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Date: Mon, 11 Jul 2011 17:37:47 -0700
From: Shirley Ma <mashirle@...ibm.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] vhost: address fixme in vhost TX zero-copy support
On Tue, 2011-07-12 at 01:04 +0300, Michael S. Tsirkin wrote:
> So the following should do it, on top of Shirleys's patch, I think.
> I'm
> a bit not sure about using vq->upend_idx - vq->done_idx to check the
> number of outstanding DMA, Shirley, what do you think?
Yes, you can use this to track # outstanding DMAs.
> Untested.
>
> I'm also thinking about making the use of this conditinal
> on a module parameter, off by default to reduce
> stability risk while still enabling more people to
> test the feature.
> Thoughts?
Agreed.
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7de0c6e..cf8deb3 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -156,8 +156,7 @@ static void handle_tx(struct vhost_net *net)
>
> for (;;) {
> /* Release DMAs done buffers first */
> - if (atomic_read(&vq->refcnt) > VHOST_MAX_PEND)
> - vhost_zerocopy_signal_used(vq);
> + vhost_zerocopy_signal_used(vq);
>
> head = vhost_get_vq_desc(&net->dev, vq, vq->iov,
> ARRAY_SIZE(vq->iov),
> @@ -175,7 +174,7 @@ static void handle_tx(struct vhost_net *net)
> break;
> }
> /* If more outstanding DMAs, queue the work */
> - if (atomic_read(&vq->refcnt) > VHOST_MAX_PEND)
> {
> + if (vq->upend_idx - vq->done_idx >
> VHOST_MAX_PEND) {
> tx_poll_start(net, sock);
> set_bit(SOCK_ASYNC_NOSPACE,
> &sock->flags);
> break;
> @@ -214,12 +213,12 @@ static void handle_tx(struct vhost_net *net)
>
> vq->heads[vq->upend_idx].len = len;
> ubuf->callback =
> vhost_zerocopy_callback;
> - ubuf->arg = vq;
> + ubuf->arg = vq->ubufs;
> ubuf->desc = vq->upend_idx;
> msg.msg_control = ubuf;
> msg.msg_controllen = sizeof(ubuf);
> + kref_get(&vq->ubufs->kref);
> }
> - atomic_inc(&vq->refcnt);
> vq->upend_idx = (vq->upend_idx + 1) %
> UIO_MAXIOV;
> }
> /* TODO: Check specific error and bomb out unless
> ENOBUFS? */
> @@ -646,6 +645,7 @@ static long vhost_net_set_backend(struct vhost_net
> *n, unsigned index, int fd)
> {
> struct socket *sock, *oldsock;
> struct vhost_virtqueue *vq;
> + struct vhost_ubuf_ref *ubufs, *oldubufs = NULL;
> int r;
>
> mutex_lock(&n->dev.mutex);
> @@ -675,6 +675,13 @@ static long vhost_net_set_backend(struct
> vhost_net *n, unsigned index, int fd)
> oldsock = rcu_dereference_protected(vq->private_data,
>
> lockdep_is_held(&vq->mutex));
> if (sock != oldsock) {
> + ubufs = vhost_ubuf_alloc(vq, sock);
> + if (IS_ERR(ubufs)) {
> + r = PTR_ERR(ubufs);
> + goto err_ubufs;
> + }
> + oldubufs = vq->ubufs;
> + vq->ubufs = ubufs;
> vhost_net_disable_vq(n, vq);
> rcu_assign_pointer(vq->private_data, sock);
> vhost_net_enable_vq(n, vq);
> @@ -682,6 +689,9 @@ static long vhost_net_set_backend(struct vhost_net
> *n, unsigned index, int fd)
>
> mutex_unlock(&vq->mutex);
>
> + if (oldbufs)
> + vhost_ubuf_put_and_wait(oldbufs);
> +
> if (oldsock) {
> vhost_net_flush_vq(n, index);
> fput(oldsock->file);
> @@ -690,6 +700,8 @@ static long vhost_net_set_backend(struct vhost_net
> *n, unsigned index, int fd)
> mutex_unlock(&n->dev.mutex);
> return 0;
>
> +err_ubufs:
> + fput(sock);
> err_vq:
> mutex_unlock(&vq->mutex);
> err:
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index db242b1..81b1dd7 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -181,7 +181,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> vq->log_ctx = NULL;
> vq->upend_idx = 0;
> vq->done_idx = 0;
> - atomic_set(&vq->refcnt, 0);
> + vq->ubufs = NULL;
> }
>
> static int vhost_worker(void *data)
> @@ -401,7 +401,7 @@ long vhost_dev_reset_owner(struct vhost_dev *dev)
> * of used idx. Once lower device DMA done contiguously, we will
> signal KVM
> * guest used idx.
