Re: [libvirt] Supporting hypervisor specific APIs in libvirt

On Mon, Mar 22, 2010 at 02:25:00PM -0500, Anthony Liguori wrote: That is more or less our current mission. If this mission leads to QEMU creating a non-libvirt based API & telling people to use that instead, then I'd say libvirt's mission needs to change to avoid that scenario ! I strongly believe that libvirt's strategy is good for application developers over the medium to long term. We need to figure out how to get rid of the short term pain from the feature timelag, rather than inventing a new library API for them to use. Stepping back a bit first, there are the two core areas in which people can be limited by libvirt currently. 1. Monitor commands 2. Command line flags Ultimately, IIUC, you are suggesting we need to allow arbitrary passthrough for both of these in libvirt. At the libvirt level, we have 3 core requirements 1. The XML format is extend only (new elements allowed, or add attributes or children to existing elements) 2. The C library API is append only (new symbols only) 3. The RPC wire protocol is append only (maps 1-1 to the C API generally) The core question for us as libvirt developers is how we could support QEMU specific features that may change arbitrarily, without it impacting on our ability to maintain these 3 requirements for the non-hypervisor specific APIs. We don't ever want to be in a situation where a QEMU specific API will require us to change the soname of the main libvirt library, or introduce incompatible wire protocol changes. If we were to introduce QEMU specific APIs, we also need a way to easily remove those over time, as & when we have them available as generic APIs. At the C API level, this to me suggests that we'd want to introduce a separate libvirt-qemu.so library for the QEMU specific APIs. This library would not have the same requirements of fixed long term ABI that the main libvirt.so did. We'd add QEMU APIs to libvirt-qemu.so any time needed, but remove them when the equivalent functionality were in libvirt.so, and increment the soname of libvirt-qemu.so at that point. At the wire protocol level, the protocol allows us to support multiple versioned protocols in parallel over the same data stream. So again there, we could define a sub-protocol for QEMU specific features for which we don't provide the indefinite ABI compatability. Finally the XML format is "easy" - just have a versioned XML namespace for extra pieces, that's distinct from the default namespace, again without the permanent long term compatability guarentees. There are, however, some bits that are unlikely to work when QEMU is under libvirt. Specifically any of the device backends that use stdio (eg, -serial stdio, or the ncurses graphics), simply because all libvirt spawned VMs are fully daemonized & so stdio is /dev/null Other items are hard, but not entirely impossible to solve. eg, any use of the 'script=' arg for -net devices doesn't work, because libvirt clears all capabilities from the QEMU process so it'll be lacking CAP_NET_ADMIN which most TAP device setup scripts in fact need. Some parts of the C library/wire protocol here are related to another feature I'd like to introduce for libvirt, namely a administrative library. eg a API to configure and manage the libvirtd daemon itself on the fly. This could easily hook into the wire protocol, but live as a separate libvirt-daemon.so library API in similar way to what I suggest for QEMU specific API Adding yet another library in the stack isn't really going to solve the problem from the POV of libvirt users, but rather fork the community effort which I imagine we'd all rather avoid. Having to tell people to switch to a different library API to get access to a specific feature is a short term win, but with a significant long term cost/burden. This means we really do need to figure out how to better/fully support QEMU's features in libvirt, removing the feature timelag pain. Regards, Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|