(Same as on Kusama https://kusama.polkassembly.io/referenda/173)

# Fix HostConfiguration (maxCodeSize to 3 MiB)

The current Polkadot `HostConfiguration` does not fulfill all assumptions about its consistency. This proposal aims at correcting that by modifying the on-chain config.

## Problem

 The `HostConfiguration::consistency_check` function asserts that the on-chain [`max_code_size`](https://github.com/paritytech/polkadot/blob/v0.9.41/runtime/parachains/src/configuration.rs#L376) is never larger than `MAX_CODE_SIZE`. This is violated on Polkadot since the concrete `max_code_size` on-chain is 10 MiB and `MAX_CODE_SIZE` is only [3 MiB](https://github.com/paritytech/polkadot/blob/v0.9.41/primitives/src/v2/mod.rs#L331).  
 You can verify this by querying [PolkadotJS](https://polkadot.js.org/apps/#/js):
 
```js
let config = await api.query.configuration.activeConfig();
console.log(config.maxCodeSize / (1024*1024), "MiB");
// Prints 10 MiB
```

It is neccecary to correct this since otherwise the `consistency_check` will stay a No-OP with the [assumption](https://github.com/paritytech/polkadot/blob/v0.9.41/runtime/parachains/src/configuration.rs#L1324): "You cannot break something that is already broken".


## Proposal

Set the on-chain `max_code_size` back to a sane value of 3 MiB. A larger value is not expected to work due to networking restrictions. Related discussions about this are in [polkadot/#7108](https://github.com/paritytech/polkadot/issues/7108) and [polkadot/#7105](https://github.com/paritytech/polkadot/issues/7105).

## Impact on Para*s

Parachain/Parathread teams should check that all queued runtime upgrade have a size of at most 3 MiB (the `Runtime Size` as reported by `srtool`). A sweep across [all parachains](https://github.com/paritytech/polkadot/issues/7108#issuecomment-1516220831) showed that the largest is currently 1.7 MiB, that is about 57% of the new limit. So no problem here.

## Impact on Developers

\-

Set the max validation code size for incoming upgrades.

:

- The current Polkadot `HostConfiguration` does not fulfill all assumptions about its consistency. 
- This proposal aims to change the on-chain config of `maxCodeSize` from 10 MiB to 3 MiB which will ensure the `consistency_check` is not a No-OP. 
- Parachain/Parathread teams should check that all queued runtime upgrade have a size of at most 3 MiB. 
- Impact on Developers is -

#9 Fix HostConfiguration

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