cross compilation: simplify emulateBuilderQemu & use linux-megous as kernel when available

but it doesn't seem to be emulating child processes...

overlays/cross.nix

@@ -67,43 +67,152 @@ let
   };
   emulated = mkEmulated final prev;
 
+  # linuxMinimal = final.linux.override {
+  #   # customize stock linux to compile using less RAM
+  #   # default config is in:
+  #   # - <pkgs/os-specific/linux/kernel/common-config.nix>
+  #   structuredExtraConfig = with lib.kernel; {
+  #     # recommended by: <https://nixos.wiki/wiki/Linux_kernel#Too_high_ram_usage>
+  #     DEBUG_INFO_BTF = lib.mkForce no;
+
+  #     # other debug-related things i can probably disable
+  #     CC_OPTIMIZE_FOR_SIZE = lib.mkForce yes;
+  #     DEBUG_INFO = lib.mkForce no;
+  #     DEBUG_KERNEL = lib.mkForce no;
+  #     GDB_SCRIPTS = lib.mkForce no;
+  #     SCHED_DEBUG = lib.mkForce no;
+  #     SUNRPC_DEBUG = lib.mkForce no;
+
+  #     # disable un-needed features
+  #     BT = no;
+  #     CAN = no;
+  #     DRM = no;  # uses a lot of space when compiling
+  #     FPGA = no;
+  #     GNSS = no;
+  #     IIO = no;  # 500 MB
+  #     INPUT_TOUCHSCREEN = no;
+  #     MEDIA_SDR_SUPPORT = no;
+  #     NFC = no;
+  #     SND = no;  # also uses a lot of disk space when compiling
+  #     SOUND = no;
+  #     # WWAN = no;  # 1.4 GB (drivers/net/wireless)  (but WWAN=no doesn't actually disable that?)
+
+  #     # we could try disabling these, but i wonder if anything relies on them (e.g. autoconf)
+  #     # FONTS = lib.mkForce no;
+  #     # FB = lib.mkForce no;
+  #     # WAN = lib.mkForce no;
+  #     # INET = no;
+  #     # MEMTEST = lib.mkForce no;
+  #     # # NET = lib.mkForce no;  # we need net (9pnet_virtio; unix) for sharing fs with the build machine
+  #     MEDIA_ANALOG_TV_SUPPORT = lib.mkForce no;
+  #     MEDIA_CAMERA_SUPPORT = lib.mkForce no;
+  #     MEDIA_DIGITAL_TV_SUPPORT = lib.mkForce no;  # 150 MB disk space when compiling
+  #     MICROCODE = lib.mkForce no;
+  #     STAGING = lib.mkForce no;  # 450 MB disk space when compiling
+
+  #     RTC_DRV_CMOS = yes;  # something in the above config changes disabled this...
+  #   };
+  # };
   # given a package that's defined for build == host,
   # build it from the native build machine by emulating the builder.
-  emulateBuilder = pkg: let
+  emulateBuilderQemu = pkg: let
+    vmTools = final.vmTools.override {
+      kernel = final.linux-megous or final.linux;  #< HACK: guess at whatever deployed linux we're using, to avoid building two kernels
+    };
     # fix up the nixpkgs command that runs a Linux OS inside QEMU:
     # qemu_kvm doesn't support x86_64 -> aarch64; but full qemu package does.
     qemuCommandLinux = lib.replaceStrings
       [ "${final.buildPackages.qemu_kvm}" ]
-      [ "${final.buildPackages.qemu}"]
-      final.vmTools.qemuCommandLinux;
+      [ "${final.buildPackages.qemu}" ]
+      vmTools.qemuCommandLinux;
+    vmRunCommand = final.buildPackages.vmTools.vmRunCommand qemuCommandLinux;
   in
     # without binfmt emulation, leverage the `vmTools.runInLinuxVM` infrastructure:
-    # final.vmTools.runInLinuxVM pkg
+    # final.buildPackages.vmTools.runInLinuxVM pkg
     #
-    # except `runInLinuxVM` doesn't seem to support cross compilation (what's its purpose, then?)
-    # so hack its components into something which *does* handle cross compilation
+    # except `runInLinuxVM` doesn't quite work OOTB (see above),
+    # so hack its components into something which *does* work.
     lib.overrideDerivation pkg ({ builder, args, ... }: {
       builder = "${final.buildPackages.bash}/bin/sh";
-      args = ["-e" (final.buildPackages.vmTools.vmRunCommand qemuCommandLinux)];
+      args = [ "-e" vmRunCommand ];
       # orig{Builder,Args} gets used by the vmRunCommand script:
       origBuilder = builder;
       origArgs = args;
 
-      QEMU_OPTS = "-m 16386";  # MiB of RAM
+      QEMU_OPTS = "-m 16384";  # MiB of RAM
       enableParallelBuilding = true;
 
