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Re: [Qemu-devel] vhost-user graceful connect/disconnect
From: |
Marc-André Lureau |
Subject: |
Re: [Qemu-devel] vhost-user graceful connect/disconnect |
Date: |
Thu, 21 Dec 2017 06:01:29 -0500 (EST) |
Hi
----- Original Message -----
> Hi,
> Vhost-user implementations assume the slave is already running before
> the master starts. The slave is required during virtio device
> initialization (e.g. feature bit negotiation) and so it is simplest to
> assume that the master is already available and will respond immediately
> to the VHOST_USER_GET_FEATURES message.
>
> I have thought about how to let master and slave start in any order.
> Some approaches involve changes to the VIRTIO specification so that
> guest drivers can wait until the vhost-user connection is established.
>
> We can avoid spec changes using PCI hotplug:
>
> 1. Introduce a new vhost-user object that manages a connection:
>
> -chardev ...
> -object vhost-user,id=vhost-user0,chardev=chr0
>
> Note this object is *not* a NetClient. It's a resource for managing
> a vhost-user connection and can be used with any device type (net,
> scsi, blk, etc).
>
> This object tries to establish a connection. When a connection is
> established it sends vhost-user protocol messages to fetch
> information needed for virtio device initialization (like the number
> of virtqueues supported, features bits, etc). This information is
> stashed so that vhost_*() calls later on do not require synchronous
> communication with the vhost-user slave.
This share similarities with vhost-user-backend I proposed here:
https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg01014.html
(instead of spawning a slave, talk to a chardev - both could eventually
co-exist)
>
> 2. When the vhost-user connection is established the object emits a QMP
> event so management software can hotplug the virtio device:
>
> VHOST_USER_CONNECTED source=vhost-user0
>
> 3. The management software hotplugs the virtio device:
>
> (qmp) netdev_add vhost-user,id=netdev0,vhost-user=vhost-user0
> (qmp) device_add virtio-net-pci,netdev=netdev0
>
> Advantages of this approach:
>
> * Does not require spec changes.
> * Can be implemented without vhost-user.c qemu_chr_fe_read/write_all()
> calls that hang QEMU until the slave sends data.
> * Allows slave to set the number of queues, feature bits, etc via the
> vhost-user socket. It is not necessary to manually specify feature
> bitmasks and other slave-specific information on the QEMU
> command-line.
>
> I haven't thought through disconnection but I imagine the vhost-user
> object would emit a VHOST_USER_DISCONNECTED event so the management
> software can hot unplug the device.
>
> Thoughts?
I am sorry, I can't think through all the potential issues easily. But that
sounds interesting enough to start a proof-of-concept.
And we need more tests for vhost-user (including fixing the existing tests!)...
Thanks