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

[Kevin Wolf]

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 at 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?

Kevin