KVM: x86: Dirty the dest op page on cmpxchg emulation

Intel SDM says for CMPXCHG: "To simplify the interface to the processor’s bus,
the destination operand receives a write cycle without regard to the result of
the comparison.". This means the destination page should be dirtied.

Fix it to by writing back the original value if cmpxchg failed.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -2205,12 +2205,15 @@ static int em_cmpxchg(struct x86_emulate_ctxt *ctxt)
 	fastop(ctxt, em_cmp);
 
 	if (ctxt->eflags & EFLG_ZF) {
-		/* Success: write back to memory. */
+		/* Success: write back to memory; no update of EAX */
+		ctxt->src.type = OP_NONE;
 		ctxt->dst.val = ctxt->src.orig_val;
 	} else {
 		/* Failure: write the value we saw to EAX. */
-		ctxt->dst.type = OP_REG;
-		ctxt->dst.addr.reg = reg_rmw(ctxt, VCPU_REGS_RAX);
+		ctxt->src.type = OP_REG;
+		ctxt->src.addr.reg = reg_rmw(ctxt, VCPU_REGS_RAX);
+		ctxt->src.val = ctxt->dst.orig_val;
+		/* Create write-cycle to dest by writing the same value */
 		ctxt->dst.val = ctxt->dst.orig_val;
 	}
 	return X86EMUL_CONTINUE;
@@ -4157,7 +4160,7 @@ static const struct opcode twobyte_table[256] = {
 	F(DstMem | SrcReg | Src2CL | ModRM, em_shrd),
 	GD(0, &group15), F(DstReg | SrcMem | ModRM, em_imul),
 	/* 0xB0 - 0xB7 */
-	I2bv(DstMem | SrcReg | ModRM | Lock | PageTable, em_cmpxchg),
+	I2bv(DstMem | SrcReg | ModRM | Lock | PageTable | SrcWrite, em_cmpxchg),
 	I(DstReg | SrcMemFAddr | ModRM | Src2SS, em_lseg),
 	F(DstMem | SrcReg | ModRM | BitOp | Lock, em_btr),
 	I(DstReg | SrcMemFAddr | ModRM | Src2FS, em_lseg),