On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote: > > > > On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi wrote: > > > > On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote: > >>> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi wrote: > >>> > >>> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote: > >>>>> It should be accompanied by a test in tests/. PCI-level testing APIS for > >>>>> BARs, configuration space, interrupts, etc are available in > >>>>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user > >>>>> device backend interact with QEMU's vfio-user-pci implementation. > >>>> > >>>> We plan to use a libmuser-based backend for testing. This, I suppose, will > >>>> make libmuser a dependency of QEMU (either as a submodule or as a library), > >>>> which for now can be disabled in the default configuration. Is this acceptable? > >>> > >>> If there are no other dependencies and libmuser supports all host > >>> operating systems that QEMU's -device vfio-user supports, then I think > >>> it's a good idea to use libmuser for at least one in-tree test in QEMU. > >>> > >>>>> Also please let us know who is working on what so additional people can > >>>>> get involved in areas that need work! > >>>> > >>>> Swapnil and I will be working on libmuser and the test in QEMU, John and > >>>> the mp-qemu folks will be working on the patches for implementing > >>>> --device vfio-user-pci. > >>> > >>> Great! > >>> > >>> John: Will mpqemu use libmuser to implement the remote PCI host > >>> controller? > >>> > >> > >> > >> The vfio-user-pci plan is to use libmuser on the server side. > > > > Okay. Using libmuser in tests seems like a good choice in that case. > > > > We'll need to figure out the details of how to do it because the > > traditional shared library dependency approach is not well-suited to > > in-development code. It would involve shipping libmuser distro packages > > so QEMU's build system can declare a library dependency (with details > > provided in a pkg-config file). > > > > Here are approaches that are better for in-development libraries: > > 1. Keep the libmuser code in qemu.git. > > 2. A copy of libmuser in qemu.git with changes being sent upstream > > (allows more flexibility in case QEMU-specific issues require > > experimentation). > > 3. Git submodules. > > > > #1 if you're happy to use the QEMU development process for merging > > libmuser code then it's easiest to officially host the code in qemu.git. > > libmuser gets a subdirectory in the qemu.git tree and you (the > > maintainers) send pull requests. A libmuser library build target > > provides installable static and shared libraries so external > > applications can link against libmuser too. The big advantage here is > > that QEMU can instantly use the latest libmuser code changes. > > I think there's a couple of limitations here which we should keep in mind. > > 1. Does putting it in qemu.git precludes it being BSD-3? > There's been evidence of people using (or at least trying out) muser > from where it currently lives. That doesn't mean we can't move it, but > I'm wondering if it means we have to make it GPL. The 3-clause BSD license is compatible with the GPL according to Wikipedia: https://en.wikipedia.org/wiki/BSD_licenses > 2. What about other projects that need libmuser code? > What worries me more is projects like SPDK/DPDK wanting to link > against the library and having to clone the entire QEMU repo as a > submodule. That sounds a lot more expensive than option 3 and probably > have further complications if they aren't GPL. In the early stages where the vfio-user protocol and library interfaces might need changes it will be hard to use it from multiple applications without compatibility issues. If SPDK/DPDK are communicating with QEMU using a cutting-edge library then they probably need to build QEMU from source anyway. ISTR they also maintain a QEMU fork? So maybe it's not a big issue for them. > > > > #2 works best if the library is a small (just a few source files) with > > no fancy build system requirements. > > The risk here is that they go out of sync. There's the same (or even > more) maintenance burden as point 3 below, with the added risk that > someone could patch the files and make cherry-picks non-trivial. > > > > > #3 is used in QEMU for several other components. Submodules are a pain > > to sync (requires sending a qemu.git patch to move to a new commit ID), > > so this isn't good for a dependency that moves quickly. > > I argue this is no worse than option 2. It's what I think aligns best, > but let's keep weighing pros/cons and come to a conclusion together. > The list of maintainers for muser.git should be extended to include > more QEMU stakeholders and probably other projects that will use it > (as) heavily. The topic has been raised in SPDK's Slack team on > whether the client library should live in a repo of its own (eg. > libvfio-user.git). Given the reference implementation is in libmuser, > I still think muser.git is accurate (but can easily be persuaded > otherwise). Me too, no solution is perfect. My thoughts about developing it within qemu.git for now is that this will make protocol and library interface changes easy. It will also encourage applications (DPDK/SPDK) to build against a matching QEMU so that there are no compatibility problems at the protocol or library level while the code is still heavily under development. Stefan