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

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Date:	Fri, 25 Jan 2008 13:17:32 +0000
From:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
To:	Andi Kleen <ak@...e.de>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Jan Beulich <jbeulich@...ell.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...e.hu>,
	Harvey Harrison <harvey.harrison@...il.com>,
	Matt Mackall <mpm@...enic.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH UPDATE] x86: ignore spurious faults

On 25/1/08 10:19, "Andi Kleen" <ak@...e.de> wrote:

>> Whether this a problem in light of Xen spurious faults depends on whether
>> NMI handlers touch dynamically-allocated data.
> 
> How do you define dynamically-allocated data?

Anything that could have been a read-only pte or ldt page in a previous life
with no intervening TLB flush. So get_free_page(), kmalloc(), vmalloc(), ...

Actually I think we are fine, now I think about it some more, because we
only clear the software NMI-in-flight flag if the guest executes IRET via
the hypervisor. Most Xen Linux guests only do IRET via the hypervisor when
the current context is an NMI handler (additionally x86_64 also does so when
returning to ring 3). Most importantly for this case, we will *not* IRET via
the hypervisor when returning from a #PF context nested in an NMI context.
Hence the NMI-in-flight flag will not be cleared, and guest virtual NMIs
will not nest. So that's a relief!

 -- Keir

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