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Re: [Qemu-devel] Re: virtio-serial: An interface for host-guest communic

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: virtio-serial: An interface for host-guest communication
Date: Mon, 10 Aug 2009 09:20:32 -0500
User-agent: Thunderbird (X11/20090320)

Gerd Hoffmann wrote:
On 08/10/09 15:02, Anthony Liguori wrote:

I think you're missing my fundamental point. Don't use the kernel as the
guest interface.

Introduce a userspace daemon that exposes a domain socket. Then we can
have a proper protocol that uses reverse fqdns for identification.

We need nothing but (a) bidirectional byte streams and (b) name tags for them.

Do we really want design a daemon and a protocol for such a simple thing?

Yes, because we also need (c) the ability to write cross platform software that targets vmchannel.

So having a library interface is going to be extremely desirable.

Also, see the previous discussion about security. How do you sanely delegate /dev/vmchannel/org/qemu/clipboard to the current Xorg user?

Especially as requiring a daemon for that adds a few problems you don't have without them. Access control for example: For device nodes you can just use standard unix permissions and acls.

But how do you set those permissions in the first place?

You can easily do stuff like adding the logged in desktop user to the /dev/vmchannel/org/qemu/clipboard acl using existing solutions. With a daemon you have to hop through a number of loops to archive the same.

Can't we simply have guest apps open "/dev/vmchannel/$protocol" ?

/dev interfaces are only simple to kernel developers :-) Besides, why do something that can be clearly done in userspace within the kernel? It just increases the possibility of kernel bugs.

You can have a /var/run/vmchannel/$protocol.sock unix domain socket and it has all the same properties that you describe. It also Just Works with standard tools like socat. You can transparently route it over the network, have it work over slirp, a serial device, or some custom virtio device if we so choose.

It's the only sane way to support older guests too. If we really want vmchannel to be used by application developers, then we really need a libvmchannel.


Anthony Liguori


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