Re: [PATCH RFC net-next v1 5/5] net: devmem: Implement TX path

Mina Almasry wrote:

Augment dmabuf binding to be able to handle TX. Additional to all the RX binding, we also create tx_vec and tx_iter needed for the TX path.

Provide API for sendmsg to be able to send dmabufs bound to this device:

• Provide a new dmabuf_tx_cmsg which includes the dmabuf to send from, and the offset into the dmabuf to send from.
• MSG_ZEROCOPY with SCM_DEVMEM_DMABUF cmsg indicates send from dma-buf.

Devmem is uncopyable, so piggyback off the existing MSG_ZEROCOPY implementation, while disabling instances where MSG_ZEROCOPY falls back to copying.

We additionally look up the dmabuf to send from by id, then pipe the binding down to the new zerocopy_fill_skb_from_devmem which fills a TX skb with net_iov netmems instead of the traditional page netmems.

We also special case skb_frag_dma_map to return the dma-address of these dmabuf net_iovs instead of attempting to map pages.

Based on work by Stanislav Fomichev sdf@fomichev.me. A lot of the meat of the implementation came from devmem TCP RFC v1[1], which included the TX path, but Stan did all the rebasing on top of netmem/net_iov.

Cc: Stanislav Fomichev sdf@fomichev.me Signed-off-by: Kaiyuan Zhang kaiyuanz@google.com Signed-off-by: Mina Almasry almasrymina@google.com

---

include/linux/skbuff.h | 13 +++- include/net/sock.h | 2 + include/uapi/linux/uio.h | 5 ++ net/core/datagram.c | 40 ++++++++++- net/core/devmem.c | 91 +++++++++++++++++++++++-- net/core/devmem.h | 40 +++++++++-- net/core/netdev-genl.c | 65 +++++++++++++++++- net/core/skbuff.c | 8 ++- net/core/sock.c | 9 +++ net/ipv4/tcp.c | 36 +++++++--- net/vmw_vsock/virtio_transport_common.c | 4 +- 11 files changed, 281 insertions(+), 32 deletions(-)

+static int zerocopy_fill_skb_from_devmem(struct sk_buff *skb,

• 			 struct msghdr *msg,
  

• 			 struct iov_iter *from, int length)
  

+{

• int i = skb_shinfo(skb)->nr_frags;
• int orig_length = length;
• netmem_ref netmem;
• size_t size;
• while (length && iov_iter_count(from)) {

• if (i == MAX_SKB_FRAGS)
  

• 	return -EMSGSIZE;
  

• size = min_t(size_t, iter_iov_len(from), length);
  

• if (!size)
  

• 	return -EFAULT;
  

On error, should caller skb_zerocopy_iter_stream rewind from, rather than (or as well as) msg->msg_iter?

• netmem = net_iov_to_netmem(iter_iov(from)->iov_base);
  

• get_netmem(netmem);
  

• skb_add_rx_frag_netmem(skb, i, netmem, from->iov_offset, size,
  

• 		       PAGE_SIZE);
  

• iov_iter_advance(from, size);
  

• length -= size;
  

• i++;
  

• }
• iov_iter_advance(&msg->msg_iter, orig_length);

What does this do if sendmsg is called with NULL as buffer?

• return 0;

+}

int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk, struct sk_buff *skb, struct iov_iter *from,

• 	    size_t length)
  

• 	    size_t length, bool is_devmem)
  

{ unsigned long orig_size = skb->truesize; unsigned long truesize; @@ -702,6 +736,8 @@ int __zerocopy_sg_from_iter(struct msghdr *msg, struct sock *sk, if (msg && msg->msg_ubuf && msg->sg_from_iter) ret = msg->sg_from_iter(skb, from, length);

• else if (unlikely(is_devmem))

• ret = zerocopy_fill_skb_from_devmem(skb, msg, from, length);
  

  else ret = zerocopy_fill_skb_from_iter(skb, from, length);

@@ -735,7 +771,7 @@ int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *from) if (skb_copy_datagram_from_iter(skb, 0, from, copy)) return -EFAULT;

• return __zerocopy_sg_from_iter(NULL, NULL, skb, from, ~0U);

• return __zerocopy_sg_from_iter(NULL, NULL, skb, from, ~0U, NULL);

} EXPORT_SYMBOL(zerocopy_sg_from_iter);

struct net_iov * net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding) @@ -109,6 +112,13 @@ void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding) unsigned long xa_idx; unsigned int rxq_idx;

• xa_erase(&net_devmem_dmabuf_bindings, binding->id);
• /* Ensure no tx net_devmem_lookup_dmabuf() are in flight after the

• * erase.
  

• */
  

• synchronize_net();

What precisely does this protect?

synchronize_net() ensures no packet is in flight inside an rcu readside section. But a packet can still be in flight, such as posted to the device or queued in a qdisc.

if (binding->list.next) list_del(&binding->list); @@ -122,8 +132,6 @@ void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding) WARN_ON(netdev_rx_queue_restart(binding->dev, rxq_idx)); }

• xa_erase(&net_devmem_dmabuf_bindings, binding->id);
• net_devmem_dmabuf_binding_put(binding);

} @@ -174,8 +182,9 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx, } struct net_devmem_dmabuf_binding * -net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,

•        struct netlink_ext_ack *extack)
  

+net_devmem_bind_dmabuf(struct net_device *dev,

•        enum dma_data_direction direction,
  

•        unsigned int dmabuf_fd, struct netlink_ext_ack *extack)
  

