linux-kernel - Re: [PATCH v9 2/3] trace,x86: add x86 irq vector tracepoints

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Date:	Fri, 15 Feb 2013 16:09:55 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Seiji Aguchi <seiji.aguchi@....com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Thomas Gleixner (tglx@...utronix.de)" <tglx@...utronix.de>,
	"'mingo@...e.hu' (mingo@...e.hu)" <mingo@...e.hu>,
	"Borislav Petkov (bp@...en8.de)" <bp@...en8.de>,
	Satoru Moriya <satoru.moriya@....com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"Luck, Tony (tony.luck@...el.com)" <tony.luck@...el.com>
Subject: Re: [PATCH v9 2/3] trace,x86: add x86 irq vector tracepoints

On 02/04/2013 02:49 PM, Seiji Aguchi wrote:
> +
> +static void switch_to_trace_idt(void *arg)
> +{
> +	store_idt(this_cpu_ptr(&orig_idt_descr));
> +	load_idt(&trace_idt_descr);
> +
> +	return;
> +}
> +
> +static void restore_original_idt(void *arg)
> +{
> +	if (this_cpu_read(orig_idt_descr.address)) {
> +		load_idt(this_cpu_ptr(&orig_idt_descr));
> +		this_cpu_write(orig_idt_descr.address, 0);
> +	}
> +
> +	return;
> +}
> +

The base address of the IDT doesn't generally change... the one
exception is when we do the funny NMI workaround.

For that reason, I would be happier if we just restored the standard
value instead of saving/restoring stuff.

The above code is probably broken if either side is in the funny NMI
zone... unless you can guarantee it is *always* in the funny NMI zone
(in which case you can just load nmi_idt_descr instead.

Steve R., do you know what the constraints are with respect to that?

	-hpa


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