Re: [PATCH net-next v9 4/4] vsock/virtio: MSG_ZEROCOPY flag support

[Stefano Garzarella]

On Sat, Sep 16, 2023 at 04:09:18PM +0300, Arseniy Krasnov wrote:
> This adds handling of MSG_ZEROCOPY flag on transmission path:
>
> 1) If this flag is set and zerocopy transmission is possible (enabled
>   in socket options and transport allows zerocopy), then non-linear
>   skb will be created and filled with the pages of user's buffer.
>   Pages of user's buffer are locked in memory by 'get_user_pages()'.
> 2) Replaces way of skb owning: instead of 'skb_set_owner_sk_safe()' it
>   calls 'skb_set_owner_w()'. Reason of this change is that
>   '__zerocopy_sg_from_iter()' increments 'sk_wmem_alloc' of socket, so
>   to decrease this field correctly, proper skb destructor is needed:
>   'sock_wfree()'. This destructor is set by 'skb_set_owner_w()'.
> 3) Adds new callback to 'struct virtio_transport': 'can_msgzerocopy'.
>   If this callback is set, then transport needs extra check to be able
>   to send provided number of buffers in zerocopy mode. Currently, the
>   only transport that needs this callback set is virtio, because this
>   transport adds new buffers to the virtio queue and we need to check,
>   that number of these buffers is less than size of the queue (it is
>   required by virtio spec). vhost and loopback transports don't need
>   this check.
>
> Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
> ---
> Changelog:
> v5(big patchset) -> v1:
>  * Refactorings of 'if' conditions.
>  * Remove extra blank line.
>  * Remove 'frag_off' field unneeded init.
>  * Add function 'virtio_transport_fill_skb()' which fills both linear
>    and non-linear skb with provided data.
> v1 -> v2:
>  * Use original order of last four arguments in 'virtio_transport_alloc_skb()'.
> v2 -> v3:
>  * Add new transport callback: 'msgzerocopy_check_iov'. It checks that
>    provided 'iov_iter' with data could be sent in a zerocopy mode.
>    If this callback is not set in transport - transport allows to send
>    any 'iov_iter' in zerocopy mode. Otherwise - if callback returns 'true'
>    then zerocopy is allowed. Reason of this callback is that in case of
>    G2H transmission we insert whole skb to the tx virtio queue and such
>    skb must fit to the size of the virtio queue to be sent in a single
>    iteration (may be tx logic in 'virtio_transport.c' could be reworked
>    as in vhost to support partial send of current skb). This callback
>    will be enabled only for G2H path. For details pls see comment
>    'Check that tx queue...' below.
> v3 -> v4:
>  * 'msgzerocopy_check_iov' moved from 'struct vsock_transport' to
>    'struct virtio_transport' as it is virtio specific callback and
>    never needed in other transports.
> v4 -> v5:
>  * 'msgzerocopy_check_iov' renamed to 'can_msgzerocopy' and now it
>    uses number of buffers to send as input argument. I think there is
>    no need to pass iov to this callback (at least today, it is used only
>    by guest side of virtio transport), because the only thing that this
>    callback does is comparison of number of buffers to be inserted to
>    the tx queue and size of this queue.
>  * Remove any checks for type of current 'iov_iter' with payload (is it
>    'iovec' or 'ubuf'). These checks left from the earlier versions where I
>    didn't use already implemented kernel API which handles every type of
>    'iov_iter'.
> v5 -> v6:
>  * Refactor 'virtio_transport_fill_skb()'.
>  * Add 'WARN_ON_ONCE()' and comment on invalid combination of destination
>    socket and payload in 'virtio_transport_alloc_skb()'.
> v7 -> v8:
>  * Move '+1' addition from 'can_msgzerocopy' callback body to the caller.
>    This addition means packet header.
>  * In 'virtio_transport_can_zcopy()' rename 'max_to_send' argument to
>    'pkt_len'.
>  * Update commit message by adding details about new 'can_msgzerocopy'
>    callback.
>  * In 'virtio_transport_init_hdr()' move 'len' argument directly after
>    'info'.
>  * Add comment about processing last skb in tx loop.
>  * Update comment for 'can_msgzerocopy' callback for more details.
> v8 -> v9:
>  * Return and update comment for 'virtio_transport_alloc_skb()'.
>  * Pass pointer to transport ops to 'virtio_transport_can_zcopy()',
>    this allows to use it directly without calling virtio_transport_get_ops()'.
>  * Remove redundant call for 'msg_data_left()' in 'virtio_transport_fill_skb()'.
>  * Do not pass 'struct vsock_sock*' to 'virtio_transport_alloc_skb()',
>    use same pointer from already passed 'struct virtio_vsock_pkt_info*'.
>  * Fix setting 'end of message' bit for SOCK_SEQPACKET (add call for
>    'msg_data_left()' == 0).
>  * Add 'zcopy' parameter to packet allocation trace event.

Thanks for addressing the comments!
> include/linux/virtio_vsock.h                  |   9 +
> .../events/vsock_virtio_transport_common.h    |  12 +-
> net/vmw_vsock/virtio_transport.c              |  32 +++
> net/vmw_vsock/virtio_transport_common.c       | 250 ++++++++++++++----
> 4 files changed, 241 insertions(+), 62 deletions(-)

LGTM!

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>