linux-kernel - RE: [PATCH 1/2 v2] x86, mce, severity: extend the the mce

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Date:	Thu, 6 Nov 2014 17:27:14 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Borislav Petkov <bp@...en8.de>, Chen Yucong <slaoub@...il.com>
CC:	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"aravind.gopalakrishnan@....com" <aravind.gopalakrishnan@....com>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2 v2] x86, mce, severity: extend the the mce_severity

>> +int mce_severity(struct mce *m, int tolerant, char **msg, bool is_excp)
>
> You're adding a function argument which is carrying redundant info which
> is already present in *m...
>
>>  {
>> +	enum exception excp = (is_excp ? EXCP_CONTEXT : NO_EXCP);
>
> ... and so this should be:
>
>	excp = ((m->mcg_status & MCG_STATUS_MCIP) ? EXCP_CONTEXT : NO_EXCP);

That only works if you trust that MCG_STATUS.MCIP is correctly set to indicate whether
we are in MCE or CMCI context. The current code doesn't do that - we check for, and flag
it as a fatal error if we find ourselves in the MCE handler with MCIP==0. If you add the
code you suggest, then it completely neuters the severity check:

        MCESEV(
                PANIC, "MCIP not set in MCA handler",
                MCGMASK(MCG_STATUS_MCIP, 0)
                ),

I'm also a bit worried about the check for DEFERRED errors in
the severity table.  That isn't conditional on an:
  if (intel) do_onething(); else /*amd/ do_anotherthing();
So if we can misinterpret some bits on an Intel cpu as if
we had a deferred error.

Overall, this might have seemed like a good idea to begin with,
but we are piling more complexity into mce_severity() [a routine
which everyone agrees is already tough to understand].

It doesn't even buy us some simple code in the polling path.
We still have to do more checks on MCi_STATUS.MCACOD above
and beyond what we get back from mce_severity()

Boris: Do you still want to keep pushing this way? Or should
we look back fondly at version 1 of this patch?

-Tony