[libvirt] Reporting log/error messages through capabilities

When qemu is completely broken, libvirtd starts up OK but exists in a kind of broken state where no guests can possibly be run. I hit this problem ... again ... today: https://bugzilla.redhat.com/show_bug.cgi?id=1066630#c0 There is a libvirt bug here, which is that it's very hard to diagnose what is going on when qemu fails to work at all. The logging system in libvirt(d) is trememdously powerful, but ultimately confusing to use, and requires users to edit config files which makes it a non-starter for programs using libvirt through the API [1]. to the user that there is a problem two layers below in qemu. So my idea is that libvirt capabilities output should have an <info> section containing log messages/errors. <capabilities> ... <info> Could not run qemu-system-blah: "symbol lookup error: /usr/bin/qemu-system-s390x: undefined symbol: glfs_discard_async" </info> </capabilities> Libguestfs queries for libvirt capabilities anyway. If we don't get a satisfactory set of <guest/> elements, then we could list out the <info/> section. Easy for us. The problem is the <info/> element hardly fits into capabilities. If we didn't put it there, could it go some other place? Or a new API? Are there other unanticipated problems here? I think one is that libvirt doesn't appear to collect detailed log information by default, (unless the user edits log_level). That's assuming I understand the code correctly. Personally I think libvirt should always collect debug information, because you never know when it could be useful, but for the above, collecting errors & warnings unconditionally is sufficient. Rich. [1] By the way, this is a general complaint about libvirt. Please DON'T add any more stuff to the configuration file. Everything should be configurable through the API, or not at all. There are two other settings I can think of that libguestfs would like to adjust but cannot because they are only available in a configuration file. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/