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Re: [RFC PATCH 0/6] xen/abi: On wide bitfields inside primitive types


  • To: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Wed, 30 Oct 2024 10:14:12 +0100
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Christian Lindig <christian.lindig@xxxxxxxxxx>, David Scott <dave@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Wed, 30 Oct 2024 09:14:23 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 29.10.2024 19:16, Alejandro Vallejo wrote:
> Non-boolean bitfields in the hypercall ABI make it fairly inconvenient to
> create bindings for any language because (a) they are always ad-hoc and are
> subject to restrictions regular fields are not (b) require boilerplate that
> regular fields do not and (c) might not even be part of the core language,
> forcing avoidable external libraries into any sort of generic library.
> 
> This patch (it's a series merely to split roughly by maintainer) is one such
> case that I happened to spot while playing around. It's the grant_version
> field, buried under an otherwise empty grant_opts.
> 
> The invariant I'd like to (slowly) introduce and discuss is that fields may
> have bitflags (e.g: a packed array of booleans indexed by some enumerated
> type), but not be mixed with wider fields in the same primitive type. This
> ensures any field containing an integer of any kind can be referred by pointer
> and treated the same way as any other with regards to sizeof() and the like.

While I don't strictly mind, I'm also not really seeing why taking addresses
or applying sizeof() would be commonly necessary. Can you perhaps provide a
concrete example of where the present way of dealing with grant max version
is getting in the way? After all your use of the term "bitfield" doesn't
really mean C's understanding of it, so especially (c) above escapes me to a
fair degree.

> I'd like to have a certain consensus about this general point before going
> establishing this restriction in the IDL system I'm working on.
> 
> My preference would be to fold everything into a single patch if we decide to
> follow through with this particular case. As I said before, the split is
> artificial for review.

That's not just a preference, but a requirement, or else the build will break
in the middle of the series (harming bisection at the very least).

Jan



 


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