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From: Neel Natu <neel@FreeBSD.org>
Date: Thu, 3 Oct 2013 21:46:44 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-projects@freebsd.org
Subject: svn commit: r256031 - projects/bhyve_npt_pmap/sys/amd64/amd64
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Author: neel
Date: Thu Oct  3 21:46:43 2013
New Revision: 256031
URL: http://svnweb.freebsd.org/changeset/base/256031

Log:
  Add a comment in pmap_copy() explaining why we skip the function entirely
  for emulated pmaps.
  
  Requested by:	alc@

Modified:
  projects/bhyve_npt_pmap/sys/amd64/amd64/pmap.c

Modified: projects/bhyve_npt_pmap/sys/amd64/amd64/pmap.c
==============================================================================
--- projects/bhyve_npt_pmap/sys/amd64/amd64/pmap.c	Thu Oct  3 20:38:57 2013	(r256030)
+++ projects/bhyve_npt_pmap/sys/amd64/amd64/pmap.c	Thu Oct  3 21:46:43 2013	(r256031)
@@ -4762,6 +4762,14 @@ pmap_copy(pmap_t dst_pmap, pmap_t src_pm
 	if (dst_pmap->pm_type != src_pmap->pm_type)
 		return;
 
+	/*
+	 * EPT page table entries that require emulation of A/D bits are
+	 * sensitive to clearing the PG_A bit (aka EPT_PG_READ). Although
+	 * we clear PG_M (aka EPT_PG_WRITE) concomitantly, the PG_U bit
+	 * (aka EPT_PG_EXECUTE) could still be set. Since some EPT
+	 * implementations flag an EPT misconfiguration for exec-only
+	 * mappings we skip this function entirely for emulated pmaps.
+	 */
 	if (pmap_emulate_ad_bits(dst_pmap))
 		return;