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[79.34.249.199]) by smtp.gmail.com with ESMTPSA id d22sm2187908ejz.79.2021.04.14.00.07.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Apr 2021 00:07:23 -0700 (PDT) Date: Wed, 14 Apr 2021 09:07:21 +0200 From: Stefano Garzarella To: "Jiang Wang ." Subject: Re: [RFC v2] virtio-vsock: add description for datagram type Message-ID: <20210414070721.cdunbel6fvr7ieaf@steredhat> References: <20210412142133.t44pn5pjy6fdcvk4@steredhat> <20210413125853.2dkldmp23vkkc74c@steredhat> <20210413091251-mutt-send-email-mst@kernel.org> <20210413133852.ebkrlbyetiqu4uje@steredhat> <20210413094722-mutt-send-email-mst@kernel.org> <20210413140351.6vmffxqnj4azpyzx@steredhat> <20210413155635-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=sgarzare@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: cong.wang@bytedance.com, Xiongchun Duan , "Michael S. Tsirkin" , cohuck@redhat.com, virtualization@lists.linux-foundation.org, xieyongji@bytedance.com, Stefan Hajnoczi , asias@redhat.com, Arseny Krasnov X-BeenThere: virtualization@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux virtualization List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: virtualization-bounces@lists.linux-foundation.org Sender: "Virtualization" On Tue, Apr 13, 2021 at 03:00:50PM -0700, Jiang Wang . wrote: >On Tue, Apr 13, 2021 at 12:58 PM Michael S. Tsirkin wrote: >> >> On Tue, Apr 13, 2021 at 04:03:51PM +0200, Stefano Garzarella wrote: >> > On Tue, Apr 13, 2021 at 09:50:45AM -0400, Michael S. Tsirkin wrote: >> > > On Tue, Apr 13, 2021 at 03:38:52PM +0200, Stefano Garzarella wrote: >> > > > On Tue, Apr 13, 2021 at 09:16:50AM -0400, Michael S. Tsirkin wrote: >> > > > > On Tue, Apr 13, 2021 at 02:58:53PM +0200, Stefano Garzarella wrote: >> > > > > > On Mon, Apr 12, 2021 at 03:42:23PM -0700, Jiang Wang . wrote: >> > > > > > > On Mon, Apr 12, 2021 at 7:21 AM Stefano Garzarella wrote: >> > > > > > > > >> > > > > > > > On Mon, Apr 12, 2021 at 02:50:17PM +0100, Stefan Hajnoczi wrote: >> > > > > > > > >On Thu, Apr 01, 2021 at 04:36:02AM +0000, jiang.wang wrote: >> > > > > > > > >> Add supports for datagram type for virtio-vsock. Datagram >> > > > > > > > >> sockets are connectionless and unreliable. To avoid contention >> > > > > > > > >> with stream and other sockets, add two more virtqueues and >> > > > > > > > >> a new feature bit to identify if those two new queues exist or not. >> > > > > > > > >> >> > > > > > > > >> Also add descriptions for resource management of datagram, which >> > > > > > > > >> does not use the existing credit update mechanism associated with >> > > > > > > > >> stream sockets. >> > > > > > > > >> >> > > > > > > > >> Signed-off-by: Jiang Wang >> > > > > > > > >> --- >> > > > > > > > >> V2 addressed the comments for the previous version. >> > > > > > > > >> >> > > > > > > > >> virtio-vsock.tex | 62 +++++++++++++++++++++++++++++++++++++++++++++++--------- >> > > > > > > > >> 1 file changed, 52 insertions(+), 10 deletions(-) >> > > > > > > > >> >> > > > > > > > >> diff --git a/virtio-vsock.tex b/virtio-vsock.tex >> > > > > > > > >> index da7e641..62c12e0 100644 >> > > > > > > > >> --- a/virtio-vsock.tex >> > > > > > > > >> +++ b/virtio-vsock.tex >> > > > > > > > >> @@ -11,12 +11,25 @@ \subsection{Virtqueues}\label{sec:Device Types / Socket Device / Virtqueues} >> > > > > > > > >> \begin{description} >> > > > > > > > >> \item[0] rx >> > > > > > > > >> \item[1] tx >> > > > > > > > >> +\item[2] datagram rx >> > > > > > > > >> +\item[3] datagram tx >> > > > > > > > >> +\item[4] event >> > > > > > > > >> +\end{description} >> > > > > > > > >> +The virtio socket device uses 5 queues if feature bit VIRTIO_VSOCK_F_DRGAM is set. Otherwise, it >> > > > > > > > >> +only uses 3 queues, as the following. Rx and tx queues are always used for stream sockets. >> > > > > > > > >> + >> > > > > > > > >> +\begin{description} >> > > > > > > > >> +\item[0] rx >> > > > > > > > >> +\item[1] tx >> > > > > > > > >> \item[2] event >> > > > > > > > >> \end{description} >> > > > > > > > >> >> > > > > > > > > >> > > > > > > > >I suggest renaming "rx" and "tx" to "stream rx" and "stream tx" >> > > > > > > > >virtqueues and also adding this: >> > > > > > > > > >> > > > > > > > > When behavior differs between stream and datagram rx/tx virtqueues >> > > > > > > > > their full names are used. Common behavior is simply described in >> > > > > > > > > terms of rx/tx virtqueues and applies to both stream and datagram >> > > > > > > > > virtqueues. >> > > > > > > > > >> > > > > > > > >This way you won't need to duplicate portions of the spec that deal with >> > > > > > > > >populating the virtqueues, for example. >> > > > > > > > > >> > > > > > > > >It's also clearer to use a full name for stream rx/tx virtqueues instead >> > > > > > > > >of calling them rx/tx virtqueues now that we have datagram rx/tx >> > > > > > > > >virtqueues. >> > > > > > > > > >> > > > > > > > >> + >> > > > > > > > >> \subsection{Feature bits}\label{sec:Device Types / Socket Device / Feature bits} >> > > > > > > > >> >> > > > > > > > >> -There are currently no feature bits defined for this device. >> > > > > > > > >> +\begin{description} >> > > > > > > > >> +\item[VIRTIO_VSOCK_F_DGRAM (0)] Device has support for datagram socket type. >> > > > > > > > >> +\end{description} >> > > > > > > > >> >> > > > > > > > >> \subsection{Device configuration layout}\label{sec:Device Types / Socket Device / Device configuration layout} >> > > > > > > > >> >> > > > > > > > >> @@ -107,6 +120,9 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op >> > > > > > > > >> >> > > > > > > > >> \subsubsection{Virtqueue Flow Control}\label{sec:Device Types / Socket Device / Device Operation / Virtqueue Flow Control} >> > > > > > > > >> >> > > > > > > > >> +Flow control applies to stream sockets; datagram sockets do not have >> > > > > > > > >> +flow control. >> > > > > > > > >> + >> > > > > > > > >> The tx virtqueue carries packets initiated by applications and replies to >> > > > > > > > >> received packets. The rx virtqueue carries packets initiated by the device and >> > > > > > > > >> replies to previously transmitted packets. >> > > > > > > > >> @@ -140,12 +156,15 @@ \subsubsection{Addressing}\label{sec:Device Types / Socket Device / Device Opera >> > > > > > > > >> consists of a (cid, port number) tuple. The header fields used for this are >> > > > > > > > >> \field{src_cid}, \field{src_port}, \field{dst_cid}, and \field{dst_port}. >> > > > > > > > >> >> > > > > > > > >> -Currently only stream sockets are supported. \field{type} is 1 for stream >> > > > > > > > >> -socket types. >> > > > > > > > >> +Currently stream and datagram (dgram) sockets are supported. \field{type} is 1 for stream >> > > > > > > > >> +socket types. \field{type} is 3 for dgram socket types. >> > > > > > > > >> >> > > > > > > > >> Stream sockets provide in-order, guaranteed, connection-oriented delivery >> > > > > > > > >> without message boundaries. >> > > > > > > > >> >> > > > > > > > >> +Datagram sockets provide connectionless unreliable messages of >> > > > > > > > >> +a fixed maximum length. >> > > > > > > > > >> > > > > > > > >Plus unordered (?) and with message boundaries. In other words: >> > > > > > > > > >> > > > > > > > > Datagram sockets provide unordered, unreliable, connectionless message >> > > > > > > > > with message boundaries and a fixed maximum length. >> > > > > > > > > >> > > > > > > > >I didn't think of the fixed maximum length aspect before. I guess the >> > > > > > > > >intention is that the rx buffer size is the message size limit? That's >> > > > > > > > >different from UDP messages, which can be fragmented into multiple IP >> > > > > > > > >packets and can be larger than 64KiB: >> > > > > > > > >https://en.wikipedia.org/wiki/User_Datagram_Protocol#UDP_datagram_structure >> > > > > > > > > >> > > > > > > > >Is it possible to support large datagram messages in vsock? I'm a little >> > > > > > > > >concerned that applications that run successfully over UDP will not be >> > > > > > > > >portable if vsock has this limitation because it would impose extra >> > > > > > > > >message boundaries that the application protocol might not tolerate. >> > > > > > > > >> > > > > > > > Maybe we can reuse the same approach Arseny is using for SEQPACKET. >> > > > > > > > Fragment the packets according to the buffers in the virtqueue and set >> > > > > > > > the EOR flag to indicate the last packet in the message. >> > > > > > > > >> > > > > > > Agree. Another option is to use the ones for skb since we may need to >> > > > > > > use skbs for multiple transport support anyway. >> > > > > > > >> > > > > > >> > > > > > The important thing I think is to have a single flag in virtio-vsock that >> > > > > > identifies pretty much the same thing: this is the last fragment of a series >> > > > > > to rebuild a packet. >> > > > > > >> > > > > > We should reuse the same flag for DGRAM and SEQPACKET. >> > > > > > >> > > > > > Thanks, >> > > > > > Stefano >> > > > > >> > > > > Well DGRAM can drop data so I wonder whether it can work ... >> > > > > >> > > > >> > > > Yep, this is true, but the channel should not be losing packets, so if the >> > > > receiver discards packets, it knows that it must then discard all of them >> > > > until the EOR. >> > > >> > > That is not so easy - they can come mixed up from multiple sources. >> > >> > I think we can prevent mixing because virtuqueue is point to point and its >> > use is not thread safe, so the access (in the same peer) is already >> > serialized. >> > In the end the packet would be fragmented only before copying it to the >> > virtuqueue. >> > >> > But maybe I missed something... >> >> Well I ask what's the point of fragmenting then. I assume it's so we >> can pass huge messages around so you can't keep locks ... >> >I have a related question. How to determine the suitable size of a >fragment? Unlike stream or seqpacket sockets, the datagram >sockets do not know the available recv buf of the listener. Eg.if the listener >free recv buf is 64KB, and the sender wants to send a 256KB packet, how >does the sender know what the fragment size is? One option is to >use some default fragment size and try to send those fragments. But those >fragments could still be dropped by the receiver. I guess it depends on the virtqueue buffers. For the driver I don't think we need to fragment, we can queue the whole packet. For device it depends on size of buffers in virtqueue, which determine size of fragment. As I said in previous email, maybe we can allocate 64K buffers in virtqueue RX, and allow user at most 64K messages, avoiding fragmentation completely. > >> >> > > Sure linux net core does this but with fragmentation added in, >> > > I start wondering whether you are beginning to reinvent the net stack >> > > ... >> > >> > No, I hope not :-), in the end our advantage is that we have a channel that >> > doesn't lose packets, so I guess we can make assumptions that the network >> > stack can't. >> > >> > Thanks, >> > Stefano >> >> I still don't know how will credit accounting work for datagram, >> but proposals I saw seem to actually lose packets ... > >Yes, I agree that is a problem. In my mind, the accounting is >different from the credit mechanism used by the stream sockets. >The accounting is for the sender side only and the packets may >still be dropped at the receiving side if it is larger than the >available recv buf. > I don't think it's a problem if the receiver decide to drop, the important thing is that it drops all the fragments. Thanks, Stefano _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization