qmp: Added new command to retrieve eBPF blob.

Now, the binary objects may be retrieved by id.
It would require for future qmp commands that may require specific
eBPF blob.

Added command "request-ebpf". This command returns
eBPF program encoded base64. The program taken from the
skeleton and essentially is an ELF object that can be
loaded in the future with libbpf.

The reason to use the command to provide the eBPF object
instead of a separate artifact was to avoid issues related
to finding the eBPF itself. eBPF object is an ELF binary
that contains the eBPF program and eBPF map description(BTF).
Overall, eBPF object should contain the program and enough
metadata to create/load eBPF with libbpf. As the eBPF
maps/program should correspond to QEMU, the eBPF can't
be used from different QEMU build.

The first solution was a helper that comes with QEMU
and loads appropriate eBPF objects. And the issue is
to find a proper helper if the system has several
different QEMUs installed and/or built from the source,
which helpers may not be compatible.

Another issue is QEMU updating while there is a running
QEMU instance. With an updated helper, it may not be
possible to hotplug virtio-net device to the already
running QEMU. Overall, requesting the eBPF object from
QEMU itself solves possible failures with acceptable effort.

Links:
[PATCH 3/5] qmp: Added the helper stamp check.
https://lore.kernel.org/all/20230219162100.174318-4-andrew@daynix.com/

Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

3 files changed, 68 insertions, 0 deletions

diff --git a/qapi/ebpf.json b/qapi/ebpf.json
new file mode 100644
index 0000000000..f413d00154
--- /dev/null
+++ b/qapi/ebpf.json
@@ -0,0 +1,66 @@
+# -*- Mode: Python -*-
+# vim: filetype=python
+#
+# This work is licensed under the terms of the GNU GPL, version 2 or later.
+# See the COPYING file in the top-level directory.
+
+##
+# = eBPF Objects
+#
+# eBPF object is an ELF binary that contains the eBPF
+# program and eBPF map description(BTF). Overall, eBPF
+# object should contain the program and enough metadata
+# to create/load eBPF with libbpf. As the eBPF maps/program
+# should correspond to QEMU, the eBPF can't be used from
+# different QEMU build.
+#
+# Currently, there is a possible eBPF for receive-side scaling (RSS).
+#
+##
+
+##
+# @EbpfObject:
+#
+# An eBPF ELF object.
+#
+# @object: the eBPF object encoded in base64
+#
+# Since: 9.0
+##
+{ 'struct': 'EbpfObject',
+  'data': {'object': 'str'},
+  'if': 'CONFIG_EBPF' }
+
+##
+# @EbpfProgramID:
+#
+# The eBPF programs that can be gotten with request-ebpf.
+#
+# @rss: Receive side scaling, technology that allows steering traffic
+#     between queues by calculation hash.  Users may set up
+#     indirection table and hash/packet types configurations.  Used
+#     with virtio-net.
+#
+# Since: 9.0
+##
+{ 'enum': 'EbpfProgramID',
+  'if': 'CONFIG_EBPF',
+  'data': [ { 'name': 'rss' } ] }
+
+##
+# @request-ebpf:
+#
+# Retrieve an eBPF object that can be loaded with libbpf.  Management
+# applications (g.e. libvirt) may load it and pass file descriptors to
+# QEMU, so they can run running QEMU without BPF capabilities.
+#
+# @id: The ID of the program to return.
+#
+# Returns: eBPF object encoded in base64.
+#
+# Since: 9.0
+##
+{ 'command': 'request-ebpf',
+  'data': { 'id': 'EbpfProgramID' },
+  'returns': 'EbpfObject',
+  'if': 'CONFIG_EBPF' }
diff --git a/qapi/meson.build b/qapi/meson.build
index f81a37565c..375d564277 100644
--- a/qapi/meson.build
+++ b/qapi/meson.build
@@ -33,6 +33,7 @@ qapi_all_modules = [
   'crypto',
   'cxl',
   'dump',
+  'ebpf',
   'error',
   'introspect',
   'job',
diff --git a/qapi/qapi-schema.json b/qapi/qapi-schema.json
index c01ec335e6..8304d45625 100644
--- a/qapi/qapi-schema.json
+++ b/qapi/qapi-schema.json
@@ -53,6 +53,7 @@
 { 'include': 'char.json' }
 { 'include': 'dump.json' }
 { 'include': 'net.json' }
+{ 'include': 'ebpf.json' }
 { 'include': 'rdma.json' }
 { 'include': 'rocker.json' }
 { 'include': 'tpm.json' }