[v4,1/2] virtio: one way barrier for packed vring desc avail flags
Checks
Commit Message
In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the frontend
and backend are assumed to be implemented in software, that is they can
run on identical CPUs in an SMP configuration.
Thus a weak form of memory barriers like rte_smp_r/wmb, other than
rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1)
and yields better performance.
For the above case, this patch helps yielding even better performance
by replacing the two-way barriers with C11 one-way barriers for avail
flags in packed ring.
Meanwhile, a read barrier is required to ensure ordering between
descriptor's flags and content reads[1]. With C11, load-acquire can
enforce the ordering instead of rmb barrier.
[1]https://patchwork.dpdk.org/patch/49109/
Signed-off-by: Joyce Kong <joyce.kong@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
drivers/net/virtio/virtio_rxtx.c | 13 +++++++------
drivers/net/virtio/virtio_user/virtio_user_dev.c | 6 +++++-
drivers/net/virtio/virtqueue.h | 21 +++++++++++++++++++++
lib/librte_vhost/vhost.h | 2 +-
lib/librte_vhost/virtio_net.c | 11 +++++------
5 files changed, 39 insertions(+), 14 deletions(-)
Comments
On 9/17/19 7:28 AM, Joyce Kong wrote:
> In case VIRTIO_F_ORDER_PLATFORM(36) is not negotiated, then the frontend
> and backend are assumed to be implemented in software, that is they can
> run on identical CPUs in an SMP configuration.
> Thus a weak form of memory barriers like rte_smp_r/wmb, other than
> rte_cio_r/wmb, is sufficient for this case(vq->hw->weak_barriers == 1)
> and yields better performance.
> For the above case, this patch helps yielding even better performance
> by replacing the two-way barriers with C11 one-way barriers for avail
> flags in packed ring.
>
> Meanwhile, a read barrier is required to ensure ordering between
> descriptor's flags and content reads[1]. With C11, load-acquire can
> enforce the ordering instead of rmb barrier.
>
> [1]https://patchwork.dpdk.org/patch/49109/
>
> Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> Reviewed-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> ---
> drivers/net/virtio/virtio_rxtx.c | 13 +++++++------
> drivers/net/virtio/virtio_user/virtio_user_dev.c | 6 +++++-
> drivers/net/virtio/virtqueue.h | 21 +++++++++++++++++++++
> lib/librte_vhost/vhost.h | 2 +-
> lib/librte_vhost/virtio_net.c | 11 +++++------
> 5 files changed, 39 insertions(+), 14 deletions(-)
>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Thanks,
Maxime
@@ -456,8 +456,10 @@ virtqueue_enqueue_recv_refill_packed(struct virtqueue *vq,
vq->vq_desc_head_idx = dxp->next;
if (vq->vq_desc_head_idx == VQ_RING_DESC_CHAIN_END)
vq->vq_desc_tail_idx = vq->vq_desc_head_idx;
- virtio_wmb(hw->weak_barriers);
- start_dp[idx].flags = flags;
+
+ virtqueue_store_flags_packed(&start_dp[idx], flags,
+ hw->weak_barriers);
+
if (++vq->vq_avail_idx >= vq->vq_nentries) {
vq->vq_avail_idx -= vq->vq_nentries;
vq->vq_packed.cached_flags ^=
@@ -671,8 +673,7 @@ virtqueue_enqueue_xmit_packed_fast(struct virtnet_tx *txvq,
vq->vq_desc_tail_idx = VQ_RING_DESC_CHAIN_END;
}
- virtio_wmb(vq->hw->weak_barriers);
- dp->flags = flags;
+ virtqueue_store_flags_packed(dp, flags, vq->hw->weak_barriers);
}
static inline void
@@ -763,8 +764,8 @@ virtqueue_enqueue_xmit_packed(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
vq->vq_desc_tail_idx = VQ_RING_DESC_CHAIN_END;
}
- virtio_wmb(vq->hw->weak_barriers);
- head_dp->flags = head_flags;
+ virtqueue_store_flags_packed(head_dp, head_flags,
+ vq->hw->weak_barriers);
}
static inline void
@@ -624,7 +624,7 @@ virtio_user_handle_ctrl_msg(struct virtio_user_dev *dev, struct vring *vring,
static inline int
desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter)
{
- uint16_t flags = desc->flags;
+ uint16_t flags = __atomic_load_n(&desc->flags, __ATOMIC_ACQUIRE);
return wrap_counter == !!(flags & VRING_PACKED_DESC_F_AVAIL) &&
wrap_counter != !!(flags & VRING_PACKED_DESC_F_USED);
@@ -684,6 +684,10 @@ virtio_user_handle_cq_packed(struct virtio_user_dev *dev, uint16_t queue_idx)
struct vring_packed *vring = &dev->packed_vrings[queue_idx];
uint16_t n_descs, flags;
+ /* Perform a load-acquire barrier in desc_is_avail to
+ * enforce the ordering between desc flags and desc
+ * content.
+ */
while (desc_is_avail(&vring->desc[vq->used_idx],
vq->used_wrap_counter)) {
@@ -54,6 +54,27 @@ virtio_wmb(uint8_t weak_barriers)
rte_cio_wmb();
}
+static inline void
+virtqueue_store_flags_packed(struct vring_packed_desc *dp,
+ uint16_t flags, uint8_t weak_barriers)
+{
+ if (weak_barriers) {
+/* x86 prefers to using rte_smp_wmb over __atomic_store_n as it reports
+ * a better perf(~1.5%), which comes from the saved branch by the compiler.
+ * The if and else branch are identical with the smp and cio barriers both
+ * defined as compiler barriers on x86.
+ */
+#ifdef RTE_ARCH_X86_64
+ rte_smp_wmb();
+ dp->flags = flags;
+#else
+ __atomic_store_n(&dp->flags, flags, __ATOMIC_RELEASE);
+#endif
+ } else {
+ rte_cio_wmb();
+ dp->flags = flags;
+ }
+}
#ifdef RTE_PMD_PACKET_PREFETCH
#define rte_packet_prefetch(p) rte_prefetch1(p)
#else
@@ -344,7 +344,7 @@ vq_is_packed(struct virtio_net *dev)
static inline bool
desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter)
{
- uint16_t flags = *((volatile uint16_t *) &desc->flags);
+ uint16_t flags = __atomic_load_n(&desc->flags, __ATOMIC_ACQUIRE);
return wrap_counter == !!(flags & VRING_DESC_F_AVAIL) &&
wrap_counter != !!(flags & VRING_DESC_F_USED);
@@ -503,14 +503,13 @@ fill_vec_buf_packed(struct virtio_net *dev, struct vhost_virtqueue *vq,
if (avail_idx < vq->last_avail_idx)
wrap_counter ^= 1;
- if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter)))
- return -1;
-
/*
- * The ordering between desc flags and desc
- * content reads need to be enforced.
+ * Perform a load-acquire barrier in desc_is_avail to
+ * enforce the ordering between desc flags and desc
+ * content.
*/
- rte_smp_rmb();
+ if (unlikely(!desc_is_avail(&descs[avail_idx], wrap_counter)))
+ return -1;
*desc_count = 0;
*len = 0;