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

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Date:	Thu, 6 Nov 2014 19:22:06 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Chen Yucong <slaoub@...il.com>,
	"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

On Thu, Nov 06, 2014 at 05:27:14PM +0000, Luck, Tony wrote:
> >> +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 was looking at the version Chen did:

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

and then

                if (s->excp && excp != s->excp)
                        continue;

Basically, this check is being done only for machine check exceptions
only.

> 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?

You mean the one which doesn't touch mce_severity() at all and decides
on deferred errors in a separate, completely unrelated function? Yeah,
that might be cleaner after all.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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