linux-kernel - Re: [PATCH 7/9] x86: skip check for spurious faults for non-present faults

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Date:	Wed, 30 Apr 2014 13:41:20 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	<xen-devel@...ts.xenproject.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 7/9] x86: skip check for spurious faults for non-present
 faults

On 15/04/14 15:15, David Vrabel wrote:
> If a fault on a kernel address is due to a non-present page, then it
> cannot be the result of stale TLB entry from a protection change (RO
> to RW or NX to X).  Thus the pagetable walk in spurious_fault() can be
> skipped.
> 
> This avoids spurious_fault() oopsing in some cases if the pagetables
> it attempts to walk are not accessible.  This obscures the location of
> the original fault.
> 
> This also fixes a crash with Xen PV guests when they access entries in
> the M2P corresponding to device MMIO regions.  The M2P is mapped
> (read-only) by Xen into the kernel address space of the guest and this
> mapping may contains holes for non-RAM regions.  Read faults will
> result in calls to spurious_fault(), but because the page tables for
> the M2P mappings are not accessible by the guest the pagetable walk
> would fault.
> 
> This was not normally a problem as MMIO mappings would not normally
> result in a M2P lookup because of the use of the _PAGE_IOMAP bit the
> PTE.  However, removing the _PAGE_IOMAP bit requires M2P lookups for
> MMIO mappings as well.
> 
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
> Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
> x86 maintainers, this is a prerequisite for removing Xen's usage of
> _PAGE_IOMAP so I think this is best merged via the Xen tree.

Peter, any opinion on this patch?

David
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