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Subject: Re: [PATCH] x86: Remove ifdef from step.c
From: Harvey Harrison <harvey.harrison@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
       LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <20080110124041.GA15156@elte.hu>
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	 <20080110124041.GA15156@elte.hu>
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Date: Thu, 10 Jan 2008 11:29:24 -0800
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On Thu, 2008-01-10 at 13:40 +0100, Ingo Molnar wrote:
> * Harvey Harrison <harvey.harrison@gmail.com> wrote:
> 
> > @@ -148,11 +148,7 @@ static void write_debugctlmsr(struct task_struct *child, unsigned long val)
> >  	if (child != current)
> >  		return;
> >  
> > -#ifdef CONFIG_X86_64
> > -	wrmsrl(MSR_IA32_DEBUGCTLMSR, val);
> > -#else
> >  	wrmsr(MSR_IA32_DEBUGCTLMSR, val, 0);
> > -#endif
> 
> doesnt this break 64-bit? 'val' is 64-bit, but wrmsr truncates it to 
> 32-bit? [while wrmsrl() kept the full 64 bits]
> 

I wasn't totally sure about this, but believe in this case that it is
OK.  If not, we'll have to add this #ifdef back to restore_btf() in
kprobes.c which was lost Masami's unification.

But looking at how this is called, there shouldn't ever be anything in
the high 32 bits being written to (32 bit always wrote zero anyway).
In any event, it's going to be safer to use wrsmrl instead as you'll
get the zero extension on 32 bit and won't effect 64 bit.

Patch to follow.

Harvey