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Mon, 23 Mar 2026 09:58:28 -0700 (PDT) Date: Mon, 23 Mar 2026 12:58:25 -0400 From: "Michael S. Tsirkin" To: Omar Elghoul Cc: vishs@meta.com, 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: <20260323122728-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> 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: <20260323114313-mutt-send-email-mst@kernel.org> 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: > > Hi, > > > > I've been testing linux-next (tags later than 03/17) and hit new issues in > > virtio-net on s390x. I bisected the issue, and I found this patch to be the > > first buggy commit. > > > > The issue seems to only be reproducible when running in Secure Execution. > > Tested in a KVM guest, the virtio-net performance appears greatly reduced, > > and the dmesg output shows many instances of the following error messages. > > > > Partial relevant logs > > ===================== > > [   49.332028] macvtap0: bad gso: type: 0, size: 0, flags 1 tunnel 0 tnl csum 0 > > [   74.365668] macvtap0: bad gso: type: 2e, size: 27948, flags 0 tunnel 0 tnl csum 0 > > [  403.302168] macvtap0: bad csum: flags: 2, gso_type: 23 rx_tnl_csum 0 > > [  403.302271] macvtap0: bad csum: flags: 2, gso_type: e0 rx_tnl_csum 0 > > [  403.302279] macvtap0: bad csum: flags: 2, gso_type: e1 rx_tnl_csum 0 > > [  403.309492] macvtap0: bad csum: flags: 2, gso_type: 4c rx_tnl_csum 0 > > [  403.317029] macvtap0: bad csum: flags: 2, gso_type: e0 rx_tnl_csum 0 > > > > Steps to reproduce > > ================== > > 1. Boot a Linux guest implementing this patch under QEMU/KVM (*) with SE > > enabled and a virtio-net-ccw device attached. > > 2. Run dmesg. The error message is usually already present at boot time, > > but if not, it can be reproduced by creating any network traffic. > > > > (*) This patch was not tested in a non-KVM hypervisor environment. > > > > I've further confirmed that reverting this patch onto its parent commit > > resolves the issue. Please let me know if you'd like me to test a fix or if > > you would need more information. > > > > Thanks in advance. > > > > Best, > > Omar > > 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: 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