       # finally, let nix know that this package should be built by the build system
       system = final.stdenv.buildPlatform.system;
     }) // {
-      override = attrs: emulateBuilder (pkg.override attrs);
-      overrideAttrs = mergeFn: emulateBuilder (pkg.overrideAttrs mergeFn);
+      override = attrs: emulateBuilderQemu (pkg.override attrs);
+      overrideAttrs = mergeFn: emulateBuilderQemu (pkg.overrideAttrs mergeFn);
     }
-    # alternatively, `proot` could let us get per-package binfmt:
-    # - <https://proot-me.github.io/>
-    # - i.e., execute host programs *and* build programs, mixed
   ;
 
-  emulateBuildMachine =
+  # given a package that's defined for build == host,
+  # build it from a "proot": a chroot-like environment where `exec` is hooked to invoke qemu instead.
+  # this is like binfmt, but configured to run *only* the emulated host and not the build machine
+  # see: <https://proot-me.github.io/>
+  # hinted at by: <https://www.tweag.io/blog/2022-03-31-running-wasm-native-hybrid-code/>
+  emulateBuilderProot = pkg:
+    lib.overrideDerivation pkg ({ builder, args, ... }: {
+      builder = "${final.buildPackages.bash}/bin/sh";
+      args = [ "-e" prootBuilder ];
+      origBuilder = builder;
+      origArgs = args;
+
+      enableParallelBuilding = true;  # TODO: inherit from `pkg`?
+      NIX_DEBUG = "6";
+
+      # finally, let nix know that this package should be built by the build system
+      system = final.stdenv.buildPlatform.system;
+    }) // {
+      override = attrs: emulateBuilderProot (pkg.override attrs);
+      overrideAttrs = mergeFn: emulateBuilderProot (pkg.overrideAttrs mergeFn);
+    };
+
+  prootBuilder = let
+    proot = "${final.buildPackages.proot}/bin/proot";
+    # prootFlags = "-r / -b /:/";
+    prootFlags = "-b /nix:/nix -b /tmp:/tmp";
+    # prootFlags = "-b /:/ -b ${final.bash}/bin/sh:/bin/sh";  # --mixed-mode false
+    qemu = "${final.buildPackages.qemu}/bin/qemu-aarch64";
+  in
+    final.pkgs.writeText "proot-run" ''
+      echo "proot: ${proot} -q ${qemu} ${prootFlags} $origBuilder $origArgs"
+      ${proot} -q ${qemu} ${prootFlags} $origBuilder $origArgs
+      echo "exited proot"
+    '';
+
+  emulateBuilderBinfmt = pkg:
+    lib.overrideDerivation pkg ({ builder, args, ...}: {
+      builder = "${final.buildPackages.bash}/bin/sh";
+      args = [ "-e" binfmtBuilder ];
+      origBuilder = builder;
+      origArgs = args;
+
+      # finally, let nix know that this package should be built by the build system
+      system = final.stdenv.buildPlatform.system;
+    }) // {
+      override = attrs: emulateBuilderBinfmt (pkg.override attrs);
+      overrideAttrs = mergeFn: emulateBuilderBinfmt (pkg.overrideAttrs mergeFn);
+    };
+
+  binfmtBuilder = let
+    sudo = "${final.buildPackages.sudo}/bin/sudo";
+    mount = "${final.buildPackages.util-linux.mount}/bin/mount";
+  in
+    final.pkgs.writeText "binfmt-run" ''
+      echo "binfmtBuilder: mounting binfmt_misc"
+      ${sudo} ${mount} binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
+      echo "binfmtBuilder: running $origBuilder $origArgs"
+      $origBuilder $origArgs
+    '';
+
+  # given a package defined for build != host, transform it to build on the host.
+  # i.e. build using the host's stdenv.
+  buildOnHost =
     let
       # patch packages which can't ordinarily exist in buildPackages
       preFixPkg = p:
@@ -122,7 +231,7 @@ let
       unsplicePkg = p: p.__spliced.hostTarget or p;
       unsplicePkgs = ps: map (p: unsplicePkg (preFixPkg p)) ps;
     in
-      pkg: emulateBuilder ((pkg.override {
+      pkg: (pkg.override {
         inherit (emulated) stdenv;
       }).overrideAttrs (upstream: {
         # for this purpose, the naming in `depsAB` is "inputs build for A, used to create packages in B" (i think).
@@ -141,7 +250,11 @@ let
 
         nativeCheckInputs = unsplicePkgs (upstream.nativeCheckInputs or []);
         nativeInstallCheckInputs = unsplicePkgs (upstream.nativeInstallCheckInputs or []);
-      }));
+      });
+
+  buildInQemu = pkg: emulateBuilderQemu (buildOnHost pkg);
+  buildInProot = pkg: emulateBuilderProot (buildOnHost pkg);
+  buildInBinfmt = pkg: emulateBuilderBinfmt (buildOnHost pkg);
 in {
   inherit emulated;
 
