[Qemu-devel] [RFC 0/3] virtiofsd: get/set log level via DBus

[Stefan Hajnoczi <stefanha@redhat.com>]

It is likely that virtiofsd will need to support "management commands" for
reconfiguring it at runtime.  The first use case was proposed by Eryu Guan for
getting/setting the current log level.

I promised to try out DBus as the management interface because it has a rich
feature set and is accessible from most programming languages.  It should be
able to support all the use cases we come up with.

This patch series is a prototype that implements the get-log-level and
set-log-level management commands via DBus.  Use the new virtiofsctl tool to
talk to a running virtiofsd process:

  # dbus-run-session ./virtiofsd ...
  ...
  Using dbus address unix:abstract=/tmp/dbus-H9WBbpjk3O,guid=0be16acefb868e6025a8737f5d7124d2
  # export DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-H9WBbpjk3O,guid=0be16acefb868e6025a8737f5d7124d2
  # ./virtiofsctl set-log-level err

Most of the work is done by gdbus-codegen(1).  It generates code for the
org.qemu.Virtiofsd.xml interface definition.  Our code can use the simple
virtiofsd_get/set_log_level() APIs and it will make corresponding DBus calls.

I'm pretty happy with this approach because the code is straightforward.  It
hasn't even triggered seccomp failures yet :).

Error handling is a little problematic.  I noticed that virtiofsctl silently
returns success even if it cannot talk to virtiofsd.  This is due to the code
generated by gdbus-codegen(1) which has no error reporting :(.  This can be
solved by writing more low-level GDBus code instead of using the high-level
generated bindings.

What do you think about this approach?

Stefan Hajnoczi (3):
  virtiofsd: add org.qemu.Virtiofsd interface
  virtiofsd: add DBus server to handle log level changes
  virtiofsd: add virtiofsctl command-line management tool

 configure                                |   7 +
 Makefile                                 |  16 +++
 Makefile.objs                            |   1 +
 contrib/virtiofsd/Makefile.objs          |  10 +-
 contrib/virtiofsd/dbus.h                 |   9 ++
 contrib/virtiofsd/dbus.c                 | 162 +++++++++++++++++++++++
 contrib/virtiofsd/passthrough_ll.c       |   8 +-
 contrib/virtiofsd/virtiofsctl.c          |  55 ++++++++
 .gitignore                               |   1 +
 contrib/virtiofsd/org.qemu.Virtiofsd.xml |   7 +
 10 files changed, 274 insertions(+), 2 deletions(-)
 create mode 100644 contrib/virtiofsd/dbus.h
 create mode 100644 contrib/virtiofsd/dbus.c
 create mode 100644 contrib/virtiofsd/virtiofsctl.c
 create mode 100644 contrib/virtiofsd/org.qemu.Virtiofsd.xml

-- 
2.21.0