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

[Long Li]

> 
> 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>

Reviewed-by: Long Li <longli at microsoft.com>



> ---
>  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