Re: NMI vs #PF clash

[Avi Kivity <avi@redhat.com>]

On 05/22/2012 04:30 PM, Steven Rostedt wrote:
> On Tue, 2012-05-22 at 15:53 +0300, Avi Kivity wrote:
>> The recent changes to NMI allow exceptions to take place in NMI
>> handlers, but I think that a #PF (say, due to access to vmalloc space)
>> is still problematic.  Consider the sequence
>> 
>>   #PF  (cr2 set by processor)
>>     NMI
>>       ...
>>       #PF (cr2 clobbered)
>>         do_page_fault()
>>         IRET
>>       ...
>>       IRET
>>     do_page_fault()
>>       address = read_cr2()
> 
> This is still problematic. But the "allow faults in NMI" wasn't written
> for page faults, although they wont totally crash the system like they
> use to. If a NMI triggers during a page fault routine before the reading
> of the cr2, and it takes a page fault, then yes, this will corrupt the
> cr2 and cause unpredictable results (not good)
> 
> That said, we still should not be having page faults in NMI. The fault
> handling was to allow breakpoints in the NMI code, which should not be a
> problem here. There is code to handle nested breakpoints because of
> NMIs.

I thought the whole thing was started by someone adding a
vmalloc_sync_all() to prevent this scenario, and Linus wanting to
fix NMI instead.  But maybe I'm confusing two threads.

> Now if we want to handle page faults from NMI context, we could do some
> tricks to have the NMI detect that it interrupted a page fault before it
> read the cr2 and in that case, save off the cr2 register, and restore it
> before returning.
> 
> Or we could just have the NMI always restore the cr2 register.

IMO that's best.


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