Re: [PATCH net-next v7 1/4] vsock/virtio/vhost: read data from non-linear skb

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On Sun, Aug 27, 2023 at 11:54:33AM +0300, Arseniy Krasnov wrote:
This is preparation patch for MSG_ZEROCOPY support. It adds handling of
non-linear skbs by replacing direct calls of 'memcpy_to_msg()' with
'skb_copy_datagram_iter()'. Main advantage of the second one is that it
can handle paged part of the skb by using 'kmap()' on each page, but if
there are no pages in the skb, it behaves like simple copying to iov
iterator. This patch also adds new field to the control block of skb -
this value shows current offset in the skb to read next portion of data
(it doesn't matter linear it or not). Idea behind this field is that
'skb_copy_datagram_iter()' handles both types of skb internally - it
just needs an offset from which to copy data from the given skb. This
offset is incremented on each read from skb. This approach allows to
simplify handling of both linear and non-linear skbs, because for
linear skb we need to call 'skb_pull()' after reading data from it,
while in non-linear case we need to update 'data_len'.

Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
v5(big patchset) -> v1:
 * Merge 'virtio_transport_common.c' and 'vhost/vsock.c' patches into
   this single patch.
 * Commit message update: grammar fix and remark that this patch is
   MSG_ZEROCOPY preparation.
 * Use 'min_t()' instead of comparison using '<>' operators.
v1 -> v2:
 * R-b tag added.
v3 -> v4:
 * R-b tag removed due to rebase:
   * Part for 'virtio_transport_stream_do_peek()' is changed.
   * Part for 'virtio_transport_seqpacket_do_peek()' is added.
 * Comments about sleep in 'memcpy_to_msg()' now describe sleep in
v5 -> v6:
 * Commit message update.
 * Rename 'frag_off' to 'offset' in 'virtio_vsock_skb_cb'.

drivers/vhost/vsock.c                   | 14 +++++++----
include/linux/virtio_vsock.h            |  1 +
net/vmw_vsock/virtio_transport_common.c | 32 +++++++++++++++----------
3 files changed, 29 insertions(+), 18 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>

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