Re: [patch 1/2] x86_64 page fault NMI-safe

[H. Peter Anvin]

On 07/15/2010 03:26 PM, Linus Torvalds wrote:
> On Thu, Jul 15, 2010 at 3:16 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> Then you could in fact possibly test the stack pointer for whether it
>> is in the NMI stack area, and use the value of %rsp itself as the
>> flag. So you could avoid the flag entirely. Because testing %rsp is
>> valid - testing %rip is not.
>>
>> That would also avoid the race, because %rsp (as a flag) now gets
>> cleared atomically by the "iret". So that might actually solve things.
> 
> Hmm. So on x86-32, it's easy: if the NMI is nested, you can literally
> look at the current %rsp value, and see if it's within the NMI stack
> region.
> 
> But on x86-64, due to IST, you need to look at the saved-rsp value on
> the stack, since the %rsp always gets reset to the NMI stack region
> regardless of where it was before.
> 
> Why do we force IST use for NMI, btw? Maybe we shouldn't, and just use
> the normal kernel stack mechanisms?
> 

The reasons for using TSS (32 bits) or IST (64 bits) are: concern about
the size of the regular kernel stack, and a concern that the kernel
stack pointer may not be in a usable state.  The former is not a problem
here: we're doing a stack switch anyway, and so the additional overhead
on the main stack is pretty minimal, but the latter may be.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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