SIMD-0291: Commission Rate in Basis Points (#291)

* SIMD-0291: Commission Rate in Basis Points

* update simd number

* add dependent simd

proposals/0291-commission-rate-in-basis-points.md

@@ -0,0 +1,134 @@
+---
+simd: '0291'
+title: Commission Rate in Basis Points
+authors: Justin Starry (Anza)
+category: Standard
+type: Core
+status: Review
+created: 2025-05-29
+feature: (fill in with feature key and github tracking issues once accepted)
+---
+
+## Summary
+
+Allow validators to update their inflations reward commission rate with basis
+points and update commission calculation.
+
+## Motivation
+
+Validators should have more fine grained control over commission values beyond
+integer percentage values.
+
+## Dependencies
+
+This proposal depends on the following previously accepted proposals:
+
+- **[SIMD-0185]: Vote Account v4**
+
+    Introduces version 4 of the vote account state, updating the commission
+    field to use basis points for greater precision.
+
+- **[SIMD-0249]: Delay Commission Updates**
+
+    Simplifies commission update rules so that the `UpdateCommissionBps`
+    instruction will not need to restrict updates in any way.
+
+[SIMD-0185]: https://github.com/solana-foundation/solana-improvement-documents/pull/185
+[SIMD-0249]: https://github.com/solana-foundation/solana-improvement-documents/pull/249
+
+## New Terminology
+
+NA
+
+## Detailed Design
+
+### Commission Calculation
+
+After the adoption of [SIMD-0185], commission rates will begin to be stored in
+basis points but they could still only be set in whole integer percentages. As a
+result, all stored values were multiples of 100 basis points. As a result, the
+actual commission calculation logic remained unchanged: the system continued to
+divide by 100 and use whole integer percentage values.
+
+This proposal introduces support for setting commission rates with full basis
+point precision (e.g., 1,234 basis points = 12.34%). Because commission values
+are no longer limited to clean multiples of 100, the calculation logic must now
+operate directly on basis point values. To compute the inflation reward
+commission while avoiding overflow, first convert the pending inflation reward
+to a `u128` integer. Then multiply it by the lesser of the vote account’s
+commission rate or the maximum of `10,000` basis points. Lastly, divide by
+`10,000` using integer division and discard the remainder.
+
+To calculate the portion of the reward that goes to delegated stake, again
+convert to `u128`. Multiply by the greater of `0` or `10,000` minus the vote
+account’s commission rate. Lastly, divide by `10,000` and discard the remainder.
+
+### Vote Program
+
+```rust
+pub enum VoteInstruction {
+    /// # Account references
+    ///   0. `[WRITE]` Vote account to be updated with the new commission
+    ///   1. `[SIGNER]` Withdraw authority
+    UpdateCommissionBps { // 17u32
+        commission_bps: u16,
+        kind: CommissionKind,
+    },
+}
+
+#[repr(u8)]
+pub enum CommissionKind {
+    InflationRewards = 0,
+    BlockRevenue = 1,
+}
+```
+
+#### UpdateCommissionBps
+
+A new instruction for setting a commission rate denominated in basis points will
+be added to the vote program with the enum discriminant value of `17u32` little
+endian encoded in the first 4 bytes.
+
+Perform the following checks:
+
+- If the number of account inputs is less than 1, return
+`InstructionError::NotEnoughAccountKeys`
+- If the vote account (index `0`) fails to deserialize, return
+`InstructionError::InvalidAccountData`
+- If the vote account's authorized withdrawer is not an account input for the
+instruction or is not a signer, return
+`InstructionError::MissingRequiredSignature`
+
+Update the corresponding field for the specified commission kind:
+
+- `CommissionKind::InflationRewards`: update the
+`inflation_rewards_commission_bps` field
+- `CommissionKind::BlockRevenue`: return
+`InstructionError::InvalidInstructionData`
+
+Note that the commission rate is allowed to be set and stored as any `u16` value
+but as detailed above, it will capped at 10,000 during the actual commission
+calculation.
+
+## Alternatives Considered
+
+NA
+
+## Impact
+
+Validators will be able to set inflation rewards commission rates in basis
+points. 
+
+## Security Considerations
+
+NA
+
+## Drawbacks *(Optional)*
+
+Why should we not do this?
+
+## Backwards Compatibility *(Optional)*
+
+A feature gate will be used to simultaneously update the vote program to support
+commission rates in basis points as well as update the runtime's commission
+calculations at epoch boundaries.