Re: [PATCH RFC net-next v2 5/6] net: devmem: Implement TX path

Stanislav Fomichev
2025-01-30T23:07:46Z

On 01/30, Mina Almasry wrote:
> Augment dmabuf binding to be able to handle TX. Additional to all the RX
> binding, we also create tx_vec 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.
> - 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 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>
>
>
> ---
>
> v2:
> - Remove dmabuf_offset from the dmabuf cmsg.
> - Update zerocopy_fill_skb_from_devmem to interpret the
> iov_base/iter_iov_addr as the offset into the dmabuf to send from
> (Stan).
> - Remove the confusing binding->tx_iter which is not needed if we
> interpret the iov_base/iter_iov_addr as offset into the dmabuf (Stan).
> - Remove check for binding->sgt and binding->sgt->nents in dmabuf
> binding.
> - Simplify the calculation of binding->tx_vec.
> - Check in net_devmem_get_binding that the binding we're returning
> has ifindex matching the sending socket (Willem).
> ---
> include/linux/skbuff.h | 15 +++-
> include/net/sock.h | 1 +
> include/uapi/linux/uio.h | 6 +-
> net/core/datagram.c | 41 ++++++++++-
> net/core/devmem.c | 96 +++++++++++++++++++++++--
> net/core/devmem.h | 42 +++++++++--
> net/core/netdev-genl.c | 65 ++++++++++++++++-
> net/core/skbuff.c | 6 +-
> net/core/sock.c | 8 +++
> net/ipv4/tcp.c | 36 +++++++---
> net/vmw_vsock/virtio_transport_common.c | 3 +-
> 11 files changed, 285 insertions(+), 34 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index bb2b751d274a..3ff8f568c382 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1711,9 +1711,12 @@ struct ubuf_info *msg_zerocopy_realloc(struct sock *sk, size_t size,
>
> void msg_zerocopy_put_abort(struct ubuf_info *uarg, bool have_uref);
>
> +struct net_devmem_dmabuf_binding;
> +
> 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,
> + struct net_devmem_dmabuf_binding *binding);
>
> int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
> struct iov_iter *from, size_t length);
> @@ -1721,12 +1724,14 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
> static inline int skb_zerocopy_iter_dgram(struct sk_buff *skb,
> struct msghdr *msg, int len)
> {
> - return __zerocopy_sg_from_iter(msg, skb->sk, skb, &msg->msg_iter, len);
> + return __zerocopy_sg_from_iter(msg, skb->sk, skb, &msg->msg_iter, len,
> + NULL);
> }
>
> int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
> struct msghdr *msg, int len,
> - struct ubuf_info *uarg);
> + struct ubuf_info *uarg,
> + struct net_devmem_dmabuf_binding *binding);
>
> /* Internal */
> #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
> @@ -3697,6 +3702,10 @@ static inline dma_addr_t __skb_frag_dma_map(struct device *dev,
> size_t offset, size_t size,
> enum dma_data_direction dir)
> {
> + if (skb_frag_is_net_iov(frag)) {
> + return netmem_to_net_iov(frag->netmem)->dma_addr + offset +
> + frag->offset;
> + }
> return dma_map_page(dev, skb_frag_page(frag),
> skb_frag_off(frag) + offset, size, dir);
> }
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 8036b3b79cd8..09eb918525b6 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1822,6 +1822,7 @@ struct sockcm_cookie {
> u32 tsflags;
> u32 ts_opt_id;
> u32 priority;
> + u32 dmabuf_id;
> };
>
> static inline void sockcm_init(struct sockcm_cookie *sockc,
> diff --git a/include/uapi/linux/uio.h b/include/uapi/linux/uio.h
> index 649739e0c404..866bd5dfe39f 100644
> --- a/include/uapi/linux/uio.h
> +++ b/include/uapi/linux/uio.h
> @@ -38,10 +38,14 @@ struct dmabuf_token {
> __u32 token_count;
> };
>
> +struct dmabuf_tx_cmsg {
> + __u32 dmabuf_id;
> +};
> +
> /*
> * UIO_MAXIOV shall be at least 16 1003.1g (5.4.1.1)
> */
> -
> +
> #define UIO_FASTIOV 8
> #define UIO_MAXIOV 1024
>
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index f0693707aece..c989606ff58d 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -63,6 +63,8 @@
> #include <net/busy_poll.h>
> #include <crypto/hash.h>
>
> +#include "devmem.h"
> +
> /*
> * Is a socket 'connection oriented' ?
> */
> @@ -692,9 +694,42 @@ int zerocopy_fill_skb_from_iter(struct sk_buff *skb,
> return 0;
> }
>
> +static int
> +zerocopy_fill_skb_from_devmem(struct sk_buff *skb, struct iov_iter *from,
> + int length,
> + struct net_devmem_dmabuf_binding *binding)
> +{
> + int i = skb_shinfo(skb)->nr_frags;
> + size_t virt_addr, size, off;
> + struct net_iov *niov;
> +
> + while (length && iov_iter_count(from)) {
> + if (i == MAX_SKB_FRAGS)
> + return -EMSGSIZE;
> +
> + virt_addr = (size_t)iter_iov_addr(from);
> + niov = net_devmem_get_niov_at(binding, virt_addr, &off, &size);
> + if (!niov)
> + return -EFAULT;
> +
> + size = min_t(size_t, size, length);
> + size = min_t(size_t, size, iter_iov_len(from));
> +
> + get_netmem(net_iov_to_netmem(niov));
> + skb_add_rx_frag_netmem(skb, i, net_iov_to_netmem(niov), off,
> + size, PAGE_SIZE);
> + iov_iter_advance(from, size);
> + length -= size;
> + i++;
> + }
> +
> + 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,
> + struct net_devmem_dmabuf_binding *binding)
> {
> unsigned long orig_size = skb->truesize;
> unsigned long truesize;
> @@ -702,6 +737,8 @@ 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(binding))
> + ret = zerocopy_fill_skb_from_devmem(skb, from, length, binding);
> else
> ret = zerocopy_fill_skb_from_iter(skb, from, length);
>
> @@ -735,7 +772,7 @@ 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);
>
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 20985a570662..796338b1599e 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -16,6 +16,7 @@
> #include <net/netdev_queues.h>
> #include <net/netdev_rx_queue.h>
> #include <net/page_pool/helpers.h>
> +#include <net/sock.h>
> #include <trace/events/page_pool.h>
>
> #include "devmem.h"
> @@ -64,8 +65,10 @@ void __net_devmem_dmabuf_binding_free(struct net_devmem_dmabuf_binding *binding)
> dma_buf_detach(binding->dmabuf, binding->attachment);
> dma_buf_put(binding->dmabuf);
> xa_destroy(&binding->bound_rxqs);
> + kfree(binding->tx_vec);
> kfree(binding);
> }
> +EXPORT_SYMBOL(__net_devmem_dmabuf_binding_free);
>
> struct net_iov *
> net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding)
> @@ -110,6 +113,13 @@ 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();
> +
> if (binding->list.next)
> list_del(&binding->list);
>
> @@ -123,8 +133,6 @@ 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);
> }
>
> @@ -185,8 +193,9 @@ 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;
> @@ -229,7 +238,7 @@ 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");
> @@ -240,13 +249,22 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,
> * binding can be much more flexible than that. We may be able to
> * allocate MTU sized chunks here. Leave that for future work...
> */
> - binding->chunk_pool =
> - gen_pool_create(PAGE_SHIFT, dev_to_node(&dev->dev));
> + binding->chunk_pool = gen_pool_create(PAGE_SHIFT,
> + dev_to_node(&dev->dev));
> if (!binding->chunk_pool) {
> err = -ENOMEM;
> goto err_unmap;
> }
>
> + if (direction == DMA_TO_DEVICE) {
> + binding->tx_vec = kcalloc(dmabuf->size / PAGE_SIZE,
> + sizeof(struct net_iov *), GFP_KERNEL);

Let's make this kvcalloc (and kvfree in the release path)? Otherwise
we'll get ENOMEM here for a reasonably sized dmabufs..

The rest looks good, I'll try to run it on my side today/tomorrow..