Re: [PATCH v6 01/15] lib: Add TLV parser

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Tue, 2025-01-21 at 15:21 +0100, Thomas Weißschuh wrote:
> On 2025-01-21 14:48:09+0100, Roberto Sassu wrote:
> > On Tue, 2025-01-21 at 14:29 +0100, Thomas Weißschuh wrote:
> > > Hi Robert,
> > > 
> > > On 2024-11-19 11:49:08+0100, Roberto Sassu wrote:
> > > > From: Roberto Sassu <roberto.sassu@huawei.com>
> > > > 
> > > > Add a parser of a generic Type-Length-Value (TLV) format:
> > > > 
> > > > +--------------+--+---------+--------+---------+
> > > > > field1 (u16) | len1 (u32) | value1 (u8 len1) |
> > > > +--------------+------------+------------------+
> > > > >     ...      |    ...     |        ...       |
> > > > +--------------+------------+------------------+
> > > > > fieldN (u16) | lenN (u32) | valueN (u8 lenN) |
> > > > +--------------+------------+------------------+
> > > 
> > > Should mention that its big endian.
> > 
> > Ok.
> > 
> > > > Each adopter can define its own fields. The TLV parser does not need to be
> > > > aware of those, but lets the adopter obtain the data and decide how to
> > > 
> > > "adopter" -> "user".
> > 
> > Ok.
> > 
> > > > continue.
> > > > 
> > > > After processing a TLV entry, call the callback function also with the
> > > > callback data provided by the adopter. The latter can decide how to
> > > > interpret the TLV entry depending on the field ID.
> > > > 
> > > > Nesting TLVs is also possible, the callback function can call tlv_parse()
> > > > to parse the inner structure.
> > > 
> > > Given that we already have the netlink data structures, helpers and
> > > infrastructure, what is the advantage over those?
> > 
> > Sorry, I'm not too familiar on how netlink works, so I might not
> > understand your point.
> 
> Netlink is a TLV format used by the Linux networking subsystem:
> 
> struct nlmsghdr {
>     __u32 nlmsg_len;    /* Length of message including header */
>     __u16 nlmsg_type;   /* Type of message content */
>     __u16 nlmsg_flags;  /* Additional flags */
>     __u32 nlmsg_seq;    /* Sequence number */
>     __u32 nlmsg_pid;    /* Sender port ID */
> };
> 
> https://man.archlinux.org/man/core/man-pages/netlink.7.en
> 
> There are both userspace and in-kernel infrastructures to handle it.
> Looking at it again however it has some unnecessary fields, wasting
> space and uses "host" byteorder which is a problem for an on-disk
> datastructure.
> So maybe not a good alternative after all.
> 
> > I think the benefit of this data structure is the retrocompatibility.
> > If you add new data fields, you don't need to introduce a v2, v3 data
> > format.
> > 
> > New versions of the parser can consume the new information, while the
> > older can still take the ones they are able to understand.
> 
> This is also exactly how netlink is used.

Ok, perfect!

> FYI there were also some review comments inside the patch diff itself.

Ops, thanks! Missed them. Will reply to that email.

Roberto