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

[Keir Fraser <keir.xen@gmail.com>]

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
>
>
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