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Date: Sun, 7 Mar 2010 17:31:30 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Stevens <dlstevens@...ibm.com>
Cc: rusty@...tcorp.com.au, netdev@...r.kernel.org, kvm@...r.kernel.org,
virtualization@...ts.osdl.org
Subject: Re: [RFC][ PATCH 1/3] vhost-net: support multiple buffer heads in
receiver
On Tue, Mar 02, 2010 at 05:20:15PM -0700, David Stevens wrote:
> This patch generalizes buffer handling functions to
> support multiple buffer heads.
>
> In-line for viewing, attached for applying.
> Signed-off-by: David L Stevens <dlstevens@...ibm.com>
> diff -ruN net-next-p0/drivers/vhost/net.c net-next-p1/drivers/vhost/net.c
> --- net-next-p0/drivers/vhost/net.c 2010-02-24 12:59:24.000000000
> -0800
> +++ net-next-p1/drivers/vhost/net.c 2010-03-01 11:44:22.000000000
> -0800
> @@ -97,7 +97,8 @@
> static void handle_tx(struct vhost_net *net)
> {
> struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> - unsigned head, out, in, s;
> + unsigned out, in, s;
> + struct iovec head;
> struct msghdr msg = {
> .msg_name = NULL,
> .msg_namelen = 0,
> @@ -126,12 +127,10 @@
> hdr_size = vq->hdr_size;
>
> for (;;) {
> - head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> - ARRAY_SIZE(vq->iov),
> - &out, &in,
> - NULL, NULL);
> + head.iov_base = (void *)vhost_get_vq_desc(&net->dev, vq,
> + vq->iov, ARRAY_SIZE(vq->iov), &out, &in, NULL,
> NULL);
Should type for head be changed so that we do not need to cast?
I also prefer aligning descendants on the opening (.
> /* Nothing new? Wait for eventfd to tell us they
> refilled. */
> - if (head == vq->num) {
> + if (head.iov_base == (void *)vq->num) {
> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
> tx_poll_start(net, sock);
> @@ -152,7 +151,7 @@
> /* Skip header. TODO: support TSO. */
> s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, out);
> msg.msg_iovlen = out;
> - len = iov_length(vq->iov, out);
> + head.iov_len = len = iov_length(vq->iov, out);
> /* Sanity check */
> if (!len) {
> vq_err(vq, "Unexpected header len for TX: "
> @@ -163,14 +162,14 @@
> /* TODO: Check specific error and bomb out unless ENOBUFS?
> */
> err = sock->ops->sendmsg(NULL, sock, &msg, len);
> if (unlikely(err < 0)) {
> - vhost_discard_vq_desc(vq);
> + vhost_discard(vq, 1);
Isn't the original name a bit more descriptive?
> tx_poll_start(net, sock);
> break;
> }
> if (err != len)
> pr_err("Truncated TX packet: "
> " len %d != %zd\n", err, len);
> - vhost_add_used_and_signal(&net->dev, vq, head, 0);
> + vhost_add_used_and_signal(&net->dev, vq, &head, 1);
> total_len += len;
> if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
> vhost_poll_queue(&vq->poll);
> @@ -182,12 +181,22 @@
> unuse_mm(net->dev.mm);
> }
>
> +static int skb_head_len(struct sk_buff_head *skq)
> +{
> + struct sk_buff *head;
> +
> + head = skb_peek(skq);
> + if (head)
> + return head->len;
> + return 0;
> +}
> +
This is done without locking, which I think can crash
if skb is consumed after we peek it but before we read the
length.
> /* Expects to be always run from workqueue - which acts as
> * read-size critical section for our kind of RCU. */
> static void handle_rx(struct vhost_net *net)
> {
> struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_RX];
> - unsigned head, out, in, log, s;
> + unsigned in, log, s;
> struct vhost_log *vq_log;
> struct msghdr msg = {
> .msg_name = NULL,
> @@ -204,10 +213,11 @@
> };
>
> size_t len, total_len = 0;
> - int err;
> + int err, headcount, datalen;
> size_t hdr_size;
> struct socket *sock = rcu_dereference(vq->private_data);
> - if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
> +
> + if (!sock || !skb_head_len(&sock->sk->sk_receive_queue))
> return;
>
Isn't this equivalent? Do you expect zero len skbs in socket?
