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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jan 14, 2021 at 04:49:17PM +0000, Stefan Hajnoczi wrote:
> On Thu, Jan 14, 2021 at 03:22:41PM +0000, Daniel P. Berrangé wrote:
> > On Thu, Jan 14, 2021 at 04:02:56PM +0100, Kevin Wolf wrote:
> > > Am 14.01.2021 um 14:59 hat Daniel P. Berrangé geschrieben:
> > > > 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.
> > > 
> > > I have an idea for the QMP command name: chardev-snapshot-sync!
> > > 
> > > Finally we get backing file chains for chardevs! :-)
> > > 
> > > > > 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.
> > > 
> > > I think the idea was to show the QMP traffic that libvirt produces for
> > > other management applications, not for the QMP shell. These APIs
> > > probably don't allow this?
> > 
> > FWIW if you want to monitor what libvirt is sending/receiving we have
> > a script for that that uses our systemtap probe points:
> > 
> >   https://gitlab.com/libvirt/libvirt/-/blob/master/examples/systemtap/qemu-monitor.stp
> 
> Does that require root?

Yeah, systemtap generally requires root.

The same info is also written to the log files. For example:

  virt-admin daemon-log-filters "2:qemu_monitor_json"
  virt-admin daemon-lop-outputs "2:file:/var/log/libvirt/libvirtd.log"

nb, i'm using level '2' there to avoid enabling  debug logs, only
info level logs which is the level dynamic probes  log at.

Regards,
Daniel
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