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Message-ID: <f020405e-86af-1b66-c5f4-9bec98298f44@sberdevices.ru>
Date:   Thu, 27 Jul 2023 11:32:00 +0300
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Subject: Re: [PATCH net-next v3 4/4] vsock/virtio: MSG_ZEROCOPY flag support
To:     Stefano Garzarella <sgarzare@redhat.com>
CC:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        <kvm@vger.kernel.org>, <virtualization@lists.linux-foundation.org>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <kernel@sberdevices.ru>, <oxffffaa@gmail.com>
References: <20230720214245.457298-1-AVKrasnov@sberdevices.ru>
 <20230720214245.457298-5-AVKrasnov@sberdevices.ru>
 <091c067b-43a0-da7f-265f-30c8c7e62977@sberdevices.ru>
 <2k3cbz762ua3fmlben5vcm7rs624sktaltbz3ldeevwiguwk2w@klggxj5e3ueu>
 <51022d5f-5b50-b943-ad92-b06f60bef433@sberdevices.ru>
 <3d1d76c9-2fdb-3dfe-222a-b2184cf17708@sberdevices.ru>
 <o6axh6mxd6mxai2zrpax6wa25slns7ysz5xsegntskvfxl53mt@wowjgb3jazt6>
Content-Language: en-US
From:   Arseniy Krasnov <avkrasnov@sberdevices.ru>
In-Reply-To: <o6axh6mxd6mxai2zrpax6wa25slns7ysz5xsegntskvfxl53mt@wowjgb3jazt6>
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Precedence: bulk


On 25.07.2023 15:28, Stefano Garzarella wrote:
> On Tue, Jul 25, 2023 at 12:16:11PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 25.07.2023 11:46, Arseniy Krasnov wrote:
>>>
>>>
>>> On 25.07.2023 11:43, Stefano Garzarella wrote:
>>>> On Fri, Jul 21, 2023 at 08:09:03AM +0300, Arseniy Krasnov wrote:
>>>>>
>>>>>
>>>>> On 21.07.2023 00:42, Arseniy Krasnov wrote:
>>>>>> This adds handling of MSG_ZEROCOPY flag on transmission path: 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()'. Second thing that
>>>>>> this patch does is replace type of skb owning: instead of calling
>>>>>> '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()'.
>>>>>>
>>>>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
>>>>>> ---
>>>>>>  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.
>>>>>>
>>>>>>  include/net/af_vsock.h                  |   3 +
>>>>>>  net/vmw_vsock/virtio_transport.c        |  39 ++++
>>>>>>  net/vmw_vsock/virtio_transport_common.c | 257 ++++++++++++++++++------
>>>>>>  3 files changed, 241 insertions(+), 58 deletions(-)
>>>>>>
>>>>>> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
>>>>>> index 0e7504a42925..a6b346eeeb8e 100644
>>>>>> --- a/include/net/af_vsock.h
>>>>>> +++ b/include/net/af_vsock.h
>>>>>> @@ -177,6 +177,9 @@ struct vsock_transport {
>>>>>>
>>>>>>      /* Read a single skb */
>>>>>>      int (*read_skb)(struct vsock_sock *, skb_read_actor_t);
>>>>>> +
>>>>>> +    /* Zero-copy. */
>>>>>> +    bool (*msgzerocopy_check_iov)(const struct iov_iter *);
>>>>>>  };
>>>>>>
>>>>>>  /**** CORE ****/
>>>>>> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>>>>>> index 7bbcc8093e51..23cb8ed638c4 100644
>>>>>> --- a/net/vmw_vsock/virtio_transport.c
>>>>>> +++ b/net/vmw_vsock/virtio_transport.c
>>>>>> @@ -442,6 +442,43 @@ static void virtio_vsock_rx_done(struct virtqueue *vq)
>>>>>>      queue_work(virtio_vsock_workqueue, &vsock->rx_work);
>>>>>>  }
>>>>>>
>>>>>> +static bool virtio_transport_msgzerocopy_check_iov(const struct iov_iter *iov)
>>>>>> +{
>>>>>> +    struct virtio_vsock *vsock;
>>>>>> +    bool res = false;
>>>>>> +
>>>>>> +    rcu_read_lock();
>>>>>> +
>>>>>> +    vsock = rcu_dereference(the_virtio_vsock);
>>>>>> +    if (vsock) {
> 
> Just noted, what about the following to reduce the indentation?
