On Thu, Aug 24, 2023 at 04:18:06PM +0200, David Marchand wrote: > On Thu, Aug 24, 2023 at 2:47 PM Morten Brørup <m...@smartsharesystems.com> > wrote: > > > From: David Marchand [mailto:david.march...@redhat.com] > > > However, I am a bit puzzled why the prefetch change makes the compiler > > > consider this loop differently. > > > We have the same constructs everywhere in this library and x86_64 > > > builds are fine... > > > > That is indeed the relevant question here! > > > > Perhaps the compiler somehow ignores the "const" part of the parameter > > given to the "asm" (in rte_prefetch0()) for 64 bit arch, but not for 32 bit > > arch? > > It is possible to reproduce the issue with current DPDK tree (with > existing prefetch implementation in asm) and removing all > rte_prefetch0 calls from the async rx path: > > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c > index d7624d18c8..6f941cf27d 100644 > --- a/lib/vhost/virtio_net.c > +++ b/lib/vhost/virtio_net.c > @@ -748,8 +748,6 @@ map_one_desc(struct virtio_net *dev, struct > vhost_virtqueue *vq, > if (unlikely(!desc_addr)) > return -1; > > - rte_prefetch0((void *)(uintptr_t)desc_addr); > - > buf_vec[vec_id].buf_iova = desc_iova; > buf_vec[vec_id].buf_addr = desc_addr; > buf_vec[vec_id].buf_len = desc_chunck_len; > @@ -1808,8 +1806,6 @@ virtio_dev_rx_async_submit_split(struct > virtio_net *dev, struct vhost_virtqueue > */ > avail_head = __atomic_load_n(&vq->avail->idx, __ATOMIC_ACQUIRE); > > - rte_prefetch0(&vq->avail->ring[vq->last_avail_idx & (vq->size - 1)]); > - > async_iter_reset(async); > > for (pkt_idx = 0; pkt_idx < count; pkt_idx++) { > @@ -1997,7 +1993,6 @@ virtio_dev_rx_async_packed_batch_enqueue(struct > virtio_net *dev, > uint16_t i; > > vhost_for_each_try_unroll(i, 0, PACKED_BATCH_SIZE) { > - rte_prefetch0((void *)(uintptr_t)desc_addrs[i]); > desc = vhost_iova_to_vva(dev, vq, desc_addrs[i], > &lens[i], VHOST_ACCESS_RW); > hdrs[i] = (struct virtio_net_hdr_mrg_rxbuf *)(uintptr_t)desc; > lens[i] = pkts[i]->pkt_len + > @@ -2106,8 +2101,6 @@ virtio_dev_rx_async_submit_packed(struct > virtio_net *dev, struct vhost_virtqueue > uint16_t i; > > do { > - rte_prefetch0(&vq->desc_packed[vq->last_avail_idx]); > - > if (count - pkt_idx >= PACKED_BATCH_SIZE) { > if (!virtio_dev_rx_async_packed_batch(dev, vq, > &pkts[pkt_idx], > dma_id, vchan_id)) { > > > If any prefetch is left, the warning disappears. > So the asm usage probably impacts/disables some compiler feature, for > this code path.
So as an aside one of the reasons often cited as to why intrinsics are preferred over inline asm is that inline asm can't be considered when the compiler performs optimization. It could be that when moving to use the intrinsics the scope of what can be optimized around the prefetch0 calls can be used for optimization leading to the observations here. Ty > > > -- > David Marchand
