Re: [Xen-devel] Woes of NMIs and MCEs, and possibly how to fix

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.12 at 12:45, Mats Petersson <mats.petersson@xxxxxxxxxx> wrote:
> On 03/12/12 11:24, George Dunlap wrote:
>> On Fri, Nov 30, 2012 at 5:34 PM, Andrew Cooper 
>> <andrew.cooper3@xxxxxxxxxx <mailto:andrew.cooper3@xxxxxxxxxx>> wrote:
>>
>>     3) SMM mode executing an iret will re-enable NMIs.  There is
>>     nothing we
>>     can do to prevent this, and as an SMI can interrupt NMIs and MCEs, no
>>     way to predict if/when it may happen.  The best we can do is
>>     accept that
>>     it might happen, and try to deal with the after effects.
>>
>>
>> Did you actually mean IRET, or did you mean RSM?  Does it make a 
>> difference?
> 
> If, for some obscure reason, the SMM code decides, for example, to run 
> code like "int 0x21", where the int 0x21 handler ends with the rather 
> predictable IRET to return to the caller, then you would indeed "unlock" 
> the NMI blocking that happens from the NMI being taken by the processor. 
> NMI will still not interrupt the SMM code, but it WILL interrupt the 
> code that was running before SMI was taken - which could be an NMI 
> handler, that doesn't expect another NMI.
> 
> RSM doesn't, in and of itself [unless "messing" with the saved state] 
> alter the NMI state in other ways than "restore to previous value".

And isn't it this "restore to previous value" that makes this a
non-issue - after RSM, NMI-s would again be masked.

Jan


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