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Re: [Qemu-devel] [PATCH v5 2/2] mach-virt: Provide sample configuration

From: Andrea Bolognani
Subject: Re: [Qemu-devel] [PATCH v5 2/2] mach-virt: Provide sample configuration files
Date: Thu, 09 Feb 2017 14:53:52 +0100

On Wed, 2017-02-08 at 19:36 +0000, Peter Maydell wrote:
> > When -nodefaults is missing, a default virtio-net-pci
> > plugged into 00:01.0 is automatically added.
> Ah, right, default -net commands. Is that it?

I believe so, but double checking is never the wrong thing
to do ;)

> > On the other hand, I just realized that I can add
> >   [machine]
> >     graphics = "off"
> > to get rid of -nographic on the command line! \o/

... and after writing this, I actually tested it and
realized that it doesn't quite do what I want it to :(

> -nographic is kind of odd if you're taking the
> "use -nodefaults and specify everything manually"
> because it wraps up a whole pile of default config
> in one oddball legacy config switch. I would have
> expected -display none and whatever other options
> you wanted.

-nographic is just what libvirt uses: if there's a better
way to achieve the same result then I'm all for it!

Seems like '-display none' behaves the same as -nographic,
at least for my purposes. Unfortunately I still can't have
it inside the configuration file, it has to be on the
command line :(

> > Moreover, something that I haven't been able to do on
> > mach-virt (even though I could on q35, but again, I want the
> > files to be as close as possible) is to configure the serial
> > console from the configuration file.
> If there's stuff we can fix in the virt board to make
> this work the way it does on the PC model I'm happy
> to make fixes to help.

Well, with a q35 guest I can just add

  [chardev "hostconsole"]
    backend = "stdio"

  [device "console"]
    driver = "isa-serial"
    chardev = "hostconsole"

to the configuration file and get the same behavior I would
get by adding '-serial stdio' to the command line, but I
haven't been able to find any device that I can use the same
way as isa-serial on mach-virt. Maybe I just missed it?

> The trouble with using virtconsole is that you have to
> get quite a long way forward (probing and setting up
> PCI, etc) before you can get any kind of console output.
> "System doesn't boot and doesn't produce any output"
> is a common and really annoying failure mode in the
> ARM world, and I think that using virtconsole is
> asking for that kind of thing to happen more rather
> than less often. (Plus having two different serial
> ports in the guest means that now the user has to
> configure where both of them are supposed to output.)

You don't get one unless you use '-serial stdio' or
something like that in my experience, so if you use the
sample configuration file as I posted it you will only
get one serial console, the VirtIO-based one.

I agree though that the lack of boot output and especially
GRUB menu access in virtconsole is less then ideal, because
even though the users will eventually get a login prompt,
the first few seconds without any output at all might lead
them to believe the guest is not working properly.

I just wish there were a way to add the default serial
console straight from the configuration file...

Andrea Bolognani / Red Hat / Virtualization

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