Re: [V5][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

[Avi Kivity <avi@redhat.com>]

On 09/20/2011 05:43 PM, Don Zickus wrote:
> Previous patches allow the NMI subsystem to process multipe NMI events
> in one NMI.  As previously discussed this can cause issues when an event
> triggered another NMI but is processed in the current NMI.  This causes the
> next NMI to go unprocessed and become an 'unknown' NMI.
>
> To handle this, we first have to flag whether or not the NMI handler handled
> more than one event or not.  If it did, then there exists a chance that
> the next NMI might be already processed.  Once the NMI is flagged as a
> candidate to be swallowed, we next look for a back-to-back NMI condition.
>
> This is determined by looking at the %rip from pt_regs.  If it is the same
> as the previous NMI, it is assumed the cpu did not have a chance to jump
> back into a non-NMI context and execute code and instead handled another NMI.
>
> If both of those conditions are true then we will swallow any unknown NMI.
>
> There still exists a chance that we accidentally swallow a real unknown NMI,
> but for now things seem better.
>
> An optimization has also been added to the nmi notifier rountine.  Because x86
> can latch up to one NMI while currently processing an NMI, we don't have to
> worry about executing _all_ the handlers in a standalone NMI.  The idea is
> if multiple NMIs come in, the second NMI will represent them.  For those
> back-to-back NMI cases, we have the potentail to drop NMIs.  Therefore only
> execute all the handlers in the second half of a detected back-to-back NMI.
>
> V2:
>    - forgot to add the 'read' code for swallow_nmi (went into next patch)
>
> V3:
>    - redesigned the algorithm to utilize Avi's idea of detecting a back-to-back
>      NMI with %rip.
> V4:
>    - clean up fixes, like adding 'static', rename save_rip to last_nmi_rip
>
>

Missing a zeroing of last_nmi_rip in the idle path.  Otherwise, as Andi 
points out, and idle machine will always see NMIs coming in from the 
hlt/mwait address and detect them as back-to-back NMIs.

-- 
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