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Commit c7d65a78 authored by Steven Rostedt's avatar Steven Rostedt Committed by Steven Rostedt
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x86: Remove cmpxchg from i386 NMI nesting code 

I've been informed by someone on LWN called 'slashdot' that
some i386 machines do not support a true cmpxchg. The cmpxchg
used by the i386 NMI nesting code must be a true cmpxchg as
disabling interrupts will not work for NMIs (which is the work
around for i386s that do not have a true cmpxchg).

This 'slashdot' character also suggested a fix to the issue.
As the state of the nesting NMIs goes as follows:

  NOT_RUNNING -> EXECUTING
  EXECUTING   -> NOT_RUNNING
  EXECUTING   -> LATCHED
  LATCHED     -> EXECUTING

Having these states as enum values of:

  NOT_RUNNING = 0
  EXECUTING   = 1
  LATCHED     = 2

Instead of a cmpxchg to make EXECUTING -> NOT_RUNNING a
dec_and_test() would work as well. If the dec_and_test brings
the state to NOT_RUNNING, that is the same as a cmpxchg
succeeding to change EXECUTING to NOT_RUNNING. If a nested NMI
were to come in and change it to LATCHED, the dec_and_test() would
convert the state to EXECUTING (what we want it to be in such a
case anyway).

I asked 'slashdot' to post this as a patch, but it never came to
be. I decided to do the work instead.

Thanks to H. Peter Anvin for suggesting to use this_cpu_dec_and_return()
instead of local_dec_and_test(&__get_cpu_var()).

Link: http://lwn.net/Articles/484932/



Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
parent 7fbb98c5

arch/x86/kernel/nmi.c

+21 −14
Original line number Diff line number Diff line
@@ -365,8 +365,9 @@ static __kprobes void default_do_nmi(struct pt_regs *regs) 
#ifdef CONFIG_X86_32

/*

 * For i386, NMIs use the same stack as the kernel, and we can

 * add a workaround to the iret problem in C. Simply have 3 states

 * the NMI can be in.

 * add a workaround to the iret problem in C (preventing nested

 * NMIs if an NMI takes a trap). Simply have 3 states the NMI

 * can be in:

 *

 *  1) not running

 *  2) executing
@@ -383,13 +384,20 @@ static __kprobes void default_do_nmi(struct pt_regs *regs) 
 * If an NMI hits a breakpoint that executes an iret, another

 * NMI can preempt it. We do not want to allow this new NMI

 * to run, but we want to execute it when the first one finishes.

 * We set the state to "latched", and the first NMI will perform

 * an cmpxchg on the state, and if it doesn't successfully

 * reset the state to "not running" it will restart the next

 * NMI.

 * We set the state to "latched", and the exit of the first NMI will

 * perform a dec_return, if the result is zero (NOT_RUNNING), then

 * it will simply exit the NMI handler. If not, the dec_return

 * would have set the state to NMI_EXECUTING (what we want it to

 * be when we are running). In this case, we simply jump back

 * to rerun the NMI handler again, and restart the 'latched' NMI.

 *

 * No trap (breakpoint or page fault) should be hit before nmi_restart,

 * thus there is no race between the first check of state for NOT_RUNNING

 * and setting it to NMI_EXECUTING. The HW will prevent nested NMIs

 * at this point.

 */

enum nmi_states {

	NMI_NOT_RUNNING,

	NMI_NOT_RUNNING = 0,

	NMI_EXECUTING,

	NMI_LATCHED,

};
@@ -397,18 +405,17 @@ static DEFINE_PER_CPU(enum nmi_states, nmi_state); 


#define nmi_nesting_preprocess(regs)					\

	do {								\

		if (__get_cpu_var(nmi_state) != NMI_NOT_RUNNING) {	\

			__get_cpu_var(nmi_state) = NMI_LATCHED;		\

		if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {	\

			this_cpu_write(nmi_state, NMI_LATCHED);		\

			return;						\

		}							\

	nmi_restart:							\

		__get_cpu_var(nmi_state) = NMI_EXECUTING;		\

	} while (0)

		this_cpu_write(nmi_state, NMI_EXECUTING);		\

	} while (0);							\

	nmi_restart:



#define nmi_nesting_postprocess()					\

	do {								\

		if (cmpxchg(&__get_cpu_var(nmi_state),			\

		    NMI_EXECUTING, NMI_NOT_RUNNING) != NMI_EXECUTING)	\

		if (this_cpu_dec_return(nmi_state))			\

			goto nmi_restart;				\

	} while (0)

#else /* x86_64 */