{ struct net_devmem_dmabuf_binding *binding; static u32 id_alloc_next; @@ -183,6 +192,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd, struct dma_buf *dmabuf; unsigned int sg_idx, i; unsigned long virtual;

• struct iovec *iov; int err;

dmabuf = dma_buf_get(dmabuf_fd); @@ -218,13 +228,19 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd, } binding->sgt = dma_buf_map_attachment_unlocked(binding->attachment,

• 				       DMA_FROM_DEVICE);
  

• 				       direction);
  

  if (IS_ERR(binding->sgt)) { err = PTR_ERR(binding->sgt); NL_SET_ERR_MSG(extack, "Failed to map dmabuf attachment"); goto err_detach; }

• if (!binding->sgt || binding->sgt->nents == 0) {

• err = -EINVAL;
  

• NL_SET_ERR_MSG(extack, "Empty dmabuf attachment");
  

• goto err_detach;
  

• }
• /* For simplicity we expect to make PAGE_SIZE allocations, but the
  • binding can be much more flexible than that. We may be able to
  • allocate MTU sized chunks here. Leave that for future work...

@@ -236,6 +252,19 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd, goto err_unmap; }

• if (direction == DMA_TO_DEVICE) {

• virtual = 0;
  

• for_each_sgtable_dma_sg(binding->sgt, sg, sg_idx)
  

• 	virtual += sg_dma_len(sg);
  

• binding->tx_vec = kcalloc(virtual / PAGE_SIZE + 1,
  

instead of open coding this computation repeatedly, consider a local variable. And parentheses and maybe round_up().

• 			  sizeof(struct iovec), GFP_KERNEL);
  

• if (!binding->tx_vec) {
  

• 	err = -ENOMEM;
  

• 	goto err_unmap;
  

• }
  

• }
• virtual = 0; for_each_sgtable_dma_sg(binding->sgt, sg, sg_idx) { dma_addr_t dma_addr = sg_dma_address(sg);

@@ -277,11 +306,21 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd, niov->owner = owner; page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov), net_devmem_get_dma_addr(niov));

• 	if (direction == DMA_TO_DEVICE) {
  

• 		iov = &binding->tx_vec[virtual / PAGE_SIZE + i];
  

why does this start counting at virtual / PAGE_SIZE, rather than 0?

• 		iov->iov_base = niov;
  

• 		iov->iov_len = PAGE_SIZE;
  

• 	}
  

  }

virtual += len; }

• if (direction == DMA_TO_DEVICE)

• iov_iter_init(&binding->tx_iter, WRITE, binding->tx_vec,
  

• 	      virtual / PAGE_SIZE + 1, virtual);
  

• return binding;

err_free_chunks: @@ -302,6 +341,21 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd, return ERR_PTR(err); } +struct net_devmem_dmabuf_binding *net_devmem_lookup_dmabuf(u32 id) +{

• struct net_devmem_dmabuf_binding *binding;
• rcu_read_lock();
• binding = xa_load(&net_devmem_dmabuf_bindings, id);
• if (binding) {

• if (!net_devmem_dmabuf_binding_get(binding))
  

• 	binding = NULL;
  

• }
• rcu_read_unlock();
• return binding;

+}

void dev_dmabuf_uninstall(struct net_device *dev) { struct net_devmem_dmabuf_binding *binding; @@ -332,6 +386,33 @@ void net_devmem_put_net_iov(struct net_iov *niov) net_devmem_dmabuf_binding_put(niov->owner->binding); } +struct net_devmem_dmabuf_binding * +net_devmem_get_sockc_binding(struct sock *sk, struct sockcm_cookie *sockc) +{

• struct net_devmem_dmabuf_binding *binding;
• int err = 0;
• binding = net_devmem_lookup_dmabuf(sockc->dmabuf_id);

This lookup is from global xarray net_devmem_dmabuf_bindings.

Is there a check that the socket is sending out through the device to which this dmabuf was bound with netlink? Should there be? (e.g., SO_BINDTODEVICE).

• if (!binding || !binding->tx_vec) {

• err = -EINVAL;
  

• goto out_err;
  

• }
• if (sock_net(sk) != dev_net(binding->dev)) {

• err = -ENODEV;
  

• goto out_err;
  

• }
• iov_iter_advance(&binding->tx_iter, sockc->dmabuf_offset);
• return binding;

+out_err:

• if (binding)

• net_devmem_dmabuf_binding_put(binding);
  

• return ERR_PTR(err);

+}

/*** "Dmabuf devmem memory provider" ***/

@@ -1063,6 +1064,15 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size) flags = msg->msg_flags;

• sockcm_init(&sockc, sk);
• if (msg->msg_controllen) {

• err = sock_cmsg_send(sk, msg, &sockc);
  

• if (unlikely(err)) {
  

• 	err = -EINVAL;
  

• 	goto out_err;
  

• }
  

• }
• if ((flags & MSG_ZEROCOPY) && size) { if (msg->msg_ubuf) { uarg = msg->msg_ubuf;

@@ -1080,6 +1090,15 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size) else uarg_to_msgzc(uarg)->zerocopy = 0; }

• if (sockc.dmabuf_id != 0) {
  

• 	binding = net_devmem_get_sockc_binding(sk, &sockc);
  

• 	if (IS_ERR(binding)) {
  

• 		err = PTR_ERR(binding);
  

• 		binding = NULL;
  

• 		goto out_err;
  

• 	}
  

• }