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Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options

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On Sat, Jan 9, 2016 at 9:45 AM, Tony Luck <tony.luck@gmail.com> wrote:
> On Fri, Jan 8, 2016 at 5:52 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> Also, I think it would be nicer if the machine check code would invoke
>> the handler regardless of which handler (or class) is selected.  Then
>> the handlers that don't want to handle #MC can just reject them.
>
> The machine check code is currently a two pass process.
>
> First we scan all the machine check banks (on all processors
> at the moment because machine checks are broadcast). We
> assess the severity of all errors found.
>
> Then we take action. Panic if the most severe error was fatal,
> recover if not.
>
> This patch series tweaks the severity calculation. In-kernel
> errors at IPs with a EXTABLE_CLASS_FAULT handler are
> now ranked as recoverable. All other kernel errors remain
> fatal.
>
> I don't think it is right to unconditionally execute the fix code in the
> severity assessment phase.

I would argue that unconditionally calling the handler would be
cleaner.  The handler would return 0 or false to indicate that it
refuses to fix the exception.

This is similar to the logic that, for regular user memory access, we
shouldn't fix up faults other than #PF.  Given that we're adding
flexible handler callbacks, lets push all the "is this an acceptable
fault to fix up" down into the callback.  Does that make sense?

>
> Perhaps later we can revisit the two pass process?

Oh, I see.  Is it the case that the MC code can't cleanly handle the
case where the error was nominally recoverable but the kernel doesn't
know how to recover from it due to the lack of a handler that's okay
with it, because the handler's refusal to handle the fault wouldn't be
known until too late?

--Andy

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[PATCH v8 0/3] Machine check recovery when kernel accesses poison Tony Luck <tony.luck@intel.com> - 2016-01-09 01:20 +0100
  [PATCH v8 1/3] x86: Expand exception table to allow new handling  options Tony Luck <tony.luck@intel.com> - 2016-01-09 01:20 +0100
    Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Andy Lutomirski <luto@amacapital.net> - 2016-01-09 03:00 +0100
      Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Brian Gerst <brgerst@gmail.com> - 2016-01-09 04:50 +0100
        Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Brian Gerst <brgerst@gmail.com> - 2016-01-09 05:40 +0100
          Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Andy Lutomirski <luto@amacapital.net> - 2016-01-09 07:40 +0100
            Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Brian Gerst <brgerst@gmail.com> - 2016-01-12 00:10 +0100
              Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Andy Lutomirski <luto@amacapital.net> - 2016-01-12 00:30 +0100
              RE: [PATCH v8 1/3] x86: Expand exception table to allow new  handling options "Luck, Tony" <tony.luck@intel.com> - 2016-01-12 00:50 +0100
      Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Tony Luck <tony.luck@gmail.com> - 2016-01-09 18:50 +0100
        Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Andy Lutomirski <luto@amacapital.net> - 2016-01-09 19:10 +0100
          Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Tony Luck <tony.luck@gmail.com> - 2016-01-09 21:00 +0100
            Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Andy Lutomirski <luto@amacapital.net> - 2016-01-09 23:40 +0100
              Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options Tony Luck <tony.luck@gmail.com> - 2016-01-10 02:20 +0100
      Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling  options "Luck, Tony" <tony.luck@intel.com> - 2016-01-11 01:30 +0100
  [PATCH v8 2/3] x86, mce: Check for faults tagged in  EXTABLE_CLASS_FAULT exception table entries Tony Luck <tony.luck@intel.com> - 2016-01-09 01:20 +0100

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