> From: David Marchand [mailto:david.march...@redhat.com] > Sent: Thursday, 24 August 2023 16.18 > > 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.
Perhaps any asm usage causes the compiler to assume that any memory might have been modified, which then causes the compiler to not warn about the use of potentially uninitialized variables. If so, then compiling other code using asm might also not omit warnings about potentially uninitialized variables where it otherwise should have.
