Re: [Xen-devel] [PATCH] x86/nmi: lower initial watchdog frequency to avoid boot hangs

[Alexey G <x1917x@xxxxxxxxx>]

On Thu, 8 Feb 2018 12:40:41 +0000
Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>- Perf/Oprofile.  This is currently mutually exclusive with Xen using
>the watchdog, but needn't be and hopefully won't be in the future.
>
>>
>> Most of the time we deal with watchdog NMIs, while all others should
>> be somewhat rare. The thing is, we actually need to read I/O port
>> 61h on system NMIs only. 
>>
>> If the main problem lies in a flow of SMIs due to reading port 61h on
>> every NMI watchdog tick -- why not to avoid reading it?
>>
>> There are at least 2 ways to check if the NMI was due to a watchdog
>> tick:
>> - LAPIC (SDM states that "When a performance monitoring counters
>> interrupt is generated, the mask bit for its associated LVT entry is
>> set")
>> - perf MSR overflow bit
>>
>> So, if we detect it was a NMI due to a watchdog using these
>> methods (early in the NMI handler), we can avoid touching the port
>> 61h and thus triggering SMI I/O trap on it.  
>
>The problem is having multiple NMIs arriving.  Like all other edge
>triggered interrupts, extra arrivals get dropped.  By skipping the 0x61
>read if we believe it was a watchdog NMI, we've opened a race condition
>where we will completely miss the system NMI.

There shouldn't be any problem I think. NMIs don't need to be cleared
with EOI and it's a common practice to handle NMIs one-by-one (as a NMI
handler is not reentrant in a typical scenario).

Execution of SMI doesn't cause a pending (blocked) NMI to get dropped,
similar mechanisms might be employed for a single NMI which arrived in
blocked-by-NMI state. Otherwise the whole thing will break -- merely
handling arbitrary NMI will be enough to miss any other NMIs. This is a
too obvious flaw. So normally it should be just a matter which NMI of
two will be serviced first.
This assumption can be verified empirically by requesting the chipset
to send an external NMI while serving a watchdog NMI and checking if it
arrive later on.

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