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SubjectRe: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults
H. Peter Anvin wrote:
> On 04/15/2014 07:15 AM, 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.
>
> Erk... this code is screaming WTF to me. The x86 architecture is such
> that the CPU is responsible for avoiding these faults.

Not in this case...

> <dig> <dig> <dig>
>
> 5b727a3b0158a129827c21ce3bfb0ba997e8ddd0
>
> x86: ignore spurious faults
>
> When changing a kernel page from RO->RW, it's OK to leave stale TLB
> entries around, since doing a global flush is expensive and they
> pose no security problem. They can, however, generate a spurious
> fault, which we should catch and simply return from (which will
> have the side-effect of reloading the TLB to the current PTE).
>
> This can occur when running under Xen, because it frequently changes
> kernel pages from RW->RO->RW to implement Xen's pagetable semantics.
> It could also occur when using CONFIG_DEBUG_PAGEALLOC, since it
> avoids doing a global TLB flush after changing page permissions.
>
> Signed-off-by: Jeremy Fitzhardinge<jeremy@xensource.com>
> Cc: Harvey Harrison<harvey.harrison@gmail.com>
> Signed-off-by: Ingo Molnar<mingo@elte.hu>
> Signed-off-by: Thomas Gleixner<tglx@linutronix.de>
>
> Again WTF?
>
> Are we chasing hardware errata here? Or did someone go off and *assume*
> that the x86 hardware architecture work a certain way? Or is there
> something way more subtle going on?

See Intel Developer's Manual Vol 3 Section 4.10.4.3, 3rd bullet... This
is expected behaviour, probably to make copy-on-write faults faster.

-- Keir

> I guess next step is mailing list archaeology...
>
> Does anyone still have contacts with Jeremy, and if so, could they poke
> him perhaps?
>
> -hpa
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel


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