Re: [PATCH v6 1/4] tools: add xdp-forwarder

[Yureka <yuka@yuka.dev>]

On 9/27/25 10:00, Alyssa Ross wrote:
> A few more things I just found.  Happy to just make the fixes myself
> when I apply the patch if you prefer — just let me know what you think.

Yes, please do the fixups when you apply the patch. Thank you!

>
> Yureka Lilian <yureka@cyberchaos.dev> writes:
>
>> The xdp-forwarder's purpose is implementing the functionality needed
>> within the net-vm (a VM running the Linux drivers for any physical
>> interfaces on the spectrum system).
>>
>> In the future, the net-vm will load the included XDP programs on the
>> passed-through physical interfaces as well as the downstream virtio
>> interface going into the router (recognized by its special MAC address).
>>
>> The net-vm needs to multiplex between the physical interfaces, as there
>> might be several interfaces in the same IOMMU-group.
>>
>> For this, the XDP program loaded on the physical interfaces
>> (`prog_physical.o`) applies a VLAN tag corresponding to the interface id
>> and redirects the packets to the router interface (identified by the
>> `router_iface` bpf map). In the other direction the XDP program loaded on
>> the router interface (`prog_router.o`) removes one layer of VLAN tagging
>> and redirects the packets to the interface read from the VLAN tag.
>>
>> The helper program `set_router_iface` is used to update the `router_iface`
>> bpf map to point to the interface passed as argument to the program.
>>
>> Co-authored-by: Demi Marie Obenour <demiobenour@gmail.com>
>> Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>> ---
>>   pkgs/default.nix                       |   4 +
>>   release/checks/pkg-tests.nix           |   1 +
>>   tools/default.nix                      |  21 +-
> tools/shell.nix should also set driverSupport = true so libbpf is
> available in the development shell.
Ack
>
>>   tools/meson.build                      |   4 +
>>   tools/meson_options.txt                |   3 +
>>   tools/xdp-forwarder/meson.build        |  48 +++++
>>   tools/xdp-forwarder/parsing_helpers.h  | 274 +++++++++++++++++++++++++
>>   tools/xdp-forwarder/prog_physical.c    |  39 ++++
>>   tools/xdp-forwarder/prog_router.c      |  42 ++++
>>   tools/xdp-forwarder/rewrite_helpers.h  | 146 +++++++++++++
>>   tools/xdp-forwarder/set_router_iface.c |  30 +++
>>   11 files changed, 608 insertions(+), 4 deletions(-)
>>   create mode 100644 tools/xdp-forwarder/meson.build
>>   create mode 100644 tools/xdp-forwarder/parsing_helpers.h
>>   create mode 100644 tools/xdp-forwarder/prog_physical.c
>>   create mode 100644 tools/xdp-forwarder/prog_router.c
>>   create mode 100644 tools/xdp-forwarder/rewrite_helpers.h
>>   create mode 100644 tools/xdp-forwarder/set_router_iface.c
>> diff --git a/tools/xdp-forwarder/meson.build b/tools/xdp-forwarder/meson.build
>> new file mode 100644
>> index 0000000..9b70ce3
>> --- /dev/null
>> +++ b/tools/xdp-forwarder/meson.build
>> @@ -0,0 +1,48 @@
>> +# SPDX-License-Identifier: EUPL-1.2+
>> +# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
>> +# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>
>> +
>> +libbpf = dependency('libbpf', version : '1.6.2')
> This is an exact version match, which means we'll have to change it
> every time libbpf updates in Nixpkgs.  Do you know what a more
> appropriate version constraint would be?
We can change it to >=1.6.3.
>
>> +
>> +executable('set-router-iface', 'set_router_iface.c',
>> +  dependencies : libbpf,
>> +  install : true)
>> +
>> +clang = find_program('clang', native: true)
>> +
>> +bpf_o_cmd = [
>> +  clang.full_path(),
>> +  '-fno-stack-protector',
>> +  '-fno-strict-aliasing',
>> +  '-fwrapv', '-fwrapv-pointer',
>> +  '-Wall',
>> +  '-Wextra',
> When I built in a Nix shell I got warnings from
> -Wcompare-distinct-pointer-types and -Wsign-compare, coming from the
> vendored headers.  Should I just silence those?  (A quick look at the
> kernel suggests the distinct pointer type comparison is probably fine,
> but that the sign comparison issue should probably be fixed upstream.)
I agree about the severity of the warnings, and I think we can silence 
them since the -Wsign-compare warnings are all in functions which we 
never use.
>> +  '-O2',
>> +  '-target', 'bpf',
>> +  '-I', meson.current_source_dir() + '/include',
>> +  '-g',
>> +  '-c',
>> +  '-o', '@OUTPUT@',
>> +  '-MD',
>> +  '-MP',
>> +  '-MF', '@DEPFILE@',
>> +  '--',
>> +  '@INPUT@',
>> +]
>> +
>> +prog_router_o = custom_target(
>> +  input : 'prog_router.c',
>> +  output : 'prog_router.o',
>> +  depfile : 'prog_router.o.dep',
>> +  command : bpf_o_cmd,
>> +  install: true,
>> +  install_dir: 'lib/xdp')
>> +
>> +prog_physical_o = custom_target(
>> +  input : 'prog_physical.c',
>> +  output : 'prog_physical.o',
>> +  depfile : 'prog_physical.o.dep',
>> +  command : bpf_o_cmd,
>> +  install: true,
>> +  install_dir: 'lib/xdp')
>> +