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

[Vladimir Oltean <olteanv@gmail.com>]

On Fri, Oct 25, 2024 at 05:04:52PM -0700, Jacob Keller wrote:
> 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.
> 
> Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
> ---
> Changes in v2:
> - Add my missing sign-off to the first patch
> - Update the descriptions for a few patches
> - Only generate CHECK_PACKED_FIELDS_N when another module selects it
> - Add a new patch introducing wrapper structures for the packed Tx and Rx
>   queue context, suggested by Vladimir.
> - Drop the now unnecessary macros in ice, thanks to the new types
> - Link to v1: https://lore.kernel.org/r/20241011-packing-pack-fields-and-ice-implementation-v1-0-d9b1f7500740@intel.com

For the set:

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>

Thanks for working on this!