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Mon, 23 Mar 2026 16:37:59 -0700 (PDT) Date: Mon, 23 Mar 2026 19:37:55 -0400 From: "Michael S. Tsirkin" To: Vishwanath Seshagiri Cc: Omar Elghoul , andrew+netdev@lunn.ch, davem@davemloft.net, dw@davidwei.uk, edumazet@google.com, eperezma@redhat.com, ilias.apalodimas@linaro.org, jasowang@redhat.com, kernel-team@meta.com, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, technoboy85@gmail.com, virtualization@lists.linux.dev, xuanzhuo@linux.alibaba.com Subject: Re: [PATCH net-next v11] virtio_net: add page_pool support for buffer allocation Message-ID: <20260323193722-mutt-send-email-mst@kernel.org> References: <20260310183107.2822016-1-vishs@meta.com> <20260323150136.14452-1-oelghoul@linux.ibm.com> <20260323114313-mutt-send-email-mst@kernel.org> <20260323122728-mutt-send-email-mst@kernel.org> <6b6bea74-4bff-4301-8dbf-a9c162456185@linux.ibm.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Mar 23, 2026 at 01:50:29PM -0400, Vishwanath Seshagiri wrote: > > > On 3/23/26 10:39 PM, Omar Elghoul wrote: > > On 3/23/26 12:58 PM, Michael S. Tsirkin wrote: > > > > > On Mon, Mar 23, 2026 at 11:52:34AM -0400, Michael S. Tsirkin wrote: > > > > On Mon, Mar 23, 2026 at 11:01:31AM -0400, Omar Elghoul wrote: > > > > > [...] > > > > Well... I am not sure how I missed it. Obvious in hindsight: > > > > > > > > static void receive_buf(struct virtnet_info *vi, struct > > > > receive_queue *rq, > > > >                          void *buf, unsigned int len, void **ctx, > > > >                          unsigned int *xdp_xmit, > > > >                          struct virtnet_rq_stats *stats) > > > > { > > > >          struct net_device *dev = vi->dev; > > > >          struct sk_buff *skb; > > > >          u8 flags; > > > >          if (unlikely(len < vi->hdr_len + ETH_HLEN)) { > > > >                  pr_debug("%s: short packet %i\n", dev->name, len); > > > >                  DEV_STATS_INC(dev, rx_length_errors); > > > >                  virtnet_rq_free_buf(vi, rq, buf); > > > >                  return; > > > >          } > > > >          /* About the flags below: > > > >           * 1. Save the flags early, as the XDP program might > > > > overwrite them. > > > >           * These flags ensure packets marked as > > > > VIRTIO_NET_HDR_F_DATA_VALID > > > >           * stay valid after XDP processing. > > > >           * 2. XDP doesn't work with partially checksummed > > > > packets (refer to > > > >           * virtnet_xdp_set()), so packets marked as > > > >           * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP > > > > processing. > > > >           */ > > > >          if (vi->mergeable_rx_bufs) { > > > >                  flags = ((struct virtio_net_common_hdr *)buf)- > > > > >hdr.flags; > > > >                  skb = receive_mergeable(dev, vi, rq, buf, ctx, > > > > len, xdp_xmit, > > > >                                          stats); > > > >          } else if (vi->big_packets) { > > > >                  void *p = page_address((struct page *)buf); > > > >                  flags = ((struct virtio_net_common_hdr *)p)->hdr.flags; > > > >                  skb = receive_big(dev, vi, rq, buf, len, stats); > > > >          } else { > > > >                  flags = ((struct virtio_net_common_hdr *)buf)- > > > > >hdr.flags; > > > >                  skb = receive_small(dev, vi, rq, buf, ctx, len, > > > > xdp_xmit, stats); > > > >          } > > > > > > > > > > > > So we are reading the header, before dma sync, which is within > > > > receive_mergeable and friends: > > > > > > > > static struct sk_buff *receive_mergeable(struct net_device *dev, > > > >                                           struct virtnet_info *vi, > > > >                                           struct receive_queue *rq, > > > >                                           void *buf, > > > >                                           void *ctx, > > > >                                           unsigned int len, > > > >                                           unsigned int *xdp_xmit, > > > >                                           struct > > > > virtnet_rq_stats *stats) > > > > { > > > >          struct virtio_net_hdr_mrg_rxbuf *hdr = buf; > > > >          int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers); > > > >          struct page *page = virt_to_head_page(buf); > > > >          int offset = buf - page_address(page); > > > >          struct sk_buff *head_skb, *curr_skb; > > > >          unsigned int truesize = mergeable_ctx_to_truesize(ctx); > > > >          unsigned int headroom = mergeable_ctx_to_headroom(ctx); > > > >          head_skb = NULL; > > > >          if (rq->use_page_pool_dma) > > > >                  page_pool_dma_sync_for_cpu(rq->page_pool, page, > > > > offset, len); > > > > > > > > > > > > Just as a test, the below should fix it (compiled only), but the real > > > > fix is more complex since we need to be careful to avoid > > > > expensive syncing > > > > twice. > > > > > > > > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > > > index 97035b49bae7..57b4f5954bed 100644 > > > > --- a/drivers/net/virtio_net.c > > > > +++ b/drivers/net/virtio_net.c > > > > @@ -931,9 +931,19 @@ static struct sk_buff *page_to_skb(struct > > > > virtnet_info *vi, > > > >   static void *virtnet_rq_get_buf(struct receive_queue *rq, u32 > > > > *len, void **ctx) > > > >   { > > > > +    void *buf; > > > > + > > > >       BUG_ON(!rq->page_pool); > > > > -    return virtqueue_get_buf_ctx(rq->vq, len, ctx); > > > > +    buf = virtqueue_get_buf_ctx(rq->vq, len, ctx); > > > > +    if (buf && rq->use_page_pool_dma && *len) { > > > > +        struct page *page = virt_to_head_page(buf); > > > > +        int offset = buf - page_address(page); > > > > + > > > > +        page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, *len); > > > > +    } > > > > + > > > > +    return buf; > > > >   } > > > >   static void virtnet_rq_unmap_free_buf(struct virtqueue *vq, void *buf) > > > > > > > > > > > > > > > > > > > > -- > > > > MST > > > or maybe like this: > > Both of these patches resolve the issue on my end. > > > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c > > > index 97035b49bae7..835f52651006 100644 > > > --- a/drivers/net/virtio_net.c > > > +++ b/drivers/net/virtio_net.c > > > @@ -1956,13 +1956,6 @@ static struct sk_buff *receive_small(struct > > > net_device *dev, > > >        */ > > >       buf -= VIRTNET_RX_PAD + xdp_headroom; > > > -    if (rq->use_page_pool_dma) { > > > -        int offset = buf - page_address(page) + > > > -                 VIRTNET_RX_PAD + xdp_headroom; > > > - > > > -        page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, len); > > > -    } > > > - > > >       len -= vi->hdr_len; > > >       u64_stats_add(&stats->bytes, len); > > > @@ -2398,9 +2391,6 @@ static struct sk_buff > > > *receive_mergeable(struct net_device *dev, > > >       head_skb = NULL; > > > -    if (rq->use_page_pool_dma) > > > -        page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, len); > > > - > > >       u64_stats_add(&stats->bytes, len - vi->hdr_len); > > >       if (check_mergeable_len(dev, ctx, len)) > > > @@ -2563,6 +2553,13 @@ static void receive_buf(struct virtnet_info > > > *vi, struct receive_queue *rq, > > >           return; > > >       } > > > +    if (rq->use_page_pool_dma) { > > > +        struct page *page = virt_to_head_page(buf); > > > +        int offset = buf - page_address(page); > > > + > > > +        page_pool_dma_sync_for_cpu(rq->page_pool, page, offset, len); > > > +    } > > > + > > >       /* About the flags below: > > >        * 1. Save the flags early, as the XDP program might overwrite > > > them. > > >        * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID > > > > > Hello Omar. Thank you for the bisect, apologies for breaking the > functionality. Thank you Michael for the patch, I will be careful next > time about DMA sync issues. Looks like the bug is fixed, is there > anything else needed from my end? > Would be great to have you test the patch on your end and report the performace. Thanks! -- MST