[RFC 04/32] bus/vmbus: fix missing acquire on receive ring index

[Stephen Hemminger]

The receive path checks available data by reading the write index and
then copies ring data. The link between the two is only the comparison,
a control dependency, which does not order load against load on a
weakly ordered architecture: packet data can be read before the host
has finished writing it. Load the write index with acquire, pairing
with the host's release store. This is the equivalent of virt_rmb()
in the Linux hv_pkt_iter code. No change in generated code on x86.

Fixes: 831dba47bd36 ("bus/vmbus: add Hyper-V virtual bus support")
Cc: stable at dpdk.org

Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
---
 drivers/bus/vmbus/private.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/bus/vmbus/private.h b/drivers/bus/vmbus/private.h
index 6efac86b77..aca5b534f9 100644
--- a/drivers/bus/vmbus/private.h
+++ b/drivers/bus/vmbus/private.h
@@ -12,6 +12,7 @@
 #include <bus_driver.h>
 #include <bus_vmbus_driver.h>
 #include <rte_log.h>
+#include <rte_stdatomic.h>
 #include <rte_eal_paging.h>
 #include <rte_vmbus_reg.h>
 
@@ -130,7 +131,16 @@ vmbus_br_availwrite(const struct vmbus_br *br, uint32_t windex)
 static inline uint32_t
 vmbus_br_availread(const struct vmbus_br *br)
 {
-	return br->dsize - vmbus_br_availwrite(br, br->vbr->windex);
+	uint32_t windex;
+
+	/* Pairs with the host's release store of the write index;
+	 * ring data must not be read before the index.
+	 */
+	windex = rte_atomic_load_explicit(
+		(volatile uint32_t __rte_atomic *)&br->vbr->windex,
+		rte_memory_order_acquire);
+
+	return br->dsize - vmbus_br_availwrite(br, windex);
 }
 
 int vmbus_txbr_write(struct vmbus_br *tbr, const struct iovec iov[], int iovlen,
-- 
2.53.0