linux-kernel - Re: [PATCH 4/5] x86: Allow nesting of the debug stack IDT setting

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Date:	Thu, 31 May 2012 12:27:40 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Dave Jones <davej@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/5] x86: Allow nesting of the debug stack IDT setting

On 05/31/2012 12:25 PM, Steven Rostedt wrote:
>> void debug_stack_set_zero(void)
>> {
>> 	if (this_cpu_inc_return(debug_stack_use_ctr) == 1)
> 
> If an NMI comes in here, it will not update the IDT and will corrupt the
> stack. The load_idt() must happen for all calls. There's only two static
> IDT tables. It's either one or the other. Thus, if it loads it twice, it
> will just set it to the same value. The counter is to know when to set
> it back.
> 

Now I'm really confused.  Why would it have to set it if it is already set?

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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