Messages in this thread | | | Date | Thu, 15 May 2014 20:22:43 +0100 | From | Keir Fraser <> | Subject | Re: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults |
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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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