On 02/24/2014 11:30 AM, Peter Zijlstra wrote:
> 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.
Is there any way that nmi_cr2 can end up getting overwritten by multiple
nestings of some kind? I would have thought it would have made more
sense to spill cr2 onto the stack after the stack has been properly set up.
-hpa
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