Re: [PATCH 2/2] tcp_vu, udp_vu: Account for virtio net header in minimum frame size

On 2/23/26 10:57, Laurent Vivier wrote:

On 2/15/26 11:56, Stefano Brivio wrote:

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On Thu, 12 Feb 2026 12:39:32 +0100 Laurent Vivier wrote:

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In the vhost-user paths, the buffers provided by the virtio queue include the virtio net header (VNET_HLEN) prepended to the Ethernet frame. The minimum size checks using ETH_ZLEN must therefore account for this additional header length, otherwise we underestimate the minimum buffer size needed.

Use ETH_ZLEN + VNET_HLEN instead of bare ETH_ZLEN in vu_collect() calls and the corresponding ASSERT() checks.

Fixes: 0cb8f9003654 ("tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)") Signed-off-by: Laurent Vivier ---   tcp_vu.c | 8 ++++----   udp_vu.c | 2 +-   2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c index f7bda4943e43..2d593d534d68 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -90,12 +90,12 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)       vu_set_element(&flags_elem[0], NULL, &flags_iov[0]);       elem_cnt = vu_collect(vdev, vq, &flags_elem[0], 1, -                  MAX(hdrlen + sizeof(*opts), ETH_ZLEN), NULL); +                  MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN), NULL);       if (elem_cnt != 1)           return -1;       ASSERT(flags_elem[0].in_sg[0].iov_len >= -           MAX(hdrlen + sizeof(*opts), ETH_ZLEN)); +           MAX(hdrlen + sizeof(*opts), ETH_ZLEN + VNET_HLEN));       vu_set_vnethdr(vdev, flags_elem[0].in_sg[0].iov_base, 1); @@ -208,7 +208,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,           cnt = vu_collect(vdev, vq, &elem[elem_cnt],                    VIRTQUEUE_MAX_SIZE - elem_cnt, -                 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN), +                 MAX(MIN(mss, fillsize) + hdrlen, ETH_ZLEN + VNET_HLEN),                    &frame_size);           if (cnt == 0)               break; @@ -302,7 +302,7 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,       /* we guess the first iovec provided by the guest can embed        * all the headers needed by L2 frame, including any padding        */ -    ASSERT(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN)); +    ASSERT(iov[0].iov_len >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));

This triggers in the passt_vu_in_ns/tcp, "TCP/IPv4: host to guest: big transfer" test case, that is, the first time we connect to the guest (probably on the initial SYN segment).

I didn't check why.

After tcp_vu_sock_recv(), iov[0].iov_len is set to the size of hdrlen + received data, it's why in tcp_vu_prepare() iov_len can be shorter than expected. As the actual buffer size is already guaranteed by vu_collect(), I think the ASSERT() should be reverted to ASSERT(iov[0].iov_len >= hdrlen).

Moreover there is a problem with vu_pad() in tcp_vu.c that includes the VNET_HLEN (it should not), see udp_vu.c

In fact, the padding is wrong in udp_vu.c, not in tcp_vu.c Thanks, Laurent