linux-kernel - Re: [PATCH UPDATE] x86: ignore spurious faults

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Message-Id: <4799B75C.76E4.0078.0@novell.com>
Date:	Fri, 25 Jan 2008 09:18:04 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	"Keir Fraser" <Keir.Fraser@...cam.ac.uk>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Harvey Harrison" <harvey.harrison@...il.com>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Matt Mackall" <mpm@...enic.com>, "Andi Kleen" <ak@...e.de>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH UPDATE] x86: ignore spurious faults

>>> Nick Piggin <nickpiggin@...oo.com.au> 25.01.08 09:38 >>>
>On Friday 25 January 2008 19:15, Jan Beulich wrote:
>> Actually, another thought: permitting (and handling) spurious faults for
>> kernel mappings conflicts with NMI handling, i.e. great care would be
>> needed to ensure the NMI path cannot touch any such mapping. So
>> even the present Xen/Linux Dom0 implementation may have some
>> (perhaps unlikely) problems here, and it would get worse if we added
>> e.g. a virtual watchdog NMI (something I am considering, which would
>> then extend the problem to DomU-s).
>
>Can you explain how they conflict?

In the same way as vmalloc faults do (which is why vmalloc_sync_all()
got introduced): a page fault nested inside an NMI will, by virtue of
executing IRET, prematurely tell the processor that NMI handling is
done (and specifically unmask further NMIs).

Jan

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