linux-kernel - Re: [PATCH net-next v2 0/9] lib: packing: introduce and use (un)pack

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Message-ID: <20241031093054.p5gz2defkbytsjcx@DEN-DL-M70577>
Date: Thu, 31 Oct 2024 09:30:54 +0000
From: Daniel Machon <daniel.machon@...rochip.com>
To: Jacob Keller <jacob.e.keller@...el.com>
CC: Vladimir Oltean <olteanv@...il.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Tony Nguyen
	<anthony.l.nguyen@...el.com>, Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Masahiro Yamada <masahiroy@...nel.org>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next v2 0/9] lib: packing: introduce and use
 (un)pack_fields

> This series improves the packing library with a new API for packing or
> unpacking a large number of fields at once with minimal code footprint. The
> API is then used to replace bespoke packing logic in the ice driver,
> preparing it to handle unpacking in the future. Finally, the ice driver has
> a few other cleanups related to the packing logic.
> 
> The pack_fields and unpack_fields functions have the following improvements
> over the existing pack() and unpack() API:
> 
>  1. Packing or unpacking a large number of fields takes significantly less
>     code. This significantly reduces the .text size for an increase in the
>     .data size which is much smaller.
> 
>  2. The unpacked data can be stored in sizes smaller than u64 variables.
>     This reduces the storage requirement both for runtime data structures,
>     and for the rodata defining the fields. This scales with the number of
>     fields used.
> 
>  3. Most of the error checking is done at compile time, rather than
>     runtime via CHECK_PACKED_FIELD_* macros. This saves wasted computation
>     time, *and* catches errors in the field definitions immediately instead
>     of only after the offending code executes.
> 
> The actual packing and unpacking code still uses the u64 size
> variables. However, these are converted to the appropriate field sizes when
> storing or reading the data from the buffer.
> 
> One complexity is that the CHECK_PACKED_FIELD_* macros need to be defined
> one per size of the packed_fields array. This is because we don't have a
> good way to handle the ordering checks otherwise. The C pre-processor is
> unable to generate and run variable length loops at compile time.
> 
> This is a significant amount of macro code, ~22,000 lines of code. To
> ensure it is correct and to avoid needing to store this directly in the
> kernel history, this file is generated as <generated/packing-checks.h> via
> a small C program, gen_packing_checks. To generate this, we need to update
> the top level Kbuild process to include the compilation of
> gen_packing_checks and execution to generate the packing-checks.h file.
> 

Hi Jacob,

As for the rest of the patches:

Reviewed-by: Daniel Machon <daniel.machon@...rochip.com>

I can confirm that smatch does not complain anymore, after Dan's recent
commit that skips the macros.

/Daniel