Re: [PATCH RFC 0/9] A rendezvous module

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Mar 23, 2021 at 05:25:43PM +0000, Rimmer, Todd wrote:

> In studying all the related fields, in most cases if we shuffled
> everything for maximum packing, the structures would still end up
> being about the same size and this would all be for non-performance
> path ABIs.

It is not about size, it is about uABI compatability. Do not place
packing holes in uABI structures as it is *very complicated* to
guarentee those work across all the combinations of user/kernel we
have to support.

Every uABI structure should have explicit padding, it should be
organized to minimize padding, and "easy on the programmer" is not a
top concern.

You must ensure that all compilers generate the same structure byte
layout. On x86 this means a classic i386 compiler, a AMD64 x32 ABI,
and the normal AMD64 compiler.

> It should be noted, there are existing examples with small gaps or
> reserved fields in the existing kernel and RDMA stack.  A few
> examples in ib_user_verbs.h include:

Past mistakes do not excuse future errors. Most of those are not uABI
structs.

Jason