Re: [PATCHv7] add mergeable buffers support to vhost_net
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2010-05-10 17:35:51
Also in:
kvm, virtualization
On Wed, Apr 28, 2010 at 01:57:12PM -0700, David L Stevens wrote:
quoted hunk ↗ jump to hunk
@@ -218,18 +248,19 @@ static void handle_rx(struct vhost_net * use_mm(net->dev.mm); mutex_lock(&vq->mutex); vhost_disable_notify(vq); - hdr_size = vq->hdr_size; + vhost_hlen = vq->vhost_hlen; vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ? vq->log : NULL; - for (;;) { - head = vhost_get_vq_desc(&net->dev, vq, vq->iov, - ARRAY_SIZE(vq->iov), - &out, &in, - vq_log, &log); + while ((datalen = vhost_head_len(vq, sock->sk))) { + headcount = vhost_get_desc_n(vq, vq->heads, + datalen + vhost_hlen, + &in, vq_log, &log); + if (headcount < 0) + break; /* OK, now we need to know about added descriptors. */ - if (head == vq->num) { + if (!headcount) { if (unlikely(vhost_enable_notify(vq))) { /* They have slipped one in as we were * doing that: check again. */
So I think this breaks handling for a failure mode where we get an skb that is larger than the max packet guest can get. The right thing to do in this case is to drop the skb, we currently do this by passing truncate flag to recvmsg. In particular, with mergeable buffers off, if we get an skb that does not fit in a single packet, this code will spread it over multiple buffers. You should be able to reproduce this fairly easily by disabling both indirect buffers and mergeable buffers on qemu command line. With current code TCP still works by falling back on small packets. I think with your code it will get stuck forever once we get an skb that is too large for us to handle. -- MST