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Date: Mon, 24 Feb 2014 20:30:43 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Vince Weaver <vincent.weaver@maine.edu>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
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On Mon, Feb 24, 2014 at 02:13:29PM -0500, Steven Rostedt wrote:
> Ah, and x86_64 saves off the cr2 register when entering NMI and restores
> it before returning. But it seems to be missing from the i386 code.

arch/x86/kernel/nmi.c:

#define nmi_nesting_preprocess(regs)					\
	do {								\
		if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {	\
			this_cpu_write(nmi_state, NMI_LATCHED);		\
			return;						\
		}							\
		this_cpu_write(nmi_state, NMI_EXECUTING);		\
		this_cpu_write(nmi_cr2, read_cr2());			\
	} while (0);							\
	nmi_restart:

#define nmi_nesting_postprocess()					\
	do {								\
		if (unlikely(this_cpu_read(nmi_cr2) != read_cr2()))	\
			write_cr2(this_cpu_read(nmi_cr2));		\
		if (this_cpu_dec_return(nmi_state))			\
			goto nmi_restart;				\
	} while (0)

That very much looks like saving/restoring CR2 to me.

FWIW; I hate how the x86_64 and i386 versions of this NMI nesting magic
are so completely different.