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