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[Xen-devel] [PATCH V2] xen: Disable emulate.c REP optimization if introspection is active

  • To: xen-devel@xxxxxxxxxxxxx
  • From: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
  • Date: Tue, 4 Nov 2014 10:55:47 +0200
  • Cc: keir@xxxxxxx, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, jbeulich@xxxxxxxx
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Emulation for REP instructions is optimized to perform a single
write for all repeats in the current page if possible. However,
this interferes with a memory introspection application's ability
to detect suspect behaviour, since it will cause only one
mem_event to be sent per page touched.
This patch disables the optimization, gated on introspection
being active for the domain.

Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>

Changes since V1:
 - Modified comment to explain why introspection_enabled is a
   special case.
 - Removed the struct domain *currd = current->domain intermediary
 - Changed the if statement into a ternary operator line.
 - Wrapped the introspection_enabled test in an unlikely().
 xen/arch/x86/hvm/emulate.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index c0f47d2..14c1847 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -406,8 +406,13 @@ static int hvmemul_virtual_to_linear(
      * Clip repetitions to avoid overflow when multiplying by @bytes_per_rep.
      * The chosen maximum is very conservative but it's what we use in
      * hvmemul_linear_to_phys() so there is no point in using a larger value.
+     * If introspection has been enabled for this domain, *reps should be
+     * at most 1, since optimization might otherwise cause a single mem_event
+     * being triggered for repeated writes to a whole page.
-    *reps = min_t(unsigned long, *reps, 4096);
+    *reps = min_t(unsigned long, *reps,
+                           ? 1 : 4096);
     reg = hvmemul_get_seg_reg(seg, hvmemul_ctxt);

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