If yes, maybe we should discard these, not stop processing ...
> use_mm(net->dev.mm);
> @@ -218,13 +228,10 @@
> 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 = skb_head_len(&sock->sk->sk_receive_queue))) {
This peeks in the queue to figure out how much data we have.
It's a neat trick, but it does introduce new failure modes
where an skb is consumed after we did the peek:
- new skb could be shorter, we should return the unconsumed part
- new skb could be longer, this will drop a packet.
maybe this last is not an issue as the race should be rare in practice
> + headcount = vhost_get_heads(vq, datalen, &in, vq_log,
> &log);
> /* 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. */
> @@ -235,13 +242,6 @@
> * they refilled. */
> break;
> }
> - /* We don't need to be notified again. */
you find this comment unhelpful?
> - if (out) {
> - vq_err(vq, "Unexpected descriptor format for RX: "
> - "out %d, int %d\n",
> - out, in);
> - break;
> - }
we still need this check, don't we?
> /* Skip header. TODO: support TSO/mergeable rx buffers. */
> s = move_iovec_hdr(vq->iov, vq->hdr, hdr_size, in);
> msg.msg_iovlen = in;
> @@ -257,14 +257,14 @@
> len, MSG_DONTWAIT | MSG_TRUNC);
> /* TODO: Check specific error and bomb out unless EAGAIN?
> */
> if (err < 0) {
> - vhost_discard_vq_desc(vq);
> + vhost_discard(vq, 1);
> break;
> }
> /* TODO: Should check and handle checksum. */
> if (err > len) {
> pr_err("Discarded truncated rx packet: "
> " len %d > %zd\n", err, len);
> - vhost_discard_vq_desc(vq);
> + vhost_discard(vq, 1);
> continue;
> }
> len = err;
> @@ -275,7 +275,7 @@
> break;
> }
> len += hdr_size;
> - vhost_add_used_and_signal(&net->dev, vq, head, len);
> + vhost_add_used_and_signal(&net->dev, vq, vq->heads,
> headcount);
> if (unlikely(vq_log))
> vhost_log_write(vq, vq_log, log, len);
> total_len += len;
> diff -ruN net-next-p0/drivers/vhost/vhost.c
> net-next-p1/drivers/vhost/vhost.c
> --- net-next-p0/drivers/vhost/vhost.c 2010-02-15 20:08:35.000000000
> -0800
> +++ net-next-p1/drivers/vhost/vhost.c 2010-03-01 11:44:06.000000000
> -0800
> @@ -848,6 +848,38 @@
> return 0;
> }
>
> +unsigned vhost_get_heads(struct vhost_virtqueue *vq, int datalen, int
> *iovcount,
> + struct vhost_log *log, unsigned int *log_num)
Could you please document this function's parameters? It's not obvious
what iovcount, log, log_num are.
> +{
> + struct iovec *heads = vq->heads;
> + int out, in;
> + int hc = 0;
> +
> + while (datalen > 0) {
> + if (hc >= VHOST_NET_MAX_SG) {
> + vhost_discard(vq, hc);
> + return 0;
> + }
> + heads[hc].iov_base = (void *)vhost_get_vq_desc(vq->dev,
> vq,
> + vq->iov, ARRAY_SIZE(vq->iov), &out, &in, log,
> log_num);
> + if (heads[hc].iov_base == (void *)vq->num) {
> + vhost_discard(vq, hc);
> + return 0;
> + }
> + if (out || in <= 0) {
> + vq_err(vq, "unexpected descriptor format for RX: "
> + "out %d, in %d\n", out, in);
> + vhost_discard(vq, hc);
> + return 0;
> + }
> + heads[hc].iov_len = iov_length(vq->iov, in);
> + hc++;
> + datalen -= heads[hc].iov_len;
> + }
> + *iovcount = in;
Only the last value?
