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Re: qmp-shell TUI (was: Re: Call for Google Summer of Code 2021 project

From: Daniel P . Berrangé
Subject: Re: qmp-shell TUI (was: Re: Call for Google Summer of Code 2021 project ideas)
Date: Thu, 14 Jan 2021 13:59:42 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Thu, Jan 14, 2021 at 01:52:34PM +0000, Stefan Hajnoczi wrote:
> On Wed, Jan 13, 2021 at 01:59:43PM -0500, John Snow wrote:
> > On 1/13/21 3:53 AM, Stefan Hajnoczi wrote:
> > > On Tue, Jan 12, 2021 at 9:10 PM John Snow <jsnow@redhat.com> wrote:
> > > 2. Ability to watch QMP activity on a running QEMU process, e.g. even
> > > when libvirt is directly connected to the monitor.
> > > 
> > 
> > That *WOULD* be extremely cool, and moves a lot closer to how mitmproxy
> > works.
> > 
> > (Actually, mitmproxy could theoretically be taught how to read and
> > understand QMP traffic, but that's not something I know how to do or would
> > be prepared to mentor.)
> > 
> > Is this possible to do in a post-hoc fashion? Let's say you are using
> > production environment QEMU, how do we attach the QMP listener to it? Or
> > does this idea require that we start QEMU in a specific fashion with a
> > second debug socket that qmp-shell can connect to in order to listen?
> > 
> > ... Or do we engineer qmp-shell to open its own socket that libvirt connects
> > to ...?
> Here is the QEMU command-line that libvirt uses on my F33 system:
>   -chardev socket,id=charmonitor,fd=36,server,nowait
>   -mon chardev=charmonitor,id=monitor,mode=control
> Goals for this feature:
> 1. No manual steps required for setup.
> 2. Ability to start/stop monitoring traffic at runtime without
>    restarting QEMU.
> 3. Available to unprivileged users.
> I think the easiest way to achieve this is through a new QEMU monitor
> command. Approaches that come to mind:
> 1. Add a -mon debug-chardev property and a QMP command to set it at
>    runtime. The debug-chardev receives both monitor input (commands) and
>    output (responses and events). This does not allow MITM, rather it
>    mirrors traffic.
> 2. Add a chardev-get-fd command that fetches the fd from a chardev and
>    then use the existing chardev-change command to replace the monitor
>    chardev with a chardev connected to qmp-shell. This inserts qmp-shell
>    as a proxy between the QMP client and server. qmp-shell can remove
>    itself again with another chardev-change command. This approach
>    allows MITM. The downside is it assumes the QMP chardev is a file
>    descriptor, so it won't work with all types of chardev.
> 3. Add a new chardev-proxy type that aggregates 3 chardevs: 1. an origin
>    source chardev, 2. a monitoring sink chardev, and 3. a monitoring
>    source chardev. The data flow is origin <-> monitoring sink <->
>    monitoring source <-> QMP monitor. qmp-shell creates the monitoring
>    sink (for receiving incoming QMP commands) and monitoring source
>    chardev (for forwarding QMP commands or MITM commands), and then it
>    uses change-chardev to instantiate a chardev-proxy that directs the
>    original libvirt chardev through the monitoring sink and source.
>    This is the most complex but also completely contained within the
>    QEMU chardev layer.
> In all these approaches qmp-shell uses virsh qemu-monitor-command or an
> equivalent API to start/stop monitoring a running VM without manual
> setup steps.

Why go to the trouble of adding more chardevs to a running QEMU that
libvirt has.  qmp-shell can just directly use the libvirt Python API
to invoke  virDomainQemuMonitorCommand to invoke QMP commands, and
the othe API for receiving QMP events.

Essentially it just needs to be split into two layers. The upper
layer works in terms of individual QMP command/replies, and QMP
events. The lower layer provides a transport that is either a
UNIX socket, or is the libvirt QMP passthrough API.

Or alternatively, provide a virt-qmp-shim  command that listens on
a UNIX socket, accepts QMP commands and turns them into calls to
virDomainQemuMonitorCommand, and funnells back the response.

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