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Subject: Re: [virtio-comment] Re: [PATCH v1] docs/vhost-user: extend the vhost-user protocol to support the vhost-pci based inter-vm communication
On 11/08/2016 08:17 PM, Marc-André Lureau wrote:
> > Message Specification > --------------------- > > Note that all numbers are in the machine native byte order. A > vhost-user message > -consists of 3 header fields and a payload: > +consists of 4 header fields and a payload: > > ------------------------------------- > -| request | flags | size | payload | > ------------------------------------- > +---------------------------------------------- > +| request | flags | conn_id | size | payload | > +---------------------------------------------- > > * Request: 32-bit type of the request > * Flags: 32-bit bit field: > - Lower 2 bits are the version (currently 0x01) > - - Bit 2 is the reply flag - needs to be sent on each reply > from the slave > + - Bit 2 is the reply flag - needs to be sent on each reply > - Bit 3 is the need_reply flag - see > VHOST_USER_PROTOCOL_F_REPLY_ACK for > details. > + * Conn_id: 64-bit connection id to indentify a client socket > connection. It is > + introduced in version 0x02 to support the > "1-server-N-client" model > + and an asynchronous client read implementation. The > connection id, > + 0xFFFFFFFFFFFFFFFF, is used by an anonymous client > (e.g. a client who > + has not got its connection id from the server in the > initial talk) > > > I don't understand why you need a connection id, on each message. > What's the purpose? Since the communication is unicast, a single > message should be enough. Sure, please let me explain more: The QEMU socket is going to be upgraded to support 1 server socket being connected by multiple client sockets (I've made patches to achieve this). In other words, here, multiple masters will connect to one slave, and the slave creates a vhost-pci device for each master after receiving the necessary message info. The slave needs to know which master it is talking to when receiving a message, as it maintains multiple connections at the same time.You should be able to identify each connection in the slave (as a socket server), without a need for connection id: connected sockets are independent from each others.
Yes, that's doable. But why couldn't we do it from the protocol layer? I think it will be easier.
Please check below my thoughts about the implementation if we do it in the slave:
The interface for receiving a msg is - tcp_chr_read(QIOChannel *chan, GIOCondition cond, void *opaque)
QIOChannel is the one that we can use to identify the master connection who sends this msg (the socket server now has an array of QIOChannel, ioc[MAX_CLIENTS]). Everytime a msg is received, the tcp_chr_read() needs to compare *chan and the ioc[] array, to find out the id (indexed into the ioc[]), and passes the id to qemu_chr_be_write(), and all the way down to the final slave handler where the msg is parsed and handled. This needs modifications to the existing APIs, for example, the mentioned qemu_chr_be_write() will need one more parameter, "id". This will not be compatible with the existing implementation, because all other implementations which invoke qemu_chr_be_write() will need to be patched to use the new qemu_chr_be_write(,"id",).
> * Size - 32-bit size of the payload > > > @@ -97,6 +106,13 @@ Depending on the request type, payload can be: > log offset: offset from start of supplied file descriptor > where logging starts (i.e. where guest address 0 would be > logged) > > +* Device info > + -------------------- > + | virito id | uuid | > + -------------------- > + Virtio id: 16-bit virtio id of the device > + UUID: 128-bit UUID to identify the QEMU instance that creates > the device > + > > > I wonder if UUID should be a different message. > We can make uuid another message if it has other usages. Do you see any other usages of uuid?Allows to associate data/configuration with a particular VM, in a multi-master/single-slave scenario. But tbh, I don't see how this is necessary, I can imagine solving this differently (having different connection address per vm for ex).
Using connection addresses, how could you know if the two connections are from the same VM?
I would like to understand your use case.
Here is an example of the use case:VM1 has two master connections (connection X and Y) and VM2 has 1 master connection (Z). X,Y,Z - each has a connection id. But X and Y send the same uuid, uuid1, to the slave, and Z sends uuid2 to the slave. In this way, the slave know X and Y are the two connections from the same VM, and Z is a connection from a different VM.
For connection Y, the vhost-pci device will be created in a way which does not need the driver to map the memory, since it has already been mapped by device X from the same VM.
> [ Also see the section on REPLY_ACK protocol extension. ] > > +Currently, the communication also supports the Slave (server) > sending messages > +to the Master (client). Here is a list of them: > + * VHOST_USER_SET_FEATURES > > + * VHOST_USER_SET_PEER_CONNECTION (the serve may actively request > to disconnect > + with the client) > > > Oh, you are making the communication bidirectional? This is a > fundamental change in the protocol. This may be difficult to implement > in qemu, since the communication in synchronous, a request expects an > immediate reply, if it gets back a request (from the slave) in the > middle, it will fail. > Not really. Adding the above two doesn't affect the existing synchronous read() messages (basically, those VHOST_USER_GET_xx messages). Like VHOST_USER_SET_FEATURES, the _SET_ messages don't need a reply. Here, we just make the slave capable of actively sending messages to the master.Yes, that's the trouble. At any time the Master may send a request and expects an immediate reply. There is a race of getting a request from the Slave in the middle with your proposed change. I'd rather avoid making the request bidirectionnal if possible. (I proposed a second channel for Slave->Master request in the past: https://lists.gnu.org/archive/html/qemu-devel/2016-04/msg00095.html)
If the message that the slave got has a different "request" field value, it simply drops it and re-read again. The implementation is not complex also, please see the change example to vhost_user_get_u64() below:
if (vhost_user_write(dev, &msg_w, NULL, 0) < 0) { return -1; } retry: if (vhost_user_read(dev, &msg_r) < 0) { return -1; } if (msg_r.request != msg_w.request) goto retry;On the other side, the slave's request to the master is dropped due to the race. This race can be solved in the protocol layer - let the _SET_ request ask for an ACK, if no ACK is received, re-sent it. Also, this kind of race should be very rare in real usage.
