> -----Original Message-----
> From: Stephen Hemminger <[email protected]>
> Sent: Monday, September 21, 2026 2:10 AM
> To: [email protected]
> Cc: Stephen Hemminger <[email protected]>; Long Li
> <[email protected]>; Wei Hu <[email protected]>
> Subject: [EXTERNAL] [PATCH v2 05/33] bus/vmbus: replace SMP barriers with
> C11 memory fences
>
> 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 <[email protected]>
> Reviewed-by: Long Li <[email protected]>
Reviewed-by: Wei Hu <[email protected]>
> ---
> 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
