Libvirt usually launches qemu with strict permissions.
To enable eBPF RSS steering, qemu-ebpf-rss-helper was added.

Added property "ebpf_rss_fds" for "virtio-net" that allows to
initialize eBPF RSS context with passed program & maps fds.

Added qemu-ebpf-rss-helper - simple helper that loads eBPF
context and passes fds through unix socket.
Libvirt should call the helper and pass fds to qemu through
"ebpf_rss_fds" property.

Added explicit target OS check for libbpf dependency in meson.
eBPF RSS works only with Linux TAP, so there is no reason to
build eBPF loader/helper for non-Linux.

Changed Qemu updates eBPF maps to array mmaping. Mmaping allows
bypassing unprivileged BPF map update. Also, instead of 3 maps
(config, key and indirection table) there is one map that
combines everything.

Added helper stamp. To check that helper was build with qemu,
qemu would check helper symbols that should contain the stamp.
It was done similar to qemu modules, but checking was performed
by the helper's ELF parsing.

Overall, libvirt process should not be aware of the "interface"
of eBPF RSS, it will not be aware of eBPF maps/program "type" and
their quantity. That's why qemu and the helper should be from
the same build and be "synchronized". Technically each qemu may
have its own helper. That's why "query-helper-paths" qmp command
was added. Qemu should return the path to the helper that suits
and libvirt should use "that" helper for "that" emulator.

qmp sample:
C: { "execute": "query-helper-paths" }
S: { "return": [
     {
       "name": "qemu-ebpf-rss-helper",
       "path": "/usr/local/libexec/qemu-ebpf-rss-helper"
     }
    ]
   }

Changes since v1:
* Mmap() used instead if bpf_map_update_elem().
* Added helper stamp.

Andrew Melnychenko (5):
  ebpf: Added eBPF initialization by fds and map update.
  virtio-net: Added property to load eBPF RSS with fds.
  qmp: Added the helper stamp check.
  ebpf_rss_helper: Added helper for eBPF RSS.
  qmp: Added qemu-ebpf-rss-path command.

 ebpf/ebpf_rss-stub.c              |   6 +
 ebpf/ebpf_rss.c                   | 120 ++++---
 ebpf/ebpf_rss.h                   |   8 +-
 ebpf/qemu-ebpf-rss-helper.c       | 130 +++++++
 ebpf/rss.bpf.skeleton.h           | 557 +++++++++++++++---------------
 hw/net/virtio-net.c               |  77 ++++-
 include/hw/virtio/virtio-net.h    |   1 +
 meson.build                       |  47 ++-
 monitor/meson.build               |   1 +
 monitor/qemu-helper-stamp-utils.c | 297 ++++++++++++++++
 monitor/qemu-helper-stamp-utils.h |  24 ++
 monitor/qmp-cmds.c                |  32 ++
 qapi/misc.json                    |  33 ++
 tools/ebpf/rss.bpf.c              |  67 ++--
 14 files changed, 990 insertions(+), 410 deletions(-)
 create mode 100644 ebpf/qemu-ebpf-rss-helper.c
 create mode 100644 monitor/qemu-helper-stamp-utils.c
 create mode 100644 monitor/qemu-helper-stamp-utils.h

--
2.31.1