[dpdk-stable] [RFC] net/virtio: use real barriers for vDPA

[Michael S. Tsirkin]

On Fri, Dec 14, 2018 at 03:37:04PM +0300, Ilya Maximets wrote:
> SMP barriers are OK for software vhost implementation, but vDPA is a
> DMA capable hardware and we have to use at least DMA barriers to
> ensure the memory ordering.
> 
> This patch mimics the kernel virtio-net driver in part of choosing
> barriers we need.
> 
> Fixes: a3f8150eac6d ("net/ifcvf: add ifcvf vDPA driver")
> Cc: stable at dpdk.org
> 
> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>

Instead of this vendor specific hack, can you please
use VIRTIO_F_ORDER_PLATFORM?


> ---
> 
> Sending as RFC, because the patch is completely not tested. I heve no
> HW to test the real behaviour. And I actually do not know if the
> subsystem_vendor/device_id's are available at the time and has
> IFCVF_SUBSYS_* values inside the real guest system.
> 
> One more thing I want to mention that cross net client Live Migration
> is actually not possible without any support from the guest driver.
> Because migration from the software vhost to vDPA will lead to using
> weaker barriers than required.
> 
> The similar change (weak_barriers = false) should be made in the linux
> kernel virtio-net driver.

IMHO no, it should use VIRTIO_F_ORDER_PLATFORM.

> Another dirty solution could be to restrict the vDPA support to x86
> systems only.
> 
>  drivers/net/virtio/virtio_ethdev.c      |  9 +++++++
>  drivers/net/virtio/virtio_pci.h         |  1 +
>  drivers/net/virtio/virtio_rxtx.c        | 14 +++++-----
>  drivers/net/virtio/virtio_user_ethdev.c |  1 +
>  drivers/net/virtio/virtqueue.h          | 35 +++++++++++++++++++++----
>  5 files changed, 48 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
> index cb2b2e0bf..249b536fd 100644
> --- a/drivers/net/virtio/virtio_ethdev.c
> +++ b/drivers/net/virtio/virtio_ethdev.c
> @@ -1448,6 +1448,9 @@ virtio_configure_intr(struct rte_eth_dev *dev)
>  	return 0;
>  }
>  
> +#define IFCVF_SUBSYS_VENDOR_ID	0x8086
> +#define IFCVF_SUBSYS_DEVICE_ID	0x001A
> +
>  /* reset device and renegotiate features if needed */
>  static int
>  virtio_init_device(struct rte_eth_dev *eth_dev, uint64_t req_features)
> @@ -1477,6 +1480,12 @@ virtio_init_device(struct rte_eth_dev *eth_dev, uint64_t req_features)
>  	if (!hw->virtio_user_dev) {
>  		pci_dev = RTE_ETH_DEV_TO_PCI(eth_dev);
>  		rte_eth_copy_pci_info(eth_dev, pci_dev);
> +
> +		if (pci_dev->id.subsystem_vendor_id == IFCVF_SUBSYS_VENDOR_ID &&
> +		    pci_dev->id.subsystem_device_id == IFCVF_SUBSYS_DEVICE_ID)
> +			hw->weak_barriers = 0;
> +		else
> +			hw->weak_barriers = 1;
>  	}
>  
>  	/* If host does not support both status and MSI-X then disable LSC */
> diff --git a/drivers/net/virtio/virtio_pci.h b/drivers/net/virtio/virtio_pci.h
> index e961a58ca..1f8e719a9 100644
> --- a/drivers/net/virtio/virtio_pci.h
> +++ b/drivers/net/virtio/virtio_pci.h
> @@ -240,6 +240,7 @@ struct virtio_hw {
>  	uint8_t     use_simple_rx;
>  	uint8_t     use_inorder_rx;
>  	uint8_t     use_inorder_tx;
> +	uint8_t     weak_barriers;
>  	bool        has_tx_offload;
>  	bool        has_rx_offload;
>  	uint16_t    port_id;
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index cb8f89f18..66195bf47 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -906,7 +906,7 @@ virtio_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts)
>  
>  	nb_used = VIRTQUEUE_NUSED(vq);
>  
> -	virtio_rmb();
> +	virtio_rmb(hw->weak_barriers);
>  
>  	num = likely(nb_used <= nb_pkts) ? nb_used : nb_pkts;
>  	if (unlikely(num > VIRTIO_MBUF_BURST_SZ))
> @@ -1017,7 +1017,7 @@ virtio_recv_mergeable_pkts_inorder(void *rx_queue,
>  	nb_used = RTE_MIN(nb_used, nb_pkts);
>  	nb_used = RTE_MIN(nb_used, VIRTIO_MBUF_BURST_SZ);
>  
> -	virtio_rmb();
> +	virtio_rmb(hw->weak_barriers);
>  
>  	PMD_RX_LOG(DEBUG, "used:%d", nb_used);
>  
> @@ -1202,7 +1202,7 @@ virtio_recv_mergeable_pkts(void *rx_queue,
>  
>  	nb_used = VIRTQUEUE_NUSED(vq);
>  
> -	virtio_rmb();
> +	virtio_rmb(hw->weak_barriers);
>  
>  	PMD_RX_LOG(DEBUG, "used:%d", nb_used);
>  
> @@ -1365,7 +1365,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  	PMD_TX_LOG(DEBUG, "%d packets to xmit", nb_pkts);
>  	nb_used = VIRTQUEUE_NUSED(vq);
>  
> -	virtio_rmb();
> +	virtio_rmb(hw->weak_barriers);
>  	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
>  		virtio_xmit_cleanup(vq, nb_used);
>  
> @@ -1407,7 +1407,7 @@ virtio_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts, uint16_t nb_pkts)
>  		/* Positive value indicates it need free vring descriptors */
>  		if (unlikely(need > 0)) {
>  			nb_used = VIRTQUEUE_NUSED(vq);
> -			virtio_rmb();
> +			virtio_rmb(hw->weak_barriers);
>  			need = RTE_MIN(need, (int)nb_used);
>  
>  			virtio_xmit_cleanup(vq, need);
> @@ -1463,7 +1463,7 @@ virtio_xmit_pkts_inorder(void *tx_queue,
>  	PMD_TX_LOG(DEBUG, "%d packets to xmit", nb_pkts);
>  	nb_used = VIRTQUEUE_NUSED(vq);
>  
> -	virtio_rmb();
> +	virtio_rmb(hw->weak_barriers);
>  	if (likely(nb_used > vq->vq_nentries - vq->vq_free_thresh))
>  		virtio_xmit_cleanup_inorder(vq, nb_used);
>  
> @@ -1511,7 +1511,7 @@ virtio_xmit_pkts_inorder(void *tx_queue,
>  		need = slots - vq->vq_free_cnt;
>  		if (unlikely(need > 0)) {
>  			nb_used = VIRTQUEUE_NUSED(vq);
> -			virtio_rmb();
> +			virtio_rmb(hw->weak_barriers);
>  			need = RTE_MIN(need, (int)nb_used);
>  
>  			virtio_xmit_cleanup_inorder(vq, need);
> diff --git a/drivers/net/virtio/virtio_user_ethdev.c b/drivers/net/virtio/virtio_user_ethdev.c
> index f8791391a..f075774b4 100644
> --- a/drivers/net/virtio/virtio_user_ethdev.c
> +++ b/drivers/net/virtio/virtio_user_ethdev.c
> @@ -434,6 +434,7 @@ virtio_user_eth_dev_alloc(struct rte_vdev_device *vdev)
>  	hw->use_simple_rx = 0;
>  	hw->use_inorder_rx = 0;
>  	hw->use_inorder_tx = 0;
> +	hw->weak_barriers = 1;
>  	hw->virtio_user_dev = dev;
>  	return eth_dev;
>  }
> diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
> index 26518ed98..7bf17d3bf 100644
> --- a/drivers/net/virtio/virtqueue.h
> +++ b/drivers/net/virtio/virtqueue.h
> @@ -19,15 +19,40 @@
>  struct rte_mbuf;
>  
>  /*
> - * Per virtio_config.h in Linux.
> + * Per virtio_ring.h in Linux.
>   *     For virtio_pci on SMP, we don't need to order with respect to MMIO
>   *     accesses through relaxed memory I/O windows, so smp_mb() et al are
>   *     sufficient.
>   *
> + *     For using virtio to talk to real devices (eg. vDPA) we do need real
> + *     barriers.
>   */
> -#define virtio_mb()	rte_smp_mb()
> -#define virtio_rmb()	rte_smp_rmb()
> -#define virtio_wmb()	rte_smp_wmb()
> +static inline void
> +virtio_mb(uint8_t weak_barriers)
> +{
> +	if (weak_barriers)
> +		rte_smp_mb();
> +	else
> +		rte_mb();
> +}
> +
> +static inline void
> +virtio_rmb(uint8_t weak_barriers)
> +{
> +	if (weak_barriers)
> +		rte_smp_rmb();
> +	else
> +		rte_cio_rmb();
> +}
> +
> +static inline void
> +virtio_wmb(uint8_t weak_barriers)
> +{
> +	if (weak_barriers)
> +		rte_smp_wmb();
> +	else
> +		rte_cio_wmb();
> +}
>  
>  #ifdef RTE_PMD_PACKET_PREFETCH
>  #define rte_packet_prefetch(p)  rte_prefetch1(p)


Note kernel uses dma_ barriers not full barriers.
I don't know whether it applies to dpdk.

> @@ -312,7 +337,7 @@ void vq_ring_free_inorder(struct virtqueue *vq, uint16_t desc_idx,
>  static inline void
>  vq_update_avail_idx(struct virtqueue *vq)
>  {
> -	virtio_wmb();
> +	virtio_wmb(vq->hw->weak_barriers);
>  	vq->vq_ring.avail->idx = vq->vq_avail_idx;
>  }
>  
> -- 
> 2.17.1