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Re: [Qemu-devel] [PATCH 11/18] vhost-user: add shutdown support

From: Marc-André Lureau
Subject: Re: [Qemu-devel] [PATCH 11/18] vhost-user: add shutdown support
Date: Wed, 4 May 2016 15:16:44 +0200


On Mon, May 2, 2016 at 2:04 PM, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, May 02, 2016 at 01:29:18PM +0200, Marc-André Lureau wrote:
>> Hi
>> On Mon, May 2, 2016 at 12:45 PM, Michael S. Tsirkin <address@hidden> wrote:
>> > 1. Graceful disconnect
>> > - One should be able to do vmstop, disconnect, connect then vm start
>> >   This looks like a nice intermediate step.
>> > - Why would we always assume it's always remote initiating the disconnect?
>> >   Monitor commands for disconnecting seem called for.
>> Those two solutions involve VM management. This looks more complex to
>> communicate/synchronize vhost-user backend & vm management & qemu. The
>> use case I cover is request from the backend to shutdown,
> Right, but flushing buffers + closing the socket looks like
> a cleaner interface than a ton of messages going back and forth.

What do you mean by "a ton of messages"? It adds
amount of work to flush and close is the same regardless. I figured
later that if we refactor vhost-user socket handling, we may be able
to accept request from the main channel socket, without extra "slave

>> because
>> that's what the users wanted (and it is already possible to stop the
>> backend and disconnect it from qemu, we would only need to know when,
>> with a new command..)
> You assume the backend has a monitor interface to disconnect though.
> That's not a given.

What do you mean? The backend must have a way to request to close/quit
indeed. That's outside of scope how the backend get this information
(via signals or other means). It's external, having this information
from VM management layer is the same (someone has to trigger this

>> > 3. Running while disconnected
>> > - At the moment, we wait on vm start for remote to connect,
>> >   if we support disconnecting backend without stopping
>> >   we probably should also support starting it without waiting
>> >   for connection
>> That's what Tetsuya proposed in its initial series, but handling the
>> flags was quite tedious.
> That would be up to management. E.g. let backend export the list
> of flags it supports in some file, and apply to QEMU.

That makes me worry that such unfriendly connections details have to
spread outside of vhost-user to VM management layer, making usage &
maintenance harder for no clear benefit. It's a similar concern you
have with "the backend has a monitor interface", here "the backend
must have an introspection interface" or at least export vhost-user
details somehow.

>> I think this can be considered easily a
>> seperate enhancement. What I proposed is to keep waiting for the
>> initial connect, and check the flags remains compatible on reconnect.
> Seems asymmetrical unless we stop the vm.

That's the point, there will be time with and without the backend if
we keep the VM running.

>> > - Guests expect tx buffers to be used in a timely manner, thus:
>> > - If vm is running we must use them in qemu, maybe discarding packets
>> >   in the process.
>> >   There already are commands for link down, I'm not sure there's value
>> >   in doing this automatically in qemu.
>> Testing doesn't show such buffer issues when the link is down (this
>> can be tested with vubr example above)
> Not enough testing then - it's a race condition: buffers can be sent
> before link down.

Ok, I'll do more testing. In all cases, looks reasonable to discard.

>> > - Alternatively, we should also have an option to stop vm automatically 
>> > (like we do on
>> >   disk errors) to reduce number of dropped packets.
>> Why not, we would need to know if this is actually useful.
>> >
>> > 4. Reconnecting
>> > - When acting as a server, we might want an option to go back to
>> >   listening state, but it should not be the only option,
>> >   there should be a monitor command for moving it back to
>> >   listening state.
>> > - When acting as a client, auto-reconnect might be handy sometimes, but 
>> > should not be the only
>> >   option, there should be a way to manually request connect, possibly to
>> >   a different target, so a monitor command for re-connecting seems called 
>> > for.
>> > - We'll also need monitor events for disconnects so management knows it
>> >   must re-connect/restart listening.
>> > - If we stopped VM, there could be an option to restart on reconnect.
>> That's all involving a third party, adding complexity but the benefit
>> isn't so clear.
> It's rather clear to me. Let's assume you want to restart bridge,
> so you trigger disconnect.
> If qemu auto-reconnects there's a race as it might re-connect
> to the old bridge.

I would say that race can trivially be avoided, so it's a backend bug.

> Management is required to make this robust, auto-reconnect
> is handy for people bypassing management.

tbh, I don't like autoreconnect. My previous series didn't include
this and assumed the feature would be supported only when qemu is
configured to be the server. I added reconnect upon request by users.

Marc-André Lureau

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