rpc: fix deadlock caused by priority inversion

The priority inversion occured when userspace process attempting to
send a message and was pre-empted after message reservation by the
workqueue processing prior finishing the send.

Fix by changing the rpc communication to use the new MPMC algorithm and
replacing the problematic message state with an additional queue that
the kernel module uses to forward the non-ioeventfd messages for the
userspace.

Signed-off-by: Markku Ahvenjärvi <[email protected]>

accel/sel4/sel4-all.c

@@ -251,29 +251,24 @@ static inline int handle_mmio(SeL4State *s, rpcmsg_t *req)
         exit(1);
     }
 
-    err = driver_ack_mmio_finish(&s->rpc, slot, data);
-    if (err) {
-        return RPCMSG_STATE_ERROR;
-    }
-
-    return RPCMSG_STATE_FREE;
+    return driver_ack_mmio_finish(&s->rpc, slot, data);
 }
 
-static unsigned int rpc_process(rpcmsg_t *msg, void *cookie)
+static int rpc_process(rpcmsg_t *msg, void *cookie)
 {
-    unsigned int next_state = RPCMSG_STATE_ERROR;
     SeL4State *s = cookie;
+    int ret = -1;
 
     switch (QEMU_OP(msg->mr0)) {
     case QEMU_OP_MMIO:
-        next_state = handle_mmio(s, msg);
+        ret = handle_mmio(s, msg);
         break;
     default:
-        fprintf(stderr, "Unknown op %u\n", QEMU_OP(msg->mr0));
+        fprintf(stderr, "Unknown op %lu\n", QEMU_OP(msg->mr0));
         break;
     }
 
-    return next_state;
+    return ret;
 }
 
 static void *do_sel4_virtio(void *opaque)
@@ -529,7 +524,6 @@ static int sel4_init(MachineState *ms)
     rc = sel4_rpc_init(&s->rpc,
                        device_rx_queue(s->maps[SEL4_MEM_MAP_IOBUF].ptr),
                        device_tx_queue(s->maps[SEL4_MEM_MAP_IOBUF].ptr),
-                       RPCMSG_STATE_DEVICE_USER,
                        s2_fault_doorbell,
                        (void *)(((uintptr_t)s->maps[SEL4_MEM_MAP_EVENT_BAR].ptr) + EVENT_BAR_EMIT_REGISTER));
     if (rc) {