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Date: Wed, 11 Oct 2023 09:45:50 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: Johannes Berg <johannes@...solutions.net>
Cc: Jakub Kicinski <kuba@...nel.org>, Nicolas Dichtel 
 <nicolas.dichtel@...nd.com>, netdev@...r.kernel.org, fw@...len.de,
 pablo@...filter.org, jiri@...nulli.us, mkubecek@...e.cz,
 aleksander.lobakin@...el.com, Thomas Haller <thaller@...hat.com>
Subject: Re: [RFC] netlink: add variable-length / auto integers

On Wed, 11 Oct 2023 18:01:49 +0200
Johannes Berg <johannes@...solutions.net> wrote:

> On Wed, 2023-10-11 at 08:52 -0700, Jakub Kicinski wrote:
> 
> > > > > Even for arches which don't have good unaligned access - I'd think
> > > > > that access aligned to 4B *is* pretty efficient, and that's all
> > > > > we need. Plus kernel deals with unaligned input. Why can't user space?    
> > > > 
> > > > Hmm. I have a vague recollection that it was related to just not doing
> > > > it - the kernel will do get_unaligned() or similar, but userspace if it
> > > > just accesses it might take a trap on some architectures?
> > > > 
> > > > But I can't find any record of this in public discussions, so ...    
> > > If I remember well, at this time, we had some (old) architectures that triggered
> > > traps (in kernel) when a 64-bit field was accessed and unaligned. Maybe a mix
> > > between 64-bit kernel / 32-bit userspace, I don't remember exactly. The goal was
> > > to align u64 fields on 8 bytes.  
> > 
> > Reading the discussions I think we can chalk the alignment up 
> > to "old way of doing things". Discussion was about stats64, 
> > if someone wants to access stats directly in the message then yes, 
> > they care a lot about alignment.
> > 
> > Today we try to steer people towards attr-per-field, rather than
> > dumping structs. Instead of doing:
> > 
> > 	struct stats *stats = nla_data(attr);
> > 	print("A: %llu", stats->a);
> > 
> > We will do:
> > 
> > 	print("A: %llu", nla_get_u64(attrs[NLA_BLA_STAT_A]));  
> 
> Well, yes, although the "struct stats" part _still_ even exists in the
> kernel, we never fixed that with the nla_put_u64_64bit() stuff, that was
> only for something that does
> 
> 	print("A: %" PRIu64, *(uint64_t *)nla_data(attrs[NLA_BLA_STAT_A]));
> 
> > Assuming nla_get_u64() is unalign-ready the problem doesn't exist.  
> 
> Depends on the library, but at least for libnl that's true since ever.
> Same for libmnl and libnl-tiny. So I guess it only ever hit hand-coded
> implementations.

Quick check of iproute2 shows places where stats are directly
mapped without accessors. One example is print_mpls_stats().

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