Re: [PATCH RFC] NMI Re-introduce un[set]_nmi_callback

[Ingo Molnar]

* Prarit Bhargava <prarit@xxxxxxxxxx> wrote:

>> Why is the DIE_NMIWATCHDOG notifier not sufficient for this driver?
>
> Peter -- good question.  The HP systems with this HW will use the 
> hpwdt driver in place of the default nmi watchdog.  When the HW 
> detects a problem, the HW will generate a single NMI that the driver 
> will handle.  The driver doesn't want the NMI to be rejected due to a 
> reason code.  I'm sure that Thomas Mingarelli, who is cc'd, can 
> provide further details.
>
> From our quick conversation as well, you raised an interesting point 
> about oprofile, kgdb, and other subsystems that use the NMI notifier 
> chains -- they may be impacted by the NMI callback.
>
> Don (dzickus) or Aris, do you have any thoughts on how to get around 
> the second issue?  We could check to see if anything is registered on 
> the notifier chain and the fail to register the callback.

i'd much rather attack this general problem from this angle:

  static inline unsigned char get_nmi_reason(void)
  {
          return inb(0x61);
  }

that port 61H read is both arcane (on modern chipsets) and broken on 
multiple levels. It's racy and SMP unsafe to begin with, if there's any 
mixture of intentional cross-CPU or CPU self-generated NMIs mixed with 
chipset generated NMIs.

One possible approach would be to get rid of it, and to perhaps register 
a low-priority die notifier on systems where we know port 61 
reads+writes to be safe and desired. Modern systems will emit MCEs in 
most cases anyway, not NMIs.

	Ingo
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