Re: [RFC PATCH v3 05/17] vsock/virtio: MSG_ZEROCOPY flag support

[Arseniy Krasnov]

On 22.05.2023 16:09, Arseniy Krasnov wrote:
> 
> 
> On 22.05.2023 16:11, Simon Horman wrote:
>> On Mon, May 22, 2023 at 10:39:38AM +0300, Arseniy Krasnov wrote:
>>> This adds handling of MSG_ZEROCOPY flag on transmission path: if this
>>> flag is set and zerocopy transmission is possible, 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()'.
>>>
>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
>>> ---
>>>  net/vmw_vsock/virtio_transport_common.c | 305 +++++++++++++++++++-----
>>>  1 file changed, 243 insertions(+), 62 deletions(-)
>>>
>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>> index 9854f48a0544..5acf824afe41 100644
>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>> @@ -37,73 +37,161 @@ 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)
>>> +{
>>> +	struct iov_iter *iov_iter;
>>> +	size_t max_skb_cap;
>>> +	size_t bytes;
>>> +	int i;
>>>  
>>> -	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);
>>> +	if (!(info->flags & MSG_ZEROCOPY) && !info->msg->msg_ubuf)
>>> +		return false;
>>>  
>>> -	if (info->msg && len > 0) {
>>> -		payload = skb_put(skb, len);
>>> -		err = memcpy_from_msg(payload, info->msg, len);
>>> -		if (err)
>>> -			goto out;
>>> +	iov_iter = &info->msg->msg_iter;
>>> +
>>> +	if (iter_is_ubuf(iov_iter)) {
>>> +		if (offset_in_page(iov_iter->ubuf))
>>> +			return false;
>>> +
>>> +		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;
>>> +
>>> +	for (bytes = 0, i = 0; i < iov_iter->nr_segs; i++) {
>>> +		const struct iovec *iovec;
>>> +		int pages_in_elem;
>>> +
>>> +		iovec = &iov_iter->__iov[i];
>>> +
>>> +		/* Base must be page aligned. */
>>> +		if (offset_in_page(iovec->iov_base))
>>> +			return false;
>>>  
>>> -		if (msg_data_left(info->msg) == 0 &&
>>> -		    info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
>>> -			hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
>>> +		/* Only last element could have non page aligned size. */
>>> +		if (i != (iov_iter->nr_segs - 1)) {
>>> +			if (offset_in_page(iovec->iov_len))
>>> +				return false;
>>>  
>>> -			if (info->msg->msg_flags & MSG_EOR)
>>> -				hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
>>> +			pages_in_elem = iovec->iov_len >> PAGE_SHIFT;
>>> +		} else {
>>> +			pages_in_elem = round_up(iovec->iov_len, PAGE_SIZE);
>>> +			pages_in_elem >>= PAGE_SHIFT;
>>>  		}
>>> +
>>> +		bytes += (pages_in_elem * PAGE_SIZE);

^^^
This alignment (base and length) checks are not needed, because virtio supports unaligned buffers. I'll remove
it in v4.

Thanks, Arseniy

>>>  	}
>>
>> Hi Arseniy,
>>
>> bytes is set but the loop above, but seems otherwise unused in this function.
>>
>>>  
>>> -	if (info->reply)
>>> -		virtio_vsock_skb_set_reply(skb);
>>> +	/* How many bytes we can pack to single skb. Maximum packet
>>> +	 * buffer size is needed to allow vhost handle such packets,
>>> +	 * otherwise they will be dropped.
>>> +	 */
>>> +	max_skb_cap = min((unsigned int)(MAX_SKB_FRAGS * PAGE_SIZE),
>>> +			  (unsigned int)VIRTIO_VSOCK_MAX_PKT_BUF_SIZE);
>>
>> Likewise, max_skb_cap seems to be set but unused in this function.
>>
> 
> Exactly! Seems I forgot to remove it since v2. Thanks for this and above!
> 
>>>  
>>> -	trace_virtio_transport_alloc_pkt(src_cid, src_port,
>>> -					 dst_cid, dst_port,
>>> -					 len,
>>> -					 info->type,
>>> -					 info->op,
>>> -					 info->flags);
>>> +	return true;
>>> +}
>>
>> ...