Expose our packages in flake packages output

To support the new instrumentation for HWC it would be useful to already
build nOS-V with PAPI support enabled. The enablePapi switch allows it
to be disabled with `nosv.override { enablePapi = false; }`.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

flake.nix

@@ -3,15 +3,33 @@
 
   outputs = { self, nixpkgs, ...}:
   let
+    system = "x86_64-linux";
     pkgs = import nixpkgs {
+      inherit system;
       # For now we only support x86
-      system = "x86_64-linux";
       overlays = [ self.overlays.default ];
     };
   in
     {
       bscOverlay = import ./overlay.nix;
       overlays.default = self.bscOverlay;
-      legacyPackages.x86_64-linux = pkgs;
+
+      # full nixpkgs with our overlay applied
+      legacyPackages.${system} = pkgs;
+
+      # packages added by our overlay
+      packages.${system} =
+        let
+          inherit (builtins) attrNames removeAttrs;
+          inherit (nixpkgs.lib) flip filterAttrs getAttrs pipe isDerivation;
+        in
+          # extract the names of the packages from the overlay, then get the
+          # actual packages from the full package set with the overlay applied
+          pipe (self.overlays.default null null) [
+            attrNames
+            (flip getAttrs pkgs)
+            (flip removeAttrs [ "bsc" ])
+            (filterAttrs (_: isDerivation))
+          ];
     };
 }

pkgs/nosv/default.nix

@@ -6,6 +6,8 @@
 , pkg-config
 , numactl
 , hwloc
+, papi
+, enablePapi ? true
 , cacheline ? 64 # bits
 , ovni ? null
 , useGit ? false
@@ -56,5 +58,5 @@ in
       numactl
       hwloc
       ovni
-    ];
+    ] ++ lib.optionals enablePapi [ papi ];
   }