linux-kernel - Re: [PATCH 3/6] x86-mce: Clear CMCI enable on all claimed CMCI banks before reboot.

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Message-ID: <20140710162419.GK2970@pd.tnic>
Date:	Thu, 10 Jul 2014 18:24:19 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Havard Skinnemoen <hskinnemoen@...gle.com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ewout van Bekkum <ewout@...gle.com>
Subject: Re: [PATCH 3/6] x86-mce: Clear CMCI enable on all claimed CMCI banks
 before reboot.

On Wed, Jul 09, 2014 at 02:40:55PM -0700, Havard Skinnemoen wrote:
> On Wed, Jul 9, 2014 at 1:36 PM, Luck, Tony <tony.luck@...el.com> wrote:
> > +       if (!xchg(&reboot_notifier_registered, true))
> > +               register_reboot_notifier(&cmci_reboot_notifier);
> >
> > This is super-safe ... but isn't the xchg() overkill? I thought we serialized bringup
> > of other cpus.
> 
> Could be. There are spinlocks being taken elsewhere in that path
> though, and I'm not sure if I see any downside with the xchg(), unless
> this path is actually performance-critical.

Several things:

We don't need all that atomicity special fun if we register the reboot
notifier on the BSP, say from mcheck_init() which is done even pre-SMP.

If that's too early, we can add an initcall or whatever...

Also, since this is kexec-only, this all should be nicely enabled only
on KEXEC kernels. Please use the ifdeffery sparingly :-)

I'm saying that because I'm assuming BIOS will clear those MSRs upon
warm reset. If it doesn't, then we have a bigger problem.

Thanks.

-- 
Regards/Gruss,
    Boris.

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