This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


[Xen-devel] [PATCH] ept: Put locks around ept_get_entry (UPDATED)

To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] [PATCH] ept: Put locks around ept_get_entry (UPDATED)
From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
Date: Fri, 27 Aug 2010 12:59:10 +0100
Cc: george.dunlap@xxxxxxxxxxxxx
Delivery-date: Fri, 27 Aug 2010 05:03:28 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mercurial-patchbomb/1.4.3
There's a subtle race in ept_get_entry, such that if tries to read an
entry that ept_set_entry is modifying, it gets neither the old entry
nor the new entry, but empty.  In the case of multi-cpu populate-on-demand
guests, this manifests as a guest crash when one vcpu tries to read a
page which another page is trying to populate, and ept_get_entry returns

This bug can also be fixed by making both ept_set_entry and ept_next_level
access-once (i.e., ept_next_level reads full ept_entry and then works with
local value; ept_set_entry construct the entry locally and then sets it
in one write).  But there doesn't seem to be any major performance implications
of just making ept_get_entry use locks; so the simpler, the better.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

diff -r 3c4c3d48a835 -r 600bbe524a66 xen/arch/x86/mm/hap/p2m-ept.c
--- a/xen/arch/x86/mm/hap/p2m-ept.c     Thu Aug 26 11:16:56 2010 +0100
+++ b/xen/arch/x86/mm/hap/p2m-ept.c     Fri Aug 27 12:58:29 2010 +0100
@@ -431,6 +431,10 @@
     int i;
     int ret = 0;
     mfn_t mfn = _mfn(INVALID_MFN);
+    int do_locking = !p2m_locked_by_me(p2m);
+    if ( do_locking )
+        p2m_lock(p2m);
     *t = p2m_mmio_dm;
@@ -507,6 +511,8 @@
+    if ( do_locking )
+        p2m_unlock(p2m);
     return mfn;

Xen-devel mailing list

<Prev in Thread] Current Thread [Next in Thread>
  • [Xen-devel] [PATCH] ept: Put locks around ept_get_entry (UPDATED), George Dunlap <=