Re: [PATCH 00/11] plugins: enable C++ plugins

["Alex Bennée" <alex.bennee@linaro.org>]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> Writing plugins in C can be sometimes tedious, especially when using Glib to
> keep track of execution state. We can directly use the same C API but write our
> plugin in C++, benefiting from its great standard library offering strings,
> smart pointers, data structures and synchronization mechanisms.
>
> It's common for downstream QEMU forks to provide C++ for plugins, like this:
> - https://github.com/panda-re/panda/tree/dev/panda/plugins
> - https://github.com/FlorentRevest/DejaView/tree/main/src/qemu_plugin
>
> Hopefully this will help more people to use upstream QEMU, and as a benefit, get
> their contribution back and help to develop plugins ecosystem upstream directly.
>
> This series first cleans up build system for plugins, factorizing details
> between contrib/plugins and tests/tcg/plugins folders.
> Then, we perform codebase cleanups to fix conflicts between existing headers
> and C++ headers.
> After that, we can update the C++ standard used by QEMU, to benefit fully
> from latest updates of the language.
> Finally, we define an empty C++ plugin, making sure we can keep track of
> possible regression in qemu-plugin header.
>
> Note: This series is *not* a trojan horse to bring C++ in QEMU
> codebase, nor to define an alternative C++ API for plugins. It's just enabling
> more users to get the most out of existing C plugin API.

I don't have any fundamental objection to allowing this - as you say it
keep the existing API and just makes it easier for the plugin authors to
use C++.

It would be interesting to see if there are any other changes that would
also be useful for other language bindings. I suspect the most relevant
would be Rust bindings for the plugins.

>
> CI: https://gitlab.com/pbo-linaro/qemu/-/pipelines/2239199381
>
> Pierrick Bouvier (11):
>   plugins: move win32_linker.c file to plugins directory
>   plugins: factorize plugin dependencies and library details
>   plugins: use complete filename for defining plugins sources
>   plugins: define plugin API symbols as extern "C" when compiling in C++
>   include: qemu/ctype.h -> qemu/qemu-ctype.h
>   include: qemu/coroutine.h -> qemu/qemu-coroutine.h
>   meson: fix supported compiler arguments in other languages than C
>   meson: enable cpp (optionally) for plugins
>   qga/vss-win32: fix clang warning with C++20
>   meson: update C++ standard to C++23
>   contrib/plugins: add empty cpp plugin
>
>  meson.build                                   |  24 ++--
>  block/parallels.h                             |   2 +-
>  block/qcow2.h                                 |   2 +-
>  fsdev/qemu-fsdev-throttle.h                   |   2 +-
>  hw/9pfs/9p.h                                  |   2 +-
>  include/block/block-global-state.h            |   2 +-
>  include/block/block-hmp-cmds.h                |   2 +-
>  include/block/block-io.h                      |   2 +-
>  include/block/reqlist.h                       |   2 +-
>  include/block/throttle-groups.h               |   2 +-
>  include/qemu/coroutine_int.h                  |   2 +-
>  include/qemu/job.h                            |   2 +-
>  .../qemu/{coroutine.h => qemu-coroutine.h}    |   0
>  include/qemu/{ctype.h => qemu-ctype.h}        |   0
>  include/qemu/qemu-plugin.h                    |   8 ++
>  migration/migration.h                         |   2 +-
>  ui/console-priv.h                             |   2 +-
>  block.c                                       |   2 +-
>  block/block-copy.c                            |   2 +-
>  block/io_uring.c                              |   2 +-
>  block/linux-aio.c                             |   2 +-
>  block/mirror.c                                |   2 +-
>  block/progress_meter.c                        |   2 +-
>  block/ssh.c                                   |   2 +-
>  block/vdi.c                                   |   2 +-
>  block/vvfat.c                                 |   2 +-
>  chardev/char.c                                |   2 +-
>  gdbstub/gdbstub.c                             |   2 +-
>  hw/9pfs/coth.c                                |   2 +-
>  hw/block/virtio-blk.c                         |   2 +-
>  hw/core/bus.c                                 |   2 +-
>  hw/core/qdev-properties-system.c              |   2 +-
>  hw/core/qdev-properties.c                     |   2 +-
>  hw/hyperv/syndbg.c                            |   2 +-
>  hw/nvme/nguid.c                               |   2 +-
>  hw/s390x/ccw-device.c                         |   2 +-
>  hw/s390x/ipl.c                                |   2 +-
>  hw/s390x/s390-virtio-ccw.c                    |   2 +-
>  hw/scsi/scsi-generic.c                        |   2 +-
>  migration/migration.c                         |   2 +-
>  migration/rdma.c                              |   2 +-
>  monitor/fds.c                                 |   2 +-
>  monitor/hmp.c                                 |   2 +-
>  nbd/client-connection.c                       |   2 +-
>  net/colo-compare.c                            |   2 +-
>  net/net.c                                     |   2 +-
>  net/tap-solaris.c                             |   2 +-
>  {contrib/plugins => plugins}/win32_linker.c   |   0
>  qapi/qapi-util.c                              |   2 +-
>  qapi/qmp-dispatch.c                           |   2 +-
>  qobject/json-parser.c                         |   2 +-
>  target/ppc/ppc-qmp-cmds.c                     |   2 +-
>  target/riscv/cpu.c                            |   2 +-
>  target/riscv/riscv-qmp-cmds.c                 |   2 +-
>  tests/qtest/libqtest.c                        |   2 +-
>  tests/qtest/migration/migration-util.c        |   2 +-
>  tests/unit/test-aio-multithread.c             |   2 +-
>  tests/vhost-user-bridge.c                     |   2 +-
>  ui/console.c                                  |   2 +-
>  ui/keymaps.c                                  |   2 +-
>  ui/ui-qmp-cmds.c                              |   2 +-
>  util/cutils.c                                 |   2 +-
>  util/id.c                                     |   2 +-
>  util/qemu-co-shared-resource.c                |   2 +-
>  util/qemu-co-timeout.c                        |   2 +-
>  util/qemu-coroutine-io.c                      |   2 +-
>  util/readline.c                               |   2 +-
>  util/thread-pool.c                            |   2 +-
>  contrib/plugins/cpp.cpp                       | 119 ++++++++++++++++++
>  contrib/plugins/meson.build                   |  25 ++--
>  plugins/meson.build                           |  15 ++-
>  qga/vss-win32/requester.cpp                   |   6 +-
>  tests/tcg/plugins/meson.build                 |  18 +--
>  73 files changed, 237 insertions(+), 104 deletions(-)
>  rename include/qemu/{coroutine.h => qemu-coroutine.h} (100%)
>  rename include/qemu/{ctype.h => qemu-ctype.h} (100%)
>  rename {contrib/plugins => plugins}/win32_linker.c (100%)
>  create mode 100644 contrib/plugins/cpp.cpp

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro