Hi, > On Oct 13, 2020, at 10:30 AM, Stefan Hajnoczi <stefa...@redhat.com> wrote: > > On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote: >> >> >>> On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefa...@redhat.com> 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 <stefa...@redhat.com> 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
Ah, ok. That's not an issue then. > >> 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. A few things on this: Per Daniel's response on the other fork of the thread, I think we can develop faster if we're not depending on qemu-devel. With the right set of maintainers on the project, there's arguably more flexibility in working at a restricted set of code. QEMU can then update the submodule when a "checkpoint" is ready. Also, I don't see why SPDK/DPDK would need to build QEMU from source for their implementations. That is, they will need an up-to-date QEMU for enabling their development, but they don't provide a QEMU to their users; they just have to wait for a release. I talked to Ben Walker (SPDK), and he confirmed SPDK doesn't really maintain a fork of QEMU. Apparently there was one when they were doing vhost-user-nvme, but that's now abandoned. If they had to add the entire QEMU as a submodule, it could potentially be an issue, right? That's a lot of code just to get some headers. > >>> >>> #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. I suppose that challenge is up to the vfio-user/libmuser maintainers to resolve by tagging rc versions or something along those lines. Those would be the ones we can add to QEMU master as the work progresses. Thoughts? F. > > Stefan