> + return hc;
> +}
> +
> /* This looks in the virtqueue and for the first available buffer, and
> converts
> * it to an iovec for convenient access. Since descriptors consist of
> some
> * number of output then some number of input descriptors, it's actually
> two
> @@ -973,31 +1005,32 @@
> }
>
> /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> -void vhost_discard_vq_desc(struct vhost_virtqueue *vq)
> +void vhost_discard(struct vhost_virtqueue *vq, int n)
> {
> - vq->last_avail_idx--;
> + vq->last_avail_idx -= n;
> }
>
> /* After we've used one of their buffers, we tell them about it. We'll
> then
> * want to notify the guest, using eventfd. */
> -int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int
> len)
> +int vhost_add_used(struct vhost_virtqueue *vq, struct iovec *heads, int
> count)
> {
> struct vring_used_elem *used;
> + int i;
>
> - /* The virtqueue contains a ring of used buffers. Get a pointer
> to the
> - * next entry in that used ring. */
> - used = &vq->used->ring[vq->last_used_idx % vq->num];
> - if (put_user(head, &used->id)) {
> - vq_err(vq, "Failed to write used id");
> - return -EFAULT;
> - }
> - if (put_user(len, &used->len)) {
> - vq_err(vq, "Failed to write used len");
> - return -EFAULT;
> + for (i=0; i<count; i++, vq->last_used_idx++) {
whitespace damage: I prefer space around =, <.
I also use ++i, etc in this driver, better be consistent?
Also for clarity, I prefer to put vq->last_used_idx inside the loop.
> + used = &vq->used->ring[vq->last_used_idx % vq->num];
> + if (put_user((unsigned)heads[i].iov_base, &used->id)) {
> + vq_err(vq, "Failed to write used id");
> + return -EFAULT;
> + }
> + if (put_user(heads[i].iov_len, &used->len)) {
> + vq_err(vq, "Failed to write used len");
> + return -EFAULT;
> + }
If this fails, last_used_idx will still be incremented, which I think is wrong.
> }
> /* Make sure buffer is written before we update index. */
> smp_wmb();
> - if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
> + if (put_user(vq->last_used_idx, &vq->used->idx)) {
here, too
> vq_err(vq, "Failed to increment used idx");
> return -EFAULT;
> }
> @@ -1011,7 +1044,6 @@
> if (vq->log_ctx)
> eventfd_signal(vq->log_ctx, 1);
> }
> - vq->last_used_idx++;
> return 0;
> }
>
> @@ -1038,9 +1070,9 @@
> /* And here's the combo meal deal. Supersize me! */
> void vhost_add_used_and_signal(struct vhost_dev *dev,
> struct vhost_virtqueue *vq,
> - unsigned int head, int len)
> + struct iovec *heads, int count)
> {
> - vhost_add_used(vq, head, len);
> + vhost_add_used(vq, heads, count);
> vhost_signal(dev, vq);
> }
>
> diff -ruN net-next-p0/drivers/vhost/vhost.h
> net-next-p1/drivers/vhost/vhost.h
> --- net-next-p0/drivers/vhost/vhost.h 2010-02-15 20:08:35.000000000
> -0800
> +++ net-next-p1/drivers/vhost/vhost.h 2010-03-01 11:42:18.000000000
> -0800
> @@ -84,6 +84,7 @@
> struct iovec indirect[VHOST_NET_MAX_SG];
> struct iovec iov[VHOST_NET_MAX_SG];
> struct iovec hdr[VHOST_NET_MAX_SG];
> + struct iovec heads[VHOST_NET_MAX_SG];
> size_t hdr_size;
> /* We use a kind of RCU to access private pointer.
> * All readers access it from workqueue, which makes it possible
> to
> @@ -120,16 +121,18 @@
> int vhost_vq_access_ok(struct vhost_virtqueue *vq);
> int vhost_log_access_ok(struct vhost_dev *);
>
> +unsigned vhost_get_heads(struct vhost_virtqueue *, int datalen, int
> *iovcount,
> + struct vhost_log *log, unsigned int *log_num);
> unsigned vhost_get_vq_desc(struct vhost_dev *, struct vhost_virtqueue *,
> struct iovec iov[], unsigned int iov_count,
> unsigned int *out_num, unsigned int *in_num,
> struct vhost_log *log, unsigned int *log_num);
> -void vhost_discard_vq_desc(struct vhost_virtqueue *);
> +void vhost_discard(struct vhost_virtqueue *, int);
>
> -int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
> -void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
> +int vhost_add_used(struct vhost_virtqueue *, struct iovec *heads, int
> count);
> void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue
> *,
> - unsigned int head, int len);
> + struct iovec *heads, int count);
> +void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
> void vhost_disable_notify(struct vhost_virtqueue *);
> bool vhost_enable_notify(struct vhost_virtqueue *);
>
--
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