Re: [libvirt] Add support for QEMU file descriptor sets

On 01/22/2013 12:07 PM, Stefan Berger wrote: Thanks for starting on this; it is something I have also been planning to work on. First, we need more than just moving it into qemu_command.c; we also need a qemu_capabilities change to add a capability to know when the feature is available. Or even two: 'add-fd' monitor command was added in 1.2, but -add-fd command line option wasn't until 1.3. Then again, maybe it's just simpler to state that if you are targeting qemu 1.2, there is nothing worth using the new add-fd QMP command for that cannot already be done with older fallbacks, and it is only 1.3 and later where the new fd passing becomes essential. Then, based on the capability being present or absent, any code in qemu_command that wants to pass an fd to qemu needs to call into a helper routine that will either use the new -add-fd argument, fall back to older ad-hoc processing, or error out if there is no fallback for that particular usage. Then we also need to use the QMP commands to update the fdsets after qemu is started; depending on what the fd is used for, we can close the fd once qemu has dup'd it into internal use, so that qemu doesn't hang on to the fd that libvirt passed in forever even when it's internal use is hot-unplugged. We also need to figure out how to use the QMP commands during hotplug operations, instead of the command line additions at qemu startup. And there's still the nagging issue that even as of qemu 1.4, there is still no way to specify block device backing chains via fds; so even if we can pass in an fd for some use cases (like TPM), we are still waiting for qemu to give us a way to do it in all use cases (like NFS security for qcow2 backing chains across snapshots, block copy, and block commit). But one thing at a time. How much of this work are you planning on attempting, and how much do you need me to do? -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org