: Use kernel identity mapping only when CONFIG_DEBUG

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Date:	Thu, 5 Jun 2014 11:14:49 +0200
From:	Petr Mládek <pmladek@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jan Beulich <jbeulich@...e.com>, Jiri Kosina <jkosina@...e.cz>,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] ftrace/x86: Use kernel identity mapping only when
 CONFIG_DEBUG_RODATA enabled

On Wed 2014-06-04 14:37:28, Steven Rostedt wrote:
> On Wed,  4 Jun 2014 17:42:46 +0200
> Petr Mladek <pmladek@...e.cz> wrote:
> 
> > This is just a tiny optimization. The remapping is not needed when the
> > kernel text mapping is read write all the time.
> 
> This is true, but I think I rather have it still do it the same. The
> less variables in the code modification the better. The more changes
> per config option, the less things tend to get tested.
> 
> This is a micro optimization to something that is hardly a fast path.

It makes sense. Let's leave it as is.

Best Regards,
Petr

> > 
> > Suggested-by: Jan Beulich <jbeulich@...e.com>
> > Signed-off-by: Petr Mladek <pmladek@...e.cz>
> > ---
> >  arch/x86/kernel/ftrace.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > index cbc4a91b131e..a7a517bb793e 100644
> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -79,6 +79,7 @@ within(unsigned long addr, unsigned long start, unsigned long end)
> >  
> >  static unsigned long text_ip_addr(unsigned long ip)
> >  {
> > +#ifdef CONFIG_DEBUG_RODATA
> >  	/*
> >  	 * On x86_64, kernel text mappings are mapped read-only with
> >  	 * CONFIG_DEBUG_RODATA. So we use the kernel identity mapping instead
> > @@ -89,7 +90,7 @@ static unsigned long text_ip_addr(unsigned long ip)
> >  	 */
> >  	if (within(ip, (unsigned long)_text, (unsigned long)_etext))
> >  		ip = (unsigned long)__va(__pa_symbol(ip));
> > -
> > +#endif
> >  	return ip;
> >  }
> >  
> 
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