On the other hand lower cpufreq should be more power efficient (due to lower core temperatures).

I guess if you wanted to maximise power efficiency the only way of finding the sweet spot is a lot of benchmarking.

In reply to @skeuchel:matrix.org

tau: Here is a minimal flake that at least evaluates

{
  inputs = {
    nixpkgs.url = "github:AlexandreTunstall/nixpkgs/ghc-cross";
  };

  outputs = { nixpkgs, ... }:
    {
      nixosConfigurations.riscv64 = nixpkgs.lib.nixosSystem {
        modules = [({lib, pkgs, ...}: {
          nixpkgs.crossSystem.system = "riscv64-linux";
          nixpkgs.localSystem.system = "x86_64-linux";
          environment.systemPackages = [
             pkgs.haskell.compiler.native-bignum.ghc948
          ];
          # Dummy values
          fileSystems."/".device  = lib.mkDefault "/dev/sda1";
          boot.loader.grub.device = lib.mkDefault "/dev/sda";
        })];
      };
    };
}

Maybe you can use that to figure out what is going wrong.

i really don't understand why it's not building, i added the crossSystem and localSystem options,

┏━ 1 Errors: 
 ⋮ 
┃           182|       buildCommand = ''
┃              |       ^
┃           183|         mkdir -p $out/nix-support $out/sd-image
┃ 
┃        (stack trace truncated; use '--show-trace' to show the full trace)
┃ 
┃        error: Package ‘ghc-9.6.4’ in /nix/store/y45vqv6pa8bhgag1dw86rvi6rk55xhxn-source/pkgs/development/compilers/ghc/common-hadrian.nix:525 is not available o…
┃          hostPlatform.config = "riscv64-unknown-linux-gnu"
┃          package.meta.platforms = [
┃            "aarch64-darwin"
┃            "aarch64-linux"
┃            "i686-linux"
┃            "x86_64-darwin"
┃            "x86_64-linux"
┃          ]
┃          package.meta.badPlatforms = [ ]
┃        , refusing to evaluate.
┃ 
┃        a) To temporarily allow packages that are unsupported for this system, you can use an environment variable
┃           for a single invocation of the nix tools.
┃ 
┃             $ export NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1
┃ 
┃           Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
┃                 then pass `--impure` in order to allow use of environment variables.
┃ 
┃        b) For `nixos-rebuild` you can set
┃          { nixpkgs.config.allowUnsupportedSystem = true; }
┃        in configuration.nix to override this.
┃ 
┃        c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
┃          { allowUnsupportedSystem = true; }
┃        to ~/.config/nixpkgs/config.nix.
┣━━━                                                            
┗━ ∑ ⚠ Exited with 1 errors reported by nix at 00:52:48 after 7s

┏━ 1 Errors: 
┃ error:
┃        … while calling the 'derivationStrict' builtin
┃ 
┃          at /derivation-internal.nix:9:12:
┃ 
┃             8|
┃             9|   strict = derivationStrict drvAttrs;
┃              |            ^
┃            10|
┃ 
┃        … while evaluating derivation 'nixos-sd-image-24.05.20240327.2726f12-riscv64-linux-starfive-visionfive2.img'
┃          whose name attribute is located at /nix/store/y45vqv6pa8bhgag1dw86rvi6rk55xhxn-source/pkgs/stdenv/generic/make-derivation.nix:331:7
┃ 
┃        … while evaluating attribute 'buildCommand' of derivation 'nixos-sd-image-24.05.20240327.2726f12-riscv64-linux-starfive-visionfive2.img'
┃ 
┃          at /nix/store/y45vqv6pa8bhgag1dw86rvi6rk55xhxn-source/nixos/modules/installer/sd-card/sd-image.nix:182:7:
┃ 
┃           181|
┃           182|       buildCommand = ''
┃              |       ^
┃           183|         mkdir -p $out/nix-support $out/sd-image
┃ 
┃        (stack trace truncated; use '--show-trace' to show the full trace)
┃ 
┃        error: cannot bootstrap GHC on this platform ('riscv64-linux' with libc 'defaultLibc')
┣━━━                                                            
┗━ ∑ ⚠ Exited with 1 errors reported by nix at 00:53:28 after 6s

( when adding the --impure flag )

It looks like you're missing the overlay that replaces the Nixpkgs GHCs with those based off of the cross-compiled GHC.

Do you have a link to a reproducer I can look at?