[Xen-devel] [Patch] cmpxchg emulation returns wrong ZF

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

Hi,

attached patch corrects a bug in cmpxchg emulation in the hypervisor.

BS2000 running as HVM-domain on 4 vcpus (no HAP) hit an error due to this bug
after several days.


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
TSP ES&S SWE OS6                       Telephone: +49 (0) 89 636 47950
Fujitsu Technolgy Solutions               e-mail: juergen.gross@xxxxxxxxxxxxxx
Otto-Hahn-Ring 6                        Internet: ts.fujitsu.com
D-81739 Muenchen                 Company details: ts.fujitsu.com/imprint.html

The cmpxchg emulation for accesses to page tables of guests doesn't handle
races correct.
ops->cmpxchg might return X86EMUL_CMPXCHG_FAILED if the addressed memory
location changed after checking the old contents. In this case ZF was not
changed and could remain 1 instead of being set to 0.

Signed-off-by: juergen.gross@xxxxxxxxxxxxxx


# HG changeset patch
# User juergen.gross@xxxxxxxxxxxxxx
# Date 1249540842 -7200
# Node ID 26adbdb6cb1d59d95e0a65b6a0d38fa8e95b9f51
# Parent  68e8b8379244e293c55875e7dc3692fc81d3d212
handle race on cmpxchg emulation

diff -r 68e8b8379244 -r 26adbdb6cb1d xen/arch/x86/x86_emulate/x86_emulate.c
--- a/xen/arch/x86/x86_emulate/x86_emulate.c    Sun Aug 02 13:43:15 2009 +0100
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c    Thu Aug 06 08:40:42 2009 +0200
@@ -4124,6 +4124,7 @@
         op_bytes *= 2;
 
         /* Get actual old value. */
+cmpxchg_failed:
         for ( i = 0; i < (op_bytes/sizeof(long)); i++ )
             if ( (rc = read_ulong(ea.mem.seg, ea.mem.off + i*sizeof(long),
                                   &old[i], sizeof(long), ctxt, ops)) != 0 )
@@ -4151,10 +4152,13 @@
         else
         {
             /* Expected == actual: attempt atomic cmpxchg and set ZF. */
-            if ( (rc = ops->cmpxchg(ea.mem.seg, ea.mem.off, old,
-                                    new, op_bytes, ctxt)) != 0 )
-                goto done;
-            _regs.eflags |= EFLG_ZF;
+            rc = ops->cmpxchg(ea.mem.seg, ea.mem.off, old, new, op_bytes, 
ctxt);
+            if ( rc == 0 )
+                _regs.eflags |= EFLG_ZF;
+            else if ( rc == X86EMUL_CMPXCHG_FAILED )
+                goto cmpxchg_failed;
+            else
+                goto done;
         }
         break;
     }

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