Re: [PATCH net-next 0/4] netlink: add 'bitmap' attribute type and API

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Jakub Kicinski <kuba@kernel.org>
Date: Thu, 21 Jul 2022 11:13:18 -0700

> On Thu, 21 Jul 2022 17:59:46 +0200 Alexander Lobakin wrote:
> > BTW, Ethtool bitsets provide similar functionality, but it operates
> > with u32s (u64 is more convenient and optimal on most platforms) and
> > Netlink bitmaps is a generic interface providing policies and data
> > verification (Ethtool bitsets are declared simply as %NLA_BINARY),
> > generic getters/setters etc.
> 
> Are you saying we don't need the other two features ethtool bitmaps
> provide? Masking and compact vs named representations?

Nah I didn't say that. I'm not too familiar with Ethtool bitsets,
just know that they're represented as arrays of u32s.

> 
> I think that straight up bitmap with a fixed word is awkward and leads
> to too much boilerplate code. People will avoid using it. What about
> implementing a bigint type instead? Needing more than 64b is extremely
> rare, so in 99% of the cases the code outside of parsing can keep using
> its u8/u16/u32.

In-kernel code can still use single unsigned long for some flags if
it wouldn't need more than 64 bits in a couple decades and not
bother with the bitmap API. Same with userspace -- a single 64 is
fine for that API, just pass a pointer to it to send it as a bitmap
to the kernel.

Re 64b vs extremely rare -- I would say so 5 years go, but now more
and more bitfields run out of 64 bits. Link modes, netdev features,
...

Re bigint -- do you mean implementing u128 as a union, like

typedef union __u128 {
	struct {
		u32 b127_96;
		u32 b95_64;
		u32 b63_32;
		u32 b31_0;
	};
	struct {
		u64 b127_64;
		u64 b63_0;
	};
#ifdef __HAVE_INT128
	__int128 b127_0;
#endif
} u128;

? We have similar feature in one of our internal trees and planning
to present generic u128 soon, but this doesn't work well for flags
I think.
bitmap API and bitops are widely used and familiar to tons of folks,
most platforms define their own machine-optimized bitops
implementation, arrays of unsigned longs are native...

Re awkward -- all u64 <-> bitmap conversion is implemented in the
core code in 4/4 and users won't need doing anything besides one
get/set. And still use bitmap/bitops API. Userspace, as I said,
can use a single __u64 as long as it fits into 64 bits.

Summarizing, I feel like bigints would lead to much more boilerplate
in both kernel and user spaces and need to implement a whole new API
instead of using the already existing and well-used bitmap one.
Continuation of using single objects with fixed size like %NLA_U* or
%NLA_BITFIELD_U32 will lead to introducing a new Netlink attr every
32/64 bits (or even 16 like with IP tunnels, that was the initial
reason why I started working on those 3 series). As Jake wrote me
in PM earlier,

"I like the concept of an NLA_BITMAP. I could have used this for
some of the devlink interfaces we've done, and it definitely feels
a bit more natural than being forced to a single u32 bitfield."

Thanks,
Olek