> -----Original Message----- > From: Paolo Abeni [mailto:pabeni@xxxxxxxxxx] > Sent: Thursday, February 29, 2024 7:13 PM > To: wangyunjian <wangyunjian@xxxxxxxxxx>; mst@xxxxxxxxxx; > willemdebruijn.kernel@xxxxxxxxx; jasowang@xxxxxxxxxx; kuba@xxxxxxxxxx; > bjorn@xxxxxxxxxx; magnus.karlsson@xxxxxxxxx; maciej.fijalkowski@xxxxxxxxx; > jonathan.lemon@xxxxxxxxx; davem@xxxxxxxxxxxxx > Cc: bpf@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; > virtualization@xxxxxxxxxxxxxxx; xudingke <xudingke@xxxxxxxxxx>; liwei (DT) > <liwei395@xxxxxxxxxx> > Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support > > On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote: > > @@ -2661,6 +2776,54 @@ static int tun_ptr_peek_len(void *ptr) > > } > > } > > > > +static void tun_peek_xsk(struct tun_file *tfile) { > > + struct xsk_buff_pool *pool; > > + u32 i, batch, budget; > > + void *frame; > > + > > + if (!ptr_ring_empty(&tfile->tx_ring)) > > + return; > > + > > + spin_lock(&tfile->pool_lock); > > + pool = tfile->xsk_pool; > > + if (!pool) { > > + spin_unlock(&tfile->pool_lock); > > + return; > > + } > > + > > + if (tfile->nb_descs) { > > + xsk_tx_completed(pool, tfile->nb_descs); > > + if (xsk_uses_need_wakeup(pool)) > > + xsk_set_tx_need_wakeup(pool); > > + } > > + > > + spin_lock(&tfile->tx_ring.producer_lock); > > + budget = min_t(u32, tfile->tx_ring.size, TUN_XDP_BATCH); > > + > > + batch = xsk_tx_peek_release_desc_batch(pool, budget); > > + if (!batch) { > > This branch looks like an unneeded "optimization". The generic loop below > should have the same effect with no measurable perf delta - and smaller code. > Just remove this. > > > + tfile->nb_descs = 0; > > + spin_unlock(&tfile->tx_ring.producer_lock); > > + spin_unlock(&tfile->pool_lock); > > + return; > > + } > > + > > + tfile->nb_descs = batch; > > + for (i = 0; i < batch; i++) { > > + /* Encode the XDP DESC flag into lowest bit for consumer to differ > > + * XDP desc from XDP buffer and sk_buff. > > + */ > > + frame = tun_xdp_desc_to_ptr(&pool->tx_descs[i]); > > + /* The budget must be less than or equal to tx_ring.size, > > + * so enqueuing will not fail. > > + */ > > + __ptr_ring_produce(&tfile->tx_ring, frame); > > + } > > + spin_unlock(&tfile->tx_ring.producer_lock); > > + spin_unlock(&tfile->pool_lock); > > More related to the general design: it looks wrong. What if > get_rx_bufs() will fail (ENOBUF) after successful peeking? With no more > incoming packets, later peek will return 0 and it looks like that the > half-processed packets will stay in the ring forever??? > > I think the 'ring produce' part should be moved into tun_do_read(). Currently, the vhost-net obtains a batch descriptors/sk_buffs from the ptr_ring and enqueue the batch descriptors/sk_buffs to the virtqueue'queue, and then consumes the descriptors/sk_buffs from the virtqueue'queue in sequence. As a result, TUN does not know whether the batch descriptors have been used up, and thus does not know when to return the batch descriptors. So, I think it's reasonable that when vhost-net checks ptr_ring is empty, it calls peek_len to get new xsk's descs and return the descriptors. Thanks > > Cheers, > > Paolo
