diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 369b4754ce..d0345611c1 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -243,11 +243,10 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
         if ( !cp->feat.ibrsb && !cp->extd.ibpb )
             goto gp_fault; /* MSR available? */
 
-        /*
-         * The only defined behaviour is when writing PRED_CMD_IBPB.  In
-         * practice, real hardware accepts any value without faulting.
-         */
-        if ( v == curr && (val & PRED_CMD_IBPB) )
+        if ( val & ~PRED_CMD_IBPB )
+            goto gp_fault; /* Rsvd bit set? */
+
+        if ( v == curr )
             wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
         break;