[RFC 05/32] bus/vmbus: replace SMP barriers with C11 memory fences

[Stephen Hemminger]

Full barriers order the preceding index store against the following
load of the interrupt mask or pending size and stay seq_cst; read
barriers become acquire fences. Same code generated on x86 and arm64.

The write barrier before publishing the transmit index becomes a
release fence. Nominally stronger on arm64, but it is immediately
followed by a full barrier cmpset; converting that to a release
compare-exchange is left for the rte_atomic32 removal series.

Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
---
 drivers/bus/vmbus/vmbus_bufring.c | 6 +++---
 drivers/bus/vmbus/vmbus_channel.c | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/bus/vmbus/vmbus_bufring.c b/drivers/bus/vmbus/vmbus_bufring.c
index f622869974..31478d3bfa 100644
--- a/drivers/bus/vmbus/vmbus_bufring.c
+++ b/drivers/bus/vmbus/vmbus_bufring.c
@@ -58,11 +58,11 @@ void vmbus_br_setup(struct vmbus_br *br, void *buf, unsigned int blen)
 static inline bool
 vmbus_txbr_need_signal(const struct vmbus_bufring *vbr, uint32_t old_windex)
 {
-	rte_smp_mb();
+	rte_atomic_thread_fence(rte_memory_order_seq_cst);
 	if (vbr->imask)
 		return false;
 
-	rte_smp_rmb();
+	rte_atomic_thread_fence(rte_memory_order_acquire);
 
 	/*
 	 * This is the only case we need to signal when the
@@ -158,7 +158,7 @@ vmbus_txbr_write(struct vmbus_br *tbr, const struct iovec iov[], int iovlen,
 	RTE_ASSERT(windex == next_windex);
 
 	/* Ensure that data is available before updating host index */
-	rte_smp_wmb();
+	rte_atomic_thread_fence(rte_memory_order_release);
 
 	/* Checkin for our reservation. wait for our turn to update host */
 	while (!rte_atomic32_cmpset(&vbr->windex, old_windex, next_windex))
diff --git a/drivers/bus/vmbus/vmbus_channel.c b/drivers/bus/vmbus/vmbus_channel.c
index 2648d8e3fd..10293f59be 100644
--- a/drivers/bus/vmbus/vmbus_channel.c
+++ b/drivers/bus/vmbus/vmbus_channel.c
@@ -211,7 +211,7 @@ bool rte_vmbus_chan_rx_empty(const struct vmbus_channel *channel)
 {
 	const struct vmbus_br *br = &channel->rxbr;
 
-	rte_smp_rmb();
+	rte_atomic_thread_fence(rte_memory_order_acquire);
 	return br->vbr->rindex == br->vbr->windex;
 }
 
@@ -229,13 +229,13 @@ void rte_vmbus_chan_signal_read(struct rte_vmbus_device *dev,
 		return;
 
 	/* Make sure reading of pending happens after new read index */
-	rte_smp_mb();
+	rte_atomic_thread_fence(rte_memory_order_seq_cst);
 
 	pending_sz = rbr->vbr->pending_send;
 	if (!pending_sz)
 		return;
 
-	rte_smp_rmb();
+	rte_atomic_thread_fence(rte_memory_order_acquire);
 	write_sz = vmbus_br_availwrite(rbr, rbr->vbr->windex);
 
 	/* If there was space before then host was not blocked */
-- 
2.53.0