[Intel-wired-lan] [PATCH iwl-net] idpf: add read memory barrier when checking descriptor done bit

[Emil Tantilov]

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 has completed.

Fixes: 8077c727561a ("idpf: add controlq init and reset checks")
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel at intel.com>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala at intel.com>
Suggested-by: Lance Richardson <rlance at google.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov at intel.com>
---
 drivers/net/ethernet/intel/idpf/idpf_controlq.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/net/ethernet/intel/idpf/idpf_controlq.c b/drivers/net/ethernet/intel/idpf/idpf_controlq.c
index 4849590a5591..61c7fafa54a1 100644
--- a/drivers/net/ethernet/intel/idpf/idpf_controlq.c
+++ b/drivers/net/ethernet/intel/idpf/idpf_controlq.c
@@ -375,6 +375,11 @@ int idpf_ctlq_clean_sq(struct idpf_ctlq_info *cq, u16 *clean_count,
 		desc = IDPF_CTLQ_DESC(cq, ntc);
 		if (!(le16_to_cpu(desc->flags) & IDPF_CTLQ_FLAG_DD))
 			break;
+		/*
+		 * This barrier is needed to ensure that no other fields
+		 * are read until we check the DD flag.
+		 */
+		dma_rmb();
 
 		/* strip off FW internal code */
 		desc_err = le16_to_cpu(desc->ret_val) & 0xff;
@@ -562,6 +567,11 @@ int idpf_ctlq_recv(struct idpf_ctlq_info *cq, u16 *num_q_msg,
 
 		if (!(flags & IDPF_CTLQ_FLAG_DD))
 			break;
+		/*
+		 * This barrier is needed to ensure that no other fields
+		 * are read until we check the DD flag.
+		 */
+		dma_rmb();
 
 		q_msg[i].vmvf_type = (flags &
 				      (IDPF_CTLQ_FLAG_FTYPE_VM |
-- 
2.17.2