> 
>         if (!vsock) {
>             goto out;
>         }
>             ...
>             ...
>     out:
>         rcu_read_unlock();
>         return res;
> 
>>>>>> +        struct virtqueue *vq;
>>>>>> +        int iov_pages;
>>>>>> +
>>>>>> +        vq = vsock->vqs[VSOCK_VQ_TX];
>>>>>> +
>>>>>> +        iov_pages = round_up(iov->count, PAGE_SIZE) / PAGE_SIZE;
>>>>>> +
>>>>>> +        /* Check that tx queue is large enough to keep whole
>>>>>> +         * data to send. This is needed, because when there is
>>>>>> +         * not enough free space in the queue, current skb to
>>>>>> +         * send will be reinserted to the head of tx list of
>>>>>> +         * the socket to retry transmission later, so if skb
>>>>>> +         * is bigger than whole queue, it will be reinserted
>>>>>> +         * again and again, thus blocking other skbs to be sent.
>>>>>> +         * Each page of the user provided buffer will be added
>>>>>> +         * as a single buffer to the tx virtqueue, so compare
>>>>>> +         * number of pages against maximum capacity of the queue.
>>>>>> +         * +1 means buffer for the packet header.
>>>>>> +         */
>>>>>> +        if (iov_pages + 1 <= vq->num_max)
>>>>>
>>>>> I think this check is actual only for case one we don't have indirect buffer feature.
>>>>> With indirect mode whole data to send will be packed into one indirect buffer.
>>>>
>>>> I think so.
>>>> So, should we check also that here?
>>>>
>>>>>
>>>>> Thanks, Arseniy
>>>>>
>>>>>> +            res = true;
>>>>>> +    }
>>>>>> +
>>>>>> +    rcu_read_unlock();
>>>>>> +
>>>>>> +    return res;
>>>>>> +}
>>>>>> +
>>>>>>  static bool virtio_transport_seqpacket_allow(u32 remote_cid);
>>>>>>
>>>>>>  static struct virtio_transport virtio_transport = {
>>>>>> @@ -475,6 +512,8 @@ static struct virtio_transport virtio_transport = {
>>>>>>          .seqpacket_allow          = virtio_transport_seqpacket_allow,
>>>>>>          .seqpacket_has_data       = virtio_transport_seqpacket_has_data,
>>>>>>
>>>>>> +        .msgzerocopy_check_iov      = virtio_transport_msgzerocopy_check_iov,
>>>>>> +
>>>>>>          .notify_poll_in           = virtio_transport_notify_poll_in,
>>>>>>          .notify_poll_out          = virtio_transport_notify_poll_out,
>>>>>>          .notify_recv_init         = virtio_transport_notify_recv_init,
>>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>>>> index 26a4d10da205..e4e3d541aff4 100644
>>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>>> @@ -37,73 +37,122 @@ virtio_transport_get_ops(struct vsock_sock *vsk)
>>>>>>      return container_of(t, struct virtio_transport, transport);
>>>>>>  }
>>>>>>
>>>>>> -/* Returns a new packet on success, otherwise returns NULL.
>>>>>> - *
>>>>>> - * If NULL is returned, errp is set to a negative errno.
