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[Xen-devel] SMAP feature causing kernel alignment faults due to setting EFLAGS.AC

  • To: xen-devel <xen-devel@xxxxxxxxxxxxx>
  • From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
  • Date: Mon, 28 Jan 2013 10:51:13 +0000
  • Delivery-date: Mon, 28 Jan 2013 10:51:29 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

This appears to be the case in the recent messages in Debian bug #660425
[0] (thread starts at [1] and continues at [2]).

SMAP uses EFLAGS.AC in kernel mode to indicate whether accesses to
usermode pages should fault or not, which gets confused because under
64-bit Xen kernel == ring3. The obvious fix would be to clear SMAP from
the cpuid features (probably in kernel as well as hypervosr) but might
we want to do something more advanced? Not sure what, actually
implementing PV SMAP would likely require a third set of pagetables with
only the kernel mappings...


[0] http://bugs.debian.org/660425
[1] http://lists.debian.org/debian-kernel/2012/12/msg00520.html 
[2] http://lists.debian.org/debian-kernel/2013/01/msg00464.html

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