[PATCH 1/2] x86_emulate: fix emulation of CMPXCHG

[PATCH 1/2] x86_emulate: fix emulation of CMPXCHG

[Eugene Korenevsky]

CMPXCHG sets CF, PF, AF, SF, and OF flags according to the results of the
comparison the rAX with the operand of the instruction.
rAX must be the first argument of the comparison (a minuend), the operand
must be the second one (a subtrahend).

Due to improper order of comparison arguments, CF, PF, AF, SF and OF flags were
set incorrectly in the case of inequality. Need to swap them.

Signed-off-by: Eugene Korenevsky <ekorenevsky@gmail.com>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index ae32c82..7532734 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -4343,7 +4343,8 @@ x86_emulate(
         /* Save real source value, then compare EAX against destination. */
         src.orig_val = src.val;
         src.val = _regs.eax;
-        emulate_2op_SrcV("cmp", src, dst, _regs.eflags);
+        /* EAX must be a minuend, dst must be a subtrahend. */
+        emulate_2op_SrcV("cmp", dst, src, _regs.eflags);
         if ( _regs.eflags & EFLG_ZF )
         {
             /* Success: write back to memory. */
-- 
2.0.5

Re: [PATCH 1/2] x86_emulate: fix emulation of CMPXCHG

[Jan Beulich]

>>> Eugene Korenevsky <ekorenevsky@gmail.com> 04/28/15 11:54 PM >>>
>--- a/xen/arch/x86/x86_emulate/x86_emulate.c
>+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>@@ -4343,7 +4343,8 @@ x86_emulate(
>/* Save real source value, then compare EAX against destination. */
>src.orig_val = src.val;
>src.val = _regs.eax;
>-        emulate_2op_SrcV("cmp", src, dst, _regs.eflags);
>+        /* EAX must be a minuend, dst must be a subtrahend. */
>+        emulate_2op_SrcV("cmp", dst, src, _regs.eflags);

Thanks, operands indeed need to be the other way around. The comment,
however, doesn't really look that helpful to me, and hence I intend to alter
it before committing (also getting it closer to what CMPS and SCAS have).

Should you submit further patches in the future, please also remember to
Cc relevant maintainers.

Jan