netdev - Re: [PATCH net-next v6 06/15] ethtool: netlink bitset handling

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Date:   Thu, 04 Jul 2019 14:03:02 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Michal Kubecek <mkubecek@...e.cz>, netdev@...r.kernel.org
Cc:     Jiri Pirko <jiri@...nulli.us>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        John Linville <linville@...driver.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v6 06/15] ethtool: netlink bitset handling

On Thu, 2019-07-04 at 13:52 +0200, Michal Kubecek wrote:
> 
> There is still the question if it it should be implemented as a nested
> attribute which could look like the current compact form without the
> "list" flag (if there is no mask, it's a list). Or an unstructured data
> block consisting of u32 bit length 

You wouldn't really need the length, since the attribute has a length
already :-)

And then, if you just concatenate the value and mask, the existing
NLA_BITFIELD32 becomes a special case.

> and one or two bitmaps of
> corresponding length. I would prefer the nested attribute, netlink was
> designed to represent structured data, passing structures as binary goes
> against the design (just looked at VFINFO in rtnetlink few days ago,
> it's awful, IMHO).

Yeah, I dunno. On the one hand I completely agree, on the other hand
NLA_BITFIELD32 already goes the other way, and is there now...

> Either way, I would still prefer to have bitmaps represented as an array
> of 32-bit blocks in host byte order. This would be easy to handle in
> kernel both in places where we have u32 based bitmaps and unsigned long
> based ones. Other options seem less appealing:
> 
>   - u8 based: only complicates processing
>   - u64 based: have to care about alignment
>   - unsigned long based: alignment and also problems with 64-bit kernel
>     vs. 32-bit userspace

Agree with this.

johannes