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Message-ID: <e15d6221-10e7-2234-0ea5-09203734b679@sberdevices.ru>
Date:   Tue, 25 Jul 2023 16:35:12 +0300
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Subject: Re: [PATCH net-next v3 4/4] vsock/virtio: MSG_ZEROCOPY flag support
Content-Language: en-US
To:     "Michael S. Tsirkin" <mst@redhat.com>
CC:     Stefan Hajnoczi <stefanha@redhat.com>,
        Stefano Garzarella <sgarzare@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@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>
 <20230725042544-mutt-send-email-mst@kernel.org>
 <8c2343fc-0517-8436-7abb-a2f69881cea7@sberdevices.ru>
 <20230725092139-mutt-send-email-mst@kernel.org>
 <3c2a8816-30de-a543-751e-db7cbac82260@sberdevices.ru>
 <20230725093552-mutt-send-email-mst@kernel.org>
From:   Arseniy Krasnov <avkrasnov@sberdevices.ru>
In-Reply-To: <20230725093552-mutt-send-email-mst@kernel.org>
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Precedence: bulk


On 25.07.2023 16:36, Michael S. Tsirkin wrote:
> On Tue, Jul 25, 2023 at 04:28:14PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 25.07.2023 16:22, Michael S. Tsirkin wrote:
>>> On Tue, Jul 25, 2023 at 04:04:13PM +0300, Arseniy Krasnov wrote:
>>>>
>>>>
>>>> On 25.07.2023 14:50, Michael S. Tsirkin 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) {
>>>>>>> +		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.
>>>>>>
>>>>>> Thanks, Arseniy
>>>>>
>>>>> Actually the reverse. With indirect you are limited to num_max.
>>>>> Without you are limited to whatever space is left in the
>>>>> queue (which you did not check here, so you should).
>>>>
>>>> I mean that with indirect, we only need one buffer, and we can just wait
>>>> for enough space - for this single buffer ( as we discussed a little bit before).
>>>> But if indirect buffer is not supported - we need that whole packet must fit
>>>> to the size of tx queue - otherwise it never be transmitted.
>>>>
>>>> Thanks, Arseniy
>>>
>>>
>>> yes but according to virtio spec it's illegal to add s/g that is bigger
>>> than queue size.
>>
>> Aah, so even in case of indirect buffers feature, buffer descriptors stored in memory
>> pointed by indirect buffer must be accounted against queue size ?
>>
>> Thanks, Arseniy
> 
> a single indirect buffer can't exceed vq size.

I see, so i guess right way is to compare length of data "in buffers"
again 'num_max', as it is implemented. And we should not refer to whether
vq support indirect buffering or not.

Thanks, Arseniy

> 
> 
>>>
>>>>>
>>>>>
>>>>>>> +			res = true;
>>>>>>> +	}
>>>>>>> +
>>>>>>> +	rcu_read_unlock();
>>>>>
>>>>> Just curious:
>>>>> is the point of all this RCU dance to allow vsock
>>>>> to change from under us? then why is it ok to
>>>>> have it change? the virtio_transport_msgzerocopy_check_iov
>>>>> will then refer to the old vsock ...
>>>>>
>>>>>
>>>>>>> +
>>>>>>> +	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;
>>>>>>>  
>>>>>>> -			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),
>>>>>
>>>
>