@@ -352,7 +465,7 @@ in {
     unwrapped = super.unwrapped // {
       browserpass-extension = super.unwrapped.browserpass-extension.override {
         # this overlay is optional for binfmt machines, but non-binfmt can't cross-compile the modules (for use at runtime)
-        mkYarnModules = args: emulateBuildMachine {
+        mkYarnModules = args: buildInQemu {
           override = { stdenv }: (
             (final.yarn2nix-moretea.override {
               pkgs = final.pkgs.__splicedPackages // { inherit stdenv; };
@@ -791,7 +904,7 @@ in {
   koreader = (prev.koreader.override {
     # fixes runtime error: luajit: ./ffi/util.lua:757: attempt to call field 'pack' (a nil value)
     # inherit (emulated) luajit;
-    luajit = emulateBuildMachine (final.luajit.override {
+    luajit = buildInQemu (final.luajit.override {
       buildPackages.stdenv = emulated.stdenv;  # it uses buildPackages.stdenv for HOST_CC
     });
   }).overrideAttrs (upstream: {
@@ -802,7 +915,7 @@ in {
   koreader-from-src = prev.koreader-from-src.override {
     # fixes runtime error: luajit: ./ffi/util.lua:757: attempt to call field 'pack' (a nil value)
     # inherit (emulated) luajit;
-    luajit = emulateBuildMachine (final.luajit.override {
+    luajit = buildInQemu (final.luajit.override {
       buildPackages.stdenv = emulated.stdenv;  # it uses buildPackages.stdenv for HOST_CC
     });
   };
@@ -1651,7 +1764,7 @@ in {
   tangram = (prev.tangram.override {
     # N.B. blueprint-compiler is in nativeBuildInputs.
     # the trick here is to force the aarch64 versions to be used during build (via emulation),
-    blueprint-compiler = emulateBuildMachine (final.blueprint-compiler.overrideAttrs (upstream: {
+    blueprint-compiler = buildInQemu (final.blueprint-compiler.overrideAttrs (upstream: {
       # default is to propagate gobject-introspection *as a buildInput*, when it's supposed to be native.
       propagatedBuildInputs = [];
       # "Namespace Gtk not available"

pkgs/additional/linux-megous/default.nix

@@ -1,13 +1,10 @@
 { lib
 , buildLinux
-, buildPackages
 , fetchFromGitHub
-, modDirVersionArg ? null
-, nixosTests
-, perl
 , pkgs
+# something inside nixpkgs calls `override` on the kernel and passes in extra arguments
 , ...
-} @ args:
+}@args:
 
 with lib;
 
@@ -34,6 +31,16 @@ let
   kernelConfig = with lib.kernel; {
     # NB: nix adds the CONFIG_ prefix to each of these.
     # if you add the prefix yourself nix will IGNORE YOUR CONFIG.
+
+    # optimize for faster builds.
+    # see <repo:kernel.org/linux:Documentation/admin-guide/quickly-build-trimmed-linux.rst>
+    DEBUG_KERNEL = lib.mkForce no;  # option group which seems to just gate the other DEBUG_ opts?
+    DEBUG_INFO = lib.mkForce no;  # for gdb debugging
+    DEBUG_INFO_BTF = lib.mkForce no;  # BPF debug symbols. rec by <https://nixos.wiki/wiki/Linux_kernel#Too_high_ram_usage>
+    SCHED_DEBUG = lib.mkForce no;  # determines /sys/kernel/debug/sched
+    # SUNRPC_DEBUG = lib.mkForce no;  # i use NFS though
+
+    # taken from mobile-nixos config?? or upstream megous config??
     RTL8723CS = module;
     BT_HCIUART_3WIRE = yes;
     BT_HCIUART_RTL = yes;
@@ -99,35 +106,36 @@ let
   extraKernelPatches = [
     pkgs.kernelPatches.bridge_stp_helper
     pkgs.kernelPatches.request_key_helper
-    (patchDefconfig kernelConfig)
+    # (patchDefconfig kernelConfig)
   ];
 
 
   # create a kernelPatch which overrides nixos' defconfig with extra options
-  patchDefconfig = config: {
-    # defconfig options. this method comes from here:
-    # - https://discourse.nixos.org/t/the-correct-way-to-override-the-latest-kernel-config/533/9
-    name = "linux-megous-defconfig";
-    patch = null;
-    extraStructuredConfig = config;
-  };
+  # patchDefconfig = config: {
+  #   # defconfig options. this method comes from here:
+  #   # - https://discourse.nixos.org/t/the-correct-way-to-override-the-latest-kernel-config/533/9
+  #   name = "linux-megous-defconfig";
+  #   patch = null;
+  #   extraStructuredConfig = config;
+  # };
 
-  overridenArgs = args // rec {
+in buildLinux (args // {
