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

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Message-ID: <1338500328.13348.454.camel@gandalf.stny.rr.com>
Date:	Thu, 31 May 2012 17:38:48 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Thu, 2012-05-31 at 17:37 -0400, Steven Rostedt wrote:
> On Thu, 2012-05-31 at 14:09 -0700, H. Peter Anvin wrote:
> 
> > No, I'm asking what environments (alternate stacks) are permitted where.
> >  That is the important information.
> 
> I believe that the IST stack switch is just a convenient way to allow
> separate stacks for separate events instead of trying to handle all
> events on a single stack, and risk stack overflow.
> 
> Thus, the debug/int3 and NMI events use a separate stack to not stress
> the current stack that is not expected to switch. The external interrupt
> stacks are switched on entry of the interrupt and stays on that stack
> and until its finished. There's a check to see if it is already on the
> stack before it does the switch so that it doesn't have the HW switch
> issues that NMI and int3 has.
> 
> Thus, if we say we must stay on either the NMI stack or the DEBUG stack,
> then this is what happens. Well, almost.
> 
> For NMIs, if it preempted something on the debug stack, it will always
> stay on the NMI stack and never switch. A check is made for both debug
> stacks at once:
> 
> int is_debug_stack(unsigned long addr)
> {
> 	return __get_cpu_var(debug_stack_usage) ||
> 		(addr <= __get_cpu_var(debug_stack_addr) &&
> 		 addr > (__get_cpu_var(debug_stack_addr) - DEBUG_STKSZ));
> }
> 

And, yes, I can send a patch to change these to this_cpu_() if you
like ;-)

-- Steve


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