Re: [PATCH v4] introduce vfio-user protocol specification

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote:
> 
> 
> > On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefanha@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 <stefanha@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

> 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

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