idpf: add read memory barrier when checking descriptor done bit

Add read memory barrier to ensure the order of operations when accessing
control queue descriptors. Specifically, we want to avoid cases where loads
can be reordered:

1. Load #1 is dispatched to read descriptor flags.
2. Load #2 is dispatched to read some other field from the descriptor.
3. Load #2 completes, accessing memory/cache at a point in time when the DD
   flag is zero.
4. NIC DMA overwrites the descriptor, now the DD flag is one.
5. Any fields loaded before step 4 are now inconsistent with the actual
   descriptor state.

Add read memory barrier between steps 1 and 2, so that load #2 is not
executed until load #1 has completed.

Fixes: 8077c72 ("idpf: add controlq init and reset checks")
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Suggested-by: Lance Richardson <rlance@google.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/idpf/idpf_controlq.c

@@ -376,6 +376,9 @@ int idpf_ctlq_clean_sq(struct idpf_ctlq_info *cq, u16 *clean_count,
 		if (!(le16_to_cpu(desc->flags) & IDPF_CTLQ_FLAG_DD))
 			break;
 
+		/* Ensure no other fields are read until DD flag is checked */
+		dma_rmb();
+
 		/* strip off FW internal code */
 		desc_err = le16_to_cpu(desc->ret_val) & 0xff;
 
@@ -563,6 +566,9 @@ int idpf_ctlq_recv(struct idpf_ctlq_info *cq, u16 *num_q_msg,
 		if (!(flags & IDPF_CTLQ_FLAG_DD))
 			break;
 
+		/* Ensure no other fields are read until DD flag is checked */
+		dma_rmb();
+
 		q_msg[i].vmvf_type = (flags &
 				      (IDPF_CTLQ_FLAG_FTYPE_VM |
 				       IDPF_CTLQ_FLAG_FTYPE_PF)) >>