linux-kernel - Re: [V4][PATCH 4/6] x86, nmi: add in logic to handle multiple events and unknown NMIs

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Message-ID: <4E7052DD.8080305@redhat.com>
Date:	Wed, 14 Sep 2011 10:08:13 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	x86@...nel.org, Andi Kleen <andi@...stfloor.org>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <peterz@...radead.org>, ying.huang@...el.com,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	jeremy@...p.org
Subject: Re: [V4][PATCH 4/6] x86, nmi:  add in logic to handle multiple events
 and unknown NMIs

On 09/13/2011 11:58 PM, Don Zickus wrote:
> Previous patches allow the NMI subsystem to process multipe NMI events
> in one NMI.  As previously discussed this can cause issues when an event
> triggered another NMI but is processed in the current NMI.  This causes the
> next NMI to go unprocessed and become an 'unknown' NMI.
>
> To handle this, we first have to flag whether or not the NMI handler handled
> more than one event or not.  If it did, then there exists a chance that
> the next NMI might be already processed.  Once the NMI is flagged as a
> candidate to be swallowed, we next look for a back-to-back NMI condition.
>
> This is determined by looking at the %rip from pt_regs.  If it is the same
> as the previous NMI, it is assumed the cpu did not have a chance to jump
> back into a non-NMI context and execute code and instead handled another NMI.
>
> If both of those conditions are true then we will swallow any unknown NMI.
>
> There still exists a chance that we accidentally swallow a real unknown NMI,
> but for now things seem better.

Patch looks good, but the changelog is outdated.

> @@ -278,19 +294,49 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
>   	pr_emerg("Dazed and confused, but trying to continue\n");
>   }
>
> +DEFINE_PER_CPU(bool, swallow_nmi);
> +DEFINE_PER_CPU(unsigned long, save_rip);

'static'.  Also need a better name for save_rip (suggest last_nmi_rip).


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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