> > +This request should be sent only when VHOST_USER_PROTOCOL_F_VHOST_PCI > has... > > +* VHOST_USER_SET_DEV_INFO > + > + Id: 21 > + Equivalent ioctl: N/A > + Master payload: dev info > + > + The client sends the producer device info to the server. > > > "Master sends producer device info to the Slave" works, no? Yes, it works. The current dev info only contains a "virtio id" field (assume we'll take uuid out as a separate message), which tells the slave if it is a net, scsi, console or else. do you see any issue? > > Could we guarantee this message is sent before SET_VRING*? Why do we need to guarantee this?It would simplify the protocol to have expectations on when messages come. In particular, an early message with devinfo would allow to check/pre-configure the Slave for a particular device. Also VHOST_USER_SET_DEV_INFO should probably be unique (don't allow a device to be reconfigured)
Yes, it is sent in an early age of the vhost-user protocol interaction. It's implemented to be sent right after sending the VHOST_USER_SET_PROTOCOL_FEATURES msg. On the slave side, when it receives SET_DEV_INFO, it pre-configures the device in a table entry (as mentioned before, a device will be created from the table entry at a later stage of the protocol interaction).
I think it should be the implementation logic, like VHOST_USER_SET_PROTOCOL_FEATURES. why do we need to add a guarantee in the protocol to specify the order?
> > + This request should be sent only when > VHOST_USER_PROTOCOL_F_VHOST_PCI has > + been negotiated. > + > > > I think this message could be useful for other purposes than > vhost-pci, thus I would give it its own flag. Could you please give an example of other usage? Thanks.You could have a Slave that implements various devices, and pick the corresponding one dynamically (we already have implementations for net/input/gpu/scsi...)
If I understand the example correctly, the various devices still belongs to the vhost-pci series - in the future we would have vhost-pci-net, vhost-pci-scsi, vhost-pci-gpu etc. If that's the case, we may still use the VHOST_PCI flag.
> > +* VHOST_USER_SET_PEER_CONNECTION > + > + Id: 22 > + Equivalent ioctl: N/A > + Master payload: u64 > + > + The producer device requests to connect or disconnect to > the consumer device. > > > producer->Master, consummer->Slave > > How does it interact with SET_VRING_ENABLE? It's independent of SET_VRING_ENABLE: SET_VRING_ENABLE enables a virtq to be in "active". SET_PEER_CONNECTION enables the peer (slave or master) device to be in "active". The driver shouldn't send packets if the device is inactive.I fail to see the difference with SET_VRING_ENABLE, perhaps someone more familiar with the protocol could help here.
I'm not sure if another email explaning this was sent out successfully, repost the explanation here:
The SET_PEER_CONNECTION msg is ued to turn "ON/OFF" the (slave or master) device connection status. For example, when the master side VM wants to turn down, the virtio-net driver sets the virtio-net device's PEER_CONNECTION status to "OFF" - before this happens, the virtio-net device needs to sync-up with the vhost-pci-net device first, that is, sending a VHOST_USER_SET_PEER_CONNECTION(cmd=OFF) msg to the master. In return (not as a synchronous reply, because it has to sync with the driver to stop using the slave side resource first), the vhost-pci-net device sends VHOST_USER_SET_PEER_CONNECTION(cmd=OFF) msg to the slave - this sets the virtio-net device's PEER_CONNECTION status to "OFF" and then the virtio driver is ready to unload. (same for the vhost-pci-net driver to unload)
SET_VRING_ENABLE controls the virtq status - the slave should not use the virtq if it's not enabled by the master. For example, a device may have 4 vitrqs, if vq[0].enabled==0, then the slave should not use vitrq 0.
> > + The consumer device may request to disconnect to the producer > device. This > + request should be sent only when > VHOST_USER_PROTOCOL_F_VHOST_PCI has been > + negotiated. > + Connection request: If the reply message indicates > "success", the vhost-pci based > + inter-VM communication channel has been established. > + Disconnection request: If the reply message indicates > "success", the vhost-pci based > + inter-VM communication channel has been destroyed. > + #define VHOST_USER_SET_PEER_CONNECTION_F_OFF 0 > + #define VHOST_USER_SET_PEER_CONNECTION_F_ON 1 > + > I think it would be better to add one more command here: #define VHOST_USER_SET_PEER_CONNECTION_F_INIT 2 The master uses this command to tell the slave it's ready to create the vhost-pci device. Regarding the implementation, it is put at the bottom of vhost_net_start() function (when all the vring info have been sent and enabled).Do you have WIP branch for qemu vhost-pci? That could help to understand the context.
Yes, I can share them. Best, Wei
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