>>>>>> - */
>>>>>> -static struct sk_buff *
>>>>>> -virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
>>>>>> -               size_t len,
>>>>>> -               u32 src_cid,
>>>>>> -               u32 src_port,
>>>>>> -               u32 dst_cid,
>>>>>> -               u32 dst_port)
>>>>>> -{
>>>>>> -    const size_t skb_len = VIRTIO_VSOCK_SKB_HEADROOM + len;
>>>>>> -    struct virtio_vsock_hdr *hdr;
>>>>>> -    struct sk_buff *skb;
>>>>>> -    void *payload;
>>>>>> -    int err;
>>>>>> +static bool virtio_transport_can_zcopy(struct virtio_vsock_pkt_info *info,
>>>>>> +                       size_t max_to_send)
>>>>>> +{
>>>>>> +    const struct vsock_transport *t;
>>>>>> +    struct iov_iter *iov_iter;
>>>>>>
>>>>>> -    skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>>>>>> -    if (!skb)
>>>>>> -        return NULL;
>>>>>> +    if (!info->msg)
>>>>>> +        return false;
>>>>>>
>>>>>> -    hdr = virtio_vsock_hdr(skb);
>>>>>> -    hdr->type    = cpu_to_le16(info->type);
>>>>>> -    hdr->op        = cpu_to_le16(info->op);
>>>>>> -    hdr->src_cid    = cpu_to_le64(src_cid);
>>>>>> -    hdr->dst_cid    = cpu_to_le64(dst_cid);
>>>>>> -    hdr->src_port    = cpu_to_le32(src_port);
>>>>>> -    hdr->dst_port    = cpu_to_le32(dst_port);
>>>>>> -    hdr->flags    = cpu_to_le32(info->flags);
>>>>>> -    hdr->len    = cpu_to_le32(len);
>>>>>> +    iov_iter = &info->msg->msg_iter;
>>>>>>
>>>>>> -    if (info->msg && len > 0) {
>>>>>> -        payload = skb_put(skb, len);
>>>>>> -        err = memcpy_from_msg(payload, info->msg, len);
>>>>>> -        if (err)
>>>>>> -            goto out;
>>>>>> +    t = vsock_core_get_transport(info->vsk);
>>>>>>
>>>>>> -        if (msg_data_left(info->msg) == 0 &&
>>>>>> -            info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
>>>>>> -            hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
>>>>>> +    if (t->msgzerocopy_check_iov &&
>>>>>> +        !t->msgzerocopy_check_iov(iov_iter))
>>>>>> +        return false;
>>>>
>>>> I'd avoid adding a new transport callback used only internally in virtio
>>>> transports.
>>>
>>> Ok, I see.
>>>
>>>>
>>>> Usually the transport callbacks are used in af_vsock.c, if we need a
>>>> callback just for virtio transports, maybe better to add it in struct
>>>> virtio_vsock_pkt_info or struct virtio_vsock_sock.
>>
>> Hm, may be I just need to move this callback from 'struct vsock_transport' to parent 'struct virtio_transport',
>> after 'send_pkt' callback. In this case:
>> 1) AF_VSOCK part is not touched.
>> 2) This callback stays in 'virtio_transport.c' and is set also in this file.
>>   vhost and loopback are unchanged - only 'send_pkt' still enabled in both
>>   files for these two transports.
> 
> Yep, this could also work!
> 
> Stefano

Great! I'll send this implementation when this patchset for MSG_PEEK will be merged
to net-next as both conflicts with each other.

https://lore.kernel.org/netdev/20230726060150-mutt-send-email-mst@kernel.org/T/#m56f3b850361a412735616145162d2d9df25f6350

Thanks, Arseniy

> 
>>
>> Thanks, Arseniy
>>
>>>>
>>>> Maybe better the last one so we don't have to allocate pointer space
>>>> for each packet and you should reach it via info.
>>>
>>> Ok, thanks, I'll try this way
>>>
>>> Thanks, Arseniy
>>>
>>>>
>>>> Thanks,
>>>> Stefano
>>>>
>>>>>>
>>>>>> -            if (info->msg->msg_flags & MSG_EOR)
>>>>>> -                hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
>>>>>> -        }
>>>>>> +    /* Data is simple buffer. */
>>>>>> +    if (iter_is_ubuf(iov_iter))
>>>>>> +        return true;
>>>>>> +
>>>>>> +    if (!iter_is_iovec(iov_iter))
>>>>>> +        return false;
>>>>>> +
>>>>>> +    if (iov_iter->iov_offset)
>>>>>> +        return false;
>>>>>> +
>>>>>> +    /* We can't send whole iov. */
>>>>>> +    if (iov_iter->count > max_to_send)
>>>>>> +        return false;
>>>>>> +
>>>>>> +    return true;
>>>>>> +}
>>>>>> +
>>>>>> +static int virtio_transport_init_zcopy_skb(struct vsock_sock *vsk,
>>>>>> +                       struct sk_buff *skb,
>>>>>> +                       struct msghdr *msg,
>>>>>> +                       bool zerocopy)
>>>>>> +{
>>>>>> +    struct ubuf_info *uarg;
>>>>>> +
>>>>>> +    if (msg->msg_ubuf) {
>>>>>> +        uarg = msg->msg_ubuf;
>>>>>> +        net_zcopy_get(uarg);
>>>>>> +    } else {
>>>>>> +        struct iov_iter *iter = &msg->msg_iter;
>>>>>> +        struct ubuf_info_msgzc *uarg_zc;
>>>>>> +        int len;
>>>>>> +
>>>>>> +        /* Only ITER_IOVEC or ITER_UBUF are allowed and
>>>>>> +         * checked before.
>>>>>> +         */
>>>>>> +        if (iter_is_iovec(iter))
>>>>>> +            len = iov_length(iter->__iov, iter->nr_segs);
>>>>>> +        else
>>>>>> +            len = iter->count;
>>>>>> +
>>>>>> +        uarg = msg_zerocopy_realloc(sk_vsock(vsk),
>>>>>> +                        len,
>>>>>> +                        NULL);
>>>>>> +        if (!uarg)
>>>>>> +            return -1;
>>>>>> +
>>>>>> +        uarg_zc = uarg_to_msgzc(uarg);
>>>>>> +        uarg_zc->zerocopy = zerocopy ? 1 : 0;
>>>>>>      }
>>>>>>
>>>>>> -    if (info->reply)
>>>>>> -        virtio_vsock_skb_set_reply(skb);
>>>>>> +    skb_zcopy_init(skb, uarg);
>>>>>>
>>>>>> -    trace_virtio_transport_alloc_pkt(src_cid, src_port,
>>>>>> -                     dst_cid, dst_port,
>>>>>> -                     len,
>>>>>> -                     info->type,
>>>>>> -                     info->op,
>>>>>> -                     info->flags);
>>>>>> +    return 0;
>>>>>> +}
>>>>>>
>>>>>> -    if (info->vsk && !skb_set_owner_sk_safe(skb, sk_vsock(info->vsk))) {
>>>>>> -        WARN_ONCE(1, "failed to allocate skb on vsock socket with sk_refcnt == 0\n");
>>>>>> -        goto out;
>>>>>> +static int virtio_transport_fill_skb(struct sk_buff *skb,
>>>>>> +                     struct virtio_vsock_pkt_info *info,
>>>>>> +                     size_t len,
>>>>>> +                     bool zcopy)
>>>>>> +{
>>>>>> +    if (zcopy) {
>>>>>> +        return __zerocopy_sg_from_iter(info->msg, NULL, skb,
>>>>>> +                          &info->msg->msg_iter,
>>>>>> +                          len);
>>>>>> +    } else {
>>>>>> +        void *payload;
>>>>>> +        int err;
>>>>>> +
>>>>>> +        payload = skb_put(skb, len);
>>>>>> +        err = memcpy_from_msg(payload, info->msg, len);
>>>>>> +        if (err)
>>>>>> +            return -1;
>>>>>> +
>>>>>> +        if (msg_data_left(info->msg))
>>>>>> +            return 0;
>>>>>> +
>>>>>> +        return 0;
>>>>>>      }
>>>>>> +}
>>>>>>
>>>>>> -    return skb;
>>>>>> +static void virtio_transport_init_hdr(struct sk_buff *skb,
>>>>>> +                      struct virtio_vsock_pkt_info *info,
