Re: [PATCH v14 7/9] vhost-user: add vhost-user

[Laurent Vivier <lvivier@redhat.com>]

On 26/11/2024 06:24, David Gibson wrote:
>> static int udp_vu_sock_recv(const struct ctx *c, int s, uint32_t events,
>> +			    bool v6, ssize_t *dlen)
>> +{
>> +	struct vu_dev *vdev = c->vdev;
>> +	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
>> +	int iov_cnt, idx, iov_used;
>> +	struct msghdr msg  = { 0 };
>> +	size_t off, hdrlen;
>> +
>> +	ASSERT(!c->no_udp);
>> +
>> +	if (!(events & EPOLLIN))
>> +		return 0;
>> +
>> +	/* compute L2 header length */
>> +	hdrlen = udp_vu_hdrlen(v6);
>> +
>> +	vu_init_elem(elem, iov_vu, VIRTQUEUE_MAX_SIZE);
>> +
>> +	iov_cnt = vu_collect(vdev, vq, elem, VIRTQUEUE_MAX_SIZE,
>> +			     IP_MAX_MTU - sizeof(struct udphdr) + hdrlen,
> I don't think this calculation is quite right, though it's probably
> safe.  At least for IPv4, IP_MAX_MTU includes the IP header itself,
> but then you count that again in hdrlen.

I think it would be semantically more correct to use "ETH_MAX_MTU +
sizeof(struct virtio_net_hdr_mrg_rxbuf)", but as ETH_MAX_MTU and IP_MAX_MTU are both 
defined to USHRT_MAX I'm not sure how to compute the segment size...

Thanks,
Laurent