Subject: Re: [RFC PATCH v5 11/19] virtio/vsock: dequeue callback for SOCK_SEQPACKET

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 24 Feb 2021 01:41:56 -0500

On Wed, Feb 24, 2021 at 08:07:48AM +0300, Arseny Krasnov wrote:
> 
> On 23.02.2021 17:17, Michael S. Tsirkin wrote:
> > On Thu, Feb 18, 2021 at 08:39:37AM +0300, Arseny Krasnov wrote:
> >> This adds transport callback and it's logic for SEQPACKET dequeue.
> >> Callback fetches RW packets from rx queue of socket until whole record
> >> is copied(if user's buffer is full, user is not woken up). This is done
> >> to not stall sender, because if we wake up user and it leaves syscall,
> >> nobody will send credit update for rest of record, and sender will wait
> >> for next enter of read syscall at receiver's side. So if user buffer is
> >> full, we just send credit update and drop data. If during copy SEQ_BEGIN
> >> was found(and not all data was copied), copying is restarted by reset
> >> user's iov iterator(previous unfinished data is dropped).
> >>
> >> Signed-off-by: Arseny Krasnov <[email-masked]>
> >> ---
> >>  include/linux/virtio_vsock.h            |  10 +++
> >>  include/uapi/linux/virtio_vsock.h       |  16 ++++
> >>  net/vmw_vsock/virtio_transport_common.c | 114 ++++++++++++++++++++++++
> >>  3 files changed, 140 insertions(+)
> >>
> >> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> >> index dc636b727179..003d06ae4a85 100644
> >> --- a/include/linux/virtio_vsock.h
> >> +++ b/include/linux/virtio_vsock.h
> >> @@ -36,6 +36,11 @@ struct virtio_vsock_sock {
> >>  	u32 rx_bytes;
> >>  	u32 buf_alloc;
> >>  	struct list_head rx_queue;
> >> +
> >> +	/* For SOCK_SEQPACKET */
> >> +	u32 user_read_seq_len;
> >> +	u32 user_read_copied;
> >> +	u32 curr_rx_msg_cnt;
> >
> > wrap these in a struct to make it's clearer they
> > are related?
> Ack
> >
> >>  };
> >>  
> >>  struct virtio_vsock_pkt {
> >> @@ -80,6 +85,11 @@ virtio_transport_dgram_dequeue(struct vsock_sock *vsk,
> >>  			       struct msghdr *msg,
> >>  			       size_t len, int flags);
> >>  
> >> +int
> >> +virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
> >> +				   struct msghdr *msg,
> >> +				   int flags,
> >> +				   bool *msg_ready);
> >>  s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
> >>  s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);
> >>  
> >> diff --git a/include/uapi/linux/virtio_vsock.h b/include/uapi/linux/virtio_vsock.h
> >> index 1d57ed3d84d2..cf9c165e5cca 100644
> >> --- a/include/uapi/linux/virtio_vsock.h
> >> +++ b/include/uapi/linux/virtio_vsock.h
> >> @@ -63,8 +63,14 @@ struct virtio_vsock_hdr {
> >>  	__le32	fwd_cnt;
> >>  } __attribute__((packed));
> >>  
> >> +struct virtio_vsock_seq_hdr {
> >> +	__le32  msg_cnt;
> >> +	__le32  msg_len;
> >> +} __attribute__((packed));
> >> +
> >>  enum virtio_vsock_type {
> >>  	VIRTIO_VSOCK_TYPE_STREAM = 1,
> >> +	VIRTIO_VSOCK_TYPE_SEQPACKET = 2,
> >>  };
> >>  
> >>  enum virtio_vsock_op {
> >> @@ -83,6 +89,11 @@ enum virtio_vsock_op {
> >>  	VIRTIO_VSOCK_OP_CREDIT_UPDATE = 6,
> >>  	/* Request the peer to send the credit info to us */
> >>  	VIRTIO_VSOCK_OP_CREDIT_REQUEST = 7,
> >> +
> >> +	/* Record begin for SOCK_SEQPACKET */
> >> +	VIRTIO_VSOCK_OP_SEQ_BEGIN = 8,
> >> +	/* Record end for SOCK_SEQPACKET */
> >> +	VIRTIO_VSOCK_OP_SEQ_END = 9,
> >>  };
> >>  
> >>  /* VIRTIO_VSOCK_OP_SHUTDOWN flags values */
> >> @@ -91,4 +102,9 @@ enum virtio_vsock_shutdown {
> >>  	VIRTIO_VSOCK_SHUTDOWN_SEND = 2,
> >>  };
> >>  
> >> +/* VIRTIO_VSOCK_OP_RW flags values */
> >> +enum virtio_vsock_rw {
> >> +	VIRTIO_VSOCK_RW_EOR = 1,
> >> +};
> >> +
> >>  #endif /* _UAPI_LINUX_VIRTIO_VSOCK_H */
> > Probably a good idea to also have a feature bit gating
> > this functionality.
> 
> IIUC this also requires some qemu patch, because in current
> 
> implementation of vsock device in qemu, there is no 'set_features'
> 
> callback for such device. This callback will handle guest's write
> 
> to feature register, by calling vhost kernel backend, where this
> 
> bit will be processed by host.

Well patching userspace to make use of a kernel feature
is par for the course, isn't it?

> 
> IMHO I'm not sure that SEQPACKET support needs feature
> 
> bit - it is just two new ops for virtio vsock protocol, and from point
> 
> of view of virtio device it is same as STREAM. May be it is needed
> 
> for cases when client tries to connect to server which doesn't support
> 
> SEQPACKET, so without bit result will be "Connection reset by peer",
> 
> and with such bit client will know that server doesn't support it and
> 
> 'socket(SOCK_SEQPACKET)' will return error?

Yes, a better error handling would be one reason to do it like this.

-- 
MST