>>>>>> +                      u32 src_cid,
>>>>>> +                      u32 src_port,
>>>>>> +                      u32 dst_cid,
>>>>>> +                      u32 dst_port,
>>>>>> +                      size_t len)
>>>>>> +{
>>>>>> +    struct virtio_vsock_hdr *hdr;
>>>>>>
>>>>>> -out:
>>>>>> -    kfree_skb(skb);
>>>>>> -    return NULL;
>>>>>> +    hdr = virtio_vsock_hdr(skb);
>>>>>> +    hdr->type    = cpu_to_le16(info->type);
>>>>>> +    hdr->op        = cpu_to_le16(info->op);
>>>>>> +    hdr->src_cid    = cpu_to_le64(src_cid);
>>>>>> +    hdr->dst_cid    = cpu_to_le64(dst_cid);
>>>>>> +    hdr->src_port    = cpu_to_le32(src_port);
>>>>>> +    hdr->dst_port    = cpu_to_le32(dst_port);
>>>>>> +    hdr->flags    = cpu_to_le32(info->flags);
>>>>>> +    hdr->len    = cpu_to_le32(len);
>>>>>>  }
>>>>>>
>>>>>>  static void virtio_transport_copy_nonlinear_skb(const struct sk_buff *skb,
>>>>>> @@ -214,6 +263,70 @@ static u16 virtio_transport_get_type(struct sock *sk)
>>>>>>          return VIRTIO_VSOCK_TYPE_SEQPACKET;
>>>>>>  }
>>>>>>
>>>>>> +static struct sk_buff *virtio_transport_alloc_skb(struct vsock_sock *vsk,
>>>>>> +                          struct virtio_vsock_pkt_info *info,
>>>>>> +                          size_t payload_len,
>>>>>> +                          bool zcopy,
>>>>>> +                          u32 src_cid,
>>>>>> +                          u32 src_port,
>>>>>> +                          u32 dst_cid,
>>>>>> +                          u32 dst_port)
>>>>>> +{
>>>>>> +    struct sk_buff *skb;
>>>>>> +    size_t skb_len;
>>>>>> +
>>>>>> +    skb_len = VIRTIO_VSOCK_SKB_HEADROOM;
>>>>>> +
>>>>>> +    if (!zcopy)
>>>>>> +        skb_len += payload_len;
>>>>>> +
>>>>>> +    skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>>>>>> +    if (!skb)
>>>>>> +        return NULL;
>>>>>> +
>>>>>> +    virtio_transport_init_hdr(skb, info, src_cid, src_port,
>>>>>> +                  dst_cid, dst_port,
>>>>>> +                  payload_len);
>>>>>> +
>>>>>> +    /* Set owner here, because '__zerocopy_sg_from_iter()' uses
>>>>>> +     * owner of skb without check to update 'sk_wmem_alloc'.
>>>>>> +     */
>>>>>> +    if (vsk)
>>>>>> +        skb_set_owner_w(skb, sk_vsock(vsk));
>>>>>> +
>>>>>> +    if (info->msg && payload_len > 0) {
>>>>>> +        int err;
>>>>>> +
>>>>>> +        err = virtio_transport_fill_skb(skb, info, payload_len, zcopy);
>>>>>> +        if (err)
>>>>>> +            goto out;
>>>>>> +
>>>>>> +        if (info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
>>>>>> +            struct virtio_vsock_hdr *hdr = virtio_vsock_hdr(skb);
>>>>>> +
>>>>>> +            hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
>>>>>> +
>>>>>> +            if (info->msg->msg_flags & MSG_EOR)
>>>>>> +                hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
>>>>>> +        }
>>>>>> +    }
>>>>>> +
>>>>>> +    if (info->reply)
>>>>>> +        virtio_vsock_skb_set_reply(skb);
>>>>>> +
>>>>>> +    trace_virtio_transport_alloc_pkt(src_cid, src_port,
>>>>>> +                     dst_cid, dst_port,
>>>>>> +                     payload_len,
>>>>>> +                     info->type,
>>>>>> +                     info->op,
>>>>>> +                     info->flags);
>>>>>> +
>>>>>> +    return skb;
>>>>>> +out:
>>>>>> +    kfree_skb(skb);
>>>>>> +    return NULL;
>>>>>> +}
>>>>>> +
>>>>>>  /* This function can only be used on connecting/connected sockets,
>>>>>>   * since a socket assigned to a transport is required.