> */
> -void vhost_zerocopy_signal_used(struct vhost_virtqueue *vq)
> +int vhost_zerocopy_signal_used(struct vhost_virtqueue *vq)
> {
> int i, j = 0;
>
> @@ -414,10 +414,9 @@ void vhost_zerocopy_signal_used(struct
> vhost_virtqueue *vq)
> } else
> break;
> }
> - if (j) {
> + if (j)
> vq->done_idx = i;
> - atomic_sub(j, &vq->refcnt);
> - }
> + return j;
> }
>
> /* Caller should have device mutex */
> @@ -430,9 +429,13 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> vhost_poll_stop(&dev->vqs[i].poll);
> vhost_poll_flush(&dev->vqs[i].poll);
> }
> - /* Wait for all lower device DMAs done (busywait
> FIXME) */
> - while (atomic_read(&dev->vqs[i].refcnt))
> - vhost_zerocopy_signal_used(&dev->vqs[i]);
> + /* Wait for all lower device DMAs done. */
> + if (dev->vqs[i].ubufs)
> + vhost_ubuf_put_and_wait(dev->vqs[i].ubufs);
> +
> + /* Signal guest as appropriate. */
> + vhost_zerocopy_signal_used(&dev->vqs[i]);
> +
> if (dev->vqs[i].error_ctx)
> eventfd_ctx_put(dev->vqs[i].error_ctx);
> if (dev->vqs[i].error)
> @@ -645,11 +648,6 @@ static long vhost_set_vring(struct vhost_dev *d,
> int ioctl, void __user *argp)
>
> mutex_lock(&vq->mutex);
>
> - /* clean up lower device outstanding DMAs, before setting ring
> - busywait FIXME */
> - while (atomic_read(&vq->refcnt))
> - vhost_zerocopy_signal_used(vq);
> -
We need to clear up outstanding DMAs here too when we set vring.
Otherwise, KVM guest remove/reload virtio_net module vring would be out
of sync with vhost.
> switch (ioctl) {
> case VHOST_SET_VRING_NUM:
> /* Resizing ring with an active backend?
> @@ -1525,12 +1523,46 @@ void vhost_disable_notify(struct vhost_dev
> *dev, struct vhost_virtqueue *vq)
> }
> }
>
> +static void vhost_zerocopy_done_signal(struct kref *kref)
> +{
> + struct vhost_ubuf_ref *ubufs = container_of(kref, struct
> vhost_ubuf_ref,
> + kref);
> + wake_up(&ubufs->wait);
> +}
> +
> +struct vhost_ubuf_ref *vhost_ubuf_alloc(struct vhost_virtqueue *vq,
> + void * private_data)
> +{
> + struct vhost_ubuf_ref *ubufs;
> + /* No backend? Nothing to count. */
> + if (!private_data)
> + return NULL;
> + ubufs = kmalloc(sizeof *ubufs, GFP_KERNEL);
> + if (!ubufs)
> + return ERR_PTR(-ENOMEM);
> + kref_init(&ubufs->kref);
> + kref_get(&ubufs->kref);
> + init_waitqueue_head(&ubufs->wait);
> + ubufs->vq = vq;
> + return ubufs;
> +}
> +
> +void vhost_ubuf_put_and_wait(struct vhost_ubuf_ref *ubufs)
> +{
> + kref_put(&ubufs->kref, vhost_zerocopy_done_signal);
> + wait_event(ubufs->wait, !atomic_read(&ubufs->kref.refcount));
> + kfree(ubufs);
> +}
> +
> void vhost_zerocopy_callback(void *arg)
> {
> struct ubuf_info *ubuf = (struct ubuf_info *)arg;
> + struct vhost_ubuf_ref *ubufs;
> struct vhost_virtqueue *vq;
>
> - vq = (struct vhost_virtqueue *)ubuf->arg;
> + ubufs = ubuf->arg;
> + vq = ubufs->vq;
> /* set len = 1 to mark this desc buffers done DMA */
> vq->heads[ubuf->desc].len = VHOST_DMA_DONE_LEN;
> + kref_put(&ubufs->kref, vhost_zerocopy_done_signal);
> }
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 883688c..b42b126 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -55,6 +55,17 @@ struct vhost_log {
> u64 len;
> };
>
> +struct vhost_virtqueue;
> +
> +struct vhost_ubuf_ref {
> + struct kref kref;
> + wait_queue_t wait;
> + struct vhost_virtqueue *vq;
> +};
> +
> +struct vhost_ubuf_ref *vhost_ubuf_alloc(struct vhost_virtqueue *,
> void *);
> +void vhost_ubuf_put_and_wait(struct vhost_ubuf_ref *);
> +
> /* The virtqueue structure describes a queue attached to a device. */
> struct vhost_virtqueue {
> struct vhost_dev *dev;
> @@ -127,6 +138,9 @@ struct vhost_virtqueue {
> int done_idx;
> /* an array of userspace buffers info */
> struct ubuf_info *ubuf_info;
> + /* Reference counting for outstanding ubufs.
> + * Protected by vq mutex. Writers must also take device mutex.
> */
> + struct vhost_ubuf_ref *ubufs;
> };
>
> struct vhost_dev {
> @@ -174,7 +188,7 @@ bool vhost_enable_notify(struct vhost_dev *,
> struct vhost_virtqueue *);
> int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log
> *log,
> unsigned int log_num, u64 len);
> void vhost_zerocopy_callback(void *arg);
> -void vhost_zerocopy_signal_used(struct vhost_virtqueue *vq);
> +int vhost_zerocopy_signal_used(struct vhost_virtqueue *vq);
>
> #define vq_err(vq, fmt, ...) do { \
> pr_debug(pr_fmt(fmt), ##__VA_ARGS__); \
> --
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