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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: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt 2012-06-07 11:03:00 -04:00 committed by Steven Rostedt
parent 7fbb98c5cb
commit c7d65a78fc
1 changed files with 21 additions and 14 deletions
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35
arch/x86/kernel/nmi.c
View File

@ -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 */