>>>>>>   *
>>>>>> @@ -222,10 +335,12 @@ static u16 virtio_transport_get_type(struct sock *sk)
>>>>>>  static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>>>>>                        struct virtio_vsock_pkt_info *info)
>>>>>>  {
>>>>>> +    u32 max_skb_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
>>>>>>      u32 src_cid, src_port, dst_cid, dst_port;
>>>>>>      const struct virtio_transport *t_ops;
>>>>>>      struct virtio_vsock_sock *vvs;
>>>>>>      u32 pkt_len = info->pkt_len;
>>>>>> +    bool can_zcopy = false;
>>>>>>      u32 rest_len;
>>>>>>      int ret;
>>>>>>
>>>>>> @@ -254,15 +369,30 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>>>>>      if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
>>>>>>          return pkt_len;
>>>>>>
>>>>>> +    if (info->msg) {
>>>>>> +        /* If zerocopy is not enabled by 'setsockopt()', we behave as
>>>>>> +         * there is no MSG_ZEROCOPY flag set.
>>>>>> +         */
>>>>>> +        if (!sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY))
>>>>>> +            info->msg->msg_flags &= ~MSG_ZEROCOPY;
>>>>>> +
>>>>>> +        if (info->msg->msg_flags & MSG_ZEROCOPY)
>>>>>> +            can_zcopy = virtio_transport_can_zcopy(info, pkt_len);
>>>>>> +
>>>>>> +        if (can_zcopy)
>>>>>> +            max_skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
>>>>>> +                        (MAX_SKB_FRAGS * PAGE_SIZE));
>>>>>> +    }
>>>>>> +
>>>>>>      rest_len = pkt_len;
>>>>>>
>>>>>>      do {
>>>>>>          struct sk_buff *skb;
>>>>>>          size_t skb_len;
>>>>>>
>>>>>> -        skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE, rest_len);
>>>>>> +        skb_len = min(max_skb_len, rest_len);
>>>>>>
>>>>>> -        skb = virtio_transport_alloc_skb(info, skb_len,
>>>>>> +        skb = virtio_transport_alloc_skb(vsk, info, skb_len, can_zcopy,
>>>>>>                           src_cid, src_port,
>>>>>>                           dst_cid, dst_port);
>>>>>>          if (!skb) {
>>>>>> @@ -270,6 +400,17 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>>>>>              break;
>>>>>>          }
>>>>>>
>>>>>> +        /* This is last skb to send this portion of data. */
>>>>>> +        if (info->msg && info->msg->msg_flags & MSG_ZEROCOPY &&
>>>>>> +            skb_len == rest_len && info->op == VIRTIO_VSOCK_OP_RW) {
>>>>>> +            if (virtio_transport_init_zcopy_skb(vsk, skb,
>>>>>> +                                info->msg,
>>>>>> +                                can_zcopy)) {
>>>>>> +                ret = -ENOMEM;
>>>>>> +                break;
>>>>>> +            }
>>>>>> +        }
>>>>>> +
>>>>>>          virtio_transport_inc_tx_pkt(vvs, skb);
>>>>>>
>>>>>>          ret = t_ops->send_pkt(skb);
>>>>>> @@ -934,7 +1075,7 @@ static int virtio_transport_reset_no_sock(const struct virtio_transport *t,
>>>>>>      if (!t)
>>>>>>          return -ENOTCONN;
>>>>>>
>>>>>> -    reply = virtio_transport_alloc_skb(&info, 0,
>>>>>> +    reply = virtio_transport_alloc_skb(NULL, &info, 0, false,
>>>>>>                         le64_to_cpu(hdr->dst_cid),
>>>>>>                         le32_to_cpu(hdr->dst_port),
>>>>>>                         le64_to_cpu(hdr->src_cid),
>>>>>
>>>>
>>
>