netdev - Re: [PATCH net-next 2/6] net: nexthop: Increase weight to u16

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Message-ID: <20240801193928.GC2495006@kernel.org>
Date: Thu, 1 Aug 2024 20:39:28 +0100
From: Simon Horman <horms@...nel.org>
To: Petr Machata <petrm@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, Ido Schimmel <idosch@...dia.com>,
	David Ahern <dsahern@...nel.org>, Donald Sharp <sharpd@...dia.com>,
	mlxsw@...dia.com
Subject: Re: [PATCH net-next 2/6] net: nexthop: Increase weight to u16

On Thu, Aug 01, 2024 at 06:23:58PM +0200, Petr Machata wrote:
> In CLOS networks, as link failures occur at various points in the network,
> ECMP weights of the involved nodes are adjusted to compensate. With high
> fan-out of the involved nodes, and overall high number of nodes,
> a (non-)ECMP weight ratio that we would like to configure does not fit into
> 8 bits. Instead of, say, 255:254, we might like to configure something like
> 1000:999. For these deployments, the 8-bit weight may not be enough.
> 
> To that end, in this patch increase the next hop weight from u8 to u16.
> 
> Increasing the width of an integral type can be tricky, because while the
> code still compiles, the types may not check out anymore, and numerical
> errors come up. To prevent this, the conversion was done in two steps.
> First the type was changed from u8 to a single-member structure, which
> invalidated all uses of the field. This allowed going through them one by
> one and audit for type correctness. Then the structure was replaced with a
> vanilla u16 again. This should ensure that no place was missed.
> 
> The UAPI for configuring nexthop group members is that an attribute
> NHA_GROUP carries an array of struct nexthop_grp entries:
> 
> 	struct nexthop_grp {
> 		__u32	id;	  /* nexthop id - must exist */
> 		__u8	weight;   /* weight of this nexthop */
> 		__u8	resvd1;
> 		__u16	resvd2;
> 	};
> 
> The field resvd1 is currently validated and required to be zero. We can
> lift this requirement and carry high-order bits of the weight in the
> reserved field:
> 
> 	struct nexthop_grp {
> 		__u32	id;	  /* nexthop id - must exist */
> 		__u8	weight;   /* weight of this nexthop */
> 		__u8	weight_high;
> 		__u16	resvd2;
> 	};
> 
> Keeping the fields split this way was chosen in case an existing userspace
> makes assumptions about the width of the weight field, and to sidestep any
> endianes issues.

nit: endianness

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