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Date: Mon, 16 Mar 2026 11:37:36 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Christoph =?UTF-8?B?QsO2aG13YWxkZXI=?=
 <christoph.boehmwalder@linbit.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Paolo Abeni
 <pabeni@redhat.com>, Simon Horman <horms@kernel.org>, Alok Tiwari
 <alok.a.tiwari@oracle.com>, Kees Cook <kees@kernel.org>,
 netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] genetlink: add multi-version family support for
 protocol negotiation
Message-ID: <20260316113736.1085a124@kernel.org>
In-Reply-To: <20260316151507.3958299-2-christoph.boehmwalder@linbit.com>
References: <20260316151507.3958299-2-christoph.boehmwalder@linbit.com>
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On Mon, 16 Mar 2026 16:15:06 +0100 Christoph B=C3=B6hmwalder wrote:
> Extend the genetlink infrastructure to allow families to advertise a
> range of supported protocol versions [min_version, max_version]. This
> enables a single kernel module to serve both old and new userspace by
> letting userspace discover the maximum version via CTRL_ATTR_MAX_VERSION
> and select the desired protocol dialect in genlmsghdr.version.
>=20
> Rename genl_family.version to .min_version across all families (pure
> rename, no behavioral change). This avoids updating all callers.
>=20
> Add genl_family.max_version. When 0 (default) the family behaves
> exactly as before. When > min_version, the controller advertises
> CTRL_ATTR_MAX_VERSION in CTRL_CMD_GETFAMILY replies.
>=20
> When sending a reply, echo the request's genlmsghdr.version, so
> userspace receives responses in the protocol version it spoke.
> Notifications (where info->nlhdr is NULL) continue to use min_version.
>=20
> Add genlmsg_put_ver() for families that need to stamp a specific
> version on notifications.
>=20
> The immediate motivation is upstreaming DRBD 9 (genl family v1)
> while keeping unmodified DRBD 8 userspace (family v2) working. The
> DRBD family would register with min_version=3D1, max_version=3D2 and
> handle both protocol dialects.

What's the delta between the families? Do you have any examples of=20
the the version ends up being used?