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
