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Re: [Qemu-devel] [PATCH] starting qemu vnc session on a pre-allocated po

From: Gilad Ben-Yossef
Subject: Re: [Qemu-devel] [PATCH] starting qemu vnc session on a pre-allocated port
Date: Sun, 01 Jul 2007 00:15:48 +0300
User-agent: Icedove (X11/20070329)

Anthony Liguori wrote:

The machine running qemu has a web based interface to start VMs.
A user asks for a new VM to start by browsing to a URL. The CGI
implmenting that URL will start a new qemu instance, send to the user
web browser an HTML page with a JAVA VNC viewer embedded and terminate.

Passing an fd is still the wrong solution due to the problems with save/restore/migrate.

There is no problem with save/restore or migration.

For save/restore, the fd is not saved in the saved state. You need to specify a (new?) fd when you restore, if that is what you want to do, that is.

It just like I can run a VM with SDL, save the state and restore it with VNC - the question if the restored session should use the same fd (for whatever definition of "the same" you want) or not is left for the user. We should be providing mechanism, not policy.

The same applies for migration - when you migrate your VM, its your call and responsibility to do the right thing. Just spawn a wrapper on the target machine that opens a new fd and exec into qemu and use *that* as your migrate command line argument instead of plain qemu, as an obvious example.

As someone wrote in their blog (this is why I write my blog in a language that's been dead for aprox. 2000 years... :-):

"Instead of just saying 'migrate hostname' you now have to construct a rather long command like 'migrate "ssh hostname qemu -loadvm -"'. A nice side effect though is that you can completely change the command line arguments in case you have NFS mounts at different locations."

I couldn't agree more. :-)

It may be interesting to have something like -vnc tcp:// to let QEMU try to find an unused port in the given range. Combined with -daemonize and having the monitor on a Unix socket, you could:

1) create a vm with qemu -vnc tcp:// -monitor unix:/path/to/socket -daemonize
2) *wait* for qemu to finish running and daemonize properly
3) connect to /path/to/socket and issue a 'info vnc' command to discover which port it's actually using
4) render that port with your HTML.

What you're saying is the same as claiming that "ls" needs to implement sorting, compression (in both gz and bzip algorithms), regexp searching and transferring it's output over an SSL connection to another machine.

We don't do that. We have pipes . VNC socket fd passing is just the old pipe concept (adapted to the situation) - nothing more.

Or in other words: you can easily implement the VNC over unix domain socket feature (if it was not in qemu already) using the fd passing method, but you can't do it the other way around (without an extra long running process, that is)

The nice thing about this is that it not only continues to work with save/restore/migrate, it's smart enough to allocate a new port to ensure that you always tend to succeed. Choosing :3 might be okay on machine A, but there's no guarantee that it's okay on machine B so you have to allow QEMU to try and find a new port after restore/migrate.

You're assuming we know what the correct behavior is. We really don't know. Maybe we want to use the same fd number on the new machine, maybe we don't. The user somehow provided a good fd on the original machine, trust him to provide a good one on the new one (via a qemu wrapper for migrate as explained above, for example).

Again it's that mechanism vs. policy thing again.,


PS. This is not some ego trip about whether you'll take the patch or not, so just tell me if this back and forth gets annoying and I'll shut up :-)

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