Re: [PATCH v2 7/8] gpu: nova-core: add NVKV typed decoding

["Gary Guo" <gary@garyguo.net>]

On Mon Sep 14, 2026 at 7:55 AM BST, Eliot Courtney wrote:
> On Mon Sep 14, 2026 at 12:46 PM JST, Alexandre Courbot wrote:
>> On Thu Aug 27, 2026 at 11:12 PM JST, Eliot Courtney wrote:
>>>  use crate::gsp::nvkv::{
>>> +    Array,
>>>      Index,
>>> +    Key,
>>>      KeyId,
>>>      Op,
>>>      Opcode, //
>>>  };
>>>  use crate::num;
>>>  
>>> +/// Defines a schema struct together with its [`Schema`] implementation that decodes into `$target`.
>>> +///
>>> +/// Each member of the struct should implement `Schema`. For every (key, index, value) triple
>>> +/// decoded from the NVKV stream, the generated parent `Schema` implementation will call each member
>>> +/// in declaration order with that triple. If a member consumes that triple, it will stop there.
>>> +/// Otherwise it will keep going until all members are tried.
>>> +///
>>> +/// The schema struct holds the state required by the schema implementation to do the decode. It's
>>> +/// recommended to use one of the existing Schema kinds (`Required`, `Accumulated`, `Key`, `Array`,
>>> +/// `Indexed`) for each member.
>>> +///
>>> +/// # Examples
>>> +///
>>> +/// ```
>>> +/// nvkv_decode! {
>>> +///     struct RequestSchema => Request {
>>> +///         id: Required<u32, 0x0001>,
>>> +///         name: Array<u8, 64, 0x0002>,
>>> +///     }
>>> +/// }
>>> +/// ```
>>> +macro_rules! nvkv_decode {
>>> +    (
>>> +        $(#[$attr:meta])*
>>> +        $vis:vis struct $name:ident => $target:ident {
>>> +            $(
>>> +                $(#[$field_attr:meta])*
>>> +                $field_vis:vis $field:ident : $ty:ty
>>> +            ),* $(,)?
>>> +        }
>>> +    ) => {
>>> +        $(#[$attr])*
>>> +        $vis struct $name {
>>> +            $(
>>> +                $(#[$field_attr])*
>>> +                $field_vis $field: $ty,
>>> +            )*
>>> +        }
>>> +
>>> +        impl $crate::gsp::nvkv::Schema for $name {
>>> +            type Target = $target;
>>> +
>>> +            fn init() -> impl ::kernel::prelude::Init<Self> {
>>> +                ::pin_init::init!(Self {
>>> +                    $( $field <- <$ty as $crate::gsp::nvkv::Schema>::init(), )*
>>> +                })
>>> +            }
>>> +
>>> +            fn visit(
>>> +                &mut self,
>>> +                key: $crate::gsp::nvkv::KeyId,
>>> +                index: $crate::gsp::nvkv::Index,
>>> +                value: $crate::gsp::nvkv::DecoderValue<'_>,
>>> +            ) -> ::kernel::error::Result<bool> {
>>> +                Ok(false
>>> +                    $( || $crate::gsp::nvkv::Schema::visit(&mut self.$field, key, index, value)? )*)
>>
>> Mmm looks like this is going to be `O(n)` with `n` being the number of
>> fields?
>>
>> This is ok for a first implementation but eventually I hope we can
>> switch to a more efficient dispatch.
>
> I thought quite a bit about this while writing this code, since we need
> the escape hatch to imperative decode (custom Schema impl basically). To
> be able to get it down to a match on the key, we need to know ahead of
> time which keys a Schema will consume. That duplicates the info from the
> visit() implementation.
>
> I thought up a few methods but it's unclear to me which one is best, so
> I just left it for now. Please LMK if you think this is urgent, I can
> try in a follow up to improve this. Here are my ideas (when I say O(1)
> lookup I mean modulo how the compiler decides to do it with the set of
> key IDs it gets):
>
> 1. current code - just visit()
> pros: key source of truth not duplicates
> cons: O(field) visit as you say
>
> 2. Associated const KEY_ID: Option<KeyId> - None if a Schema accepts multiple keys.
> You can match on each associated const in the macro.
> pros: O(1) if the current key goes to a field with KEY_ID = Some(...)
> cons: O(#fields accepting multiple keys) if current key is one of them
>
> 3. fn accepts() -> bool
> You can match on `if F::accepts(key)` for each field. We could potentially make
> this const with Gary's const traits polyfill.
> pros: O(1) if you write an inline-able+optimizable implementation.
>
> 4. Associated const KEYS table; use tricks to concat tables
> pros: O(1) lookup 
> cons: actually MSRV can't get this to optimize down to O(1) 
>   if you use slice::contains(), but stable can.

Hmm, am I missing the obvious? Why not generate a `match` expression on IDs of
fields? It looks like in the example all keys would have a known ID to the
macro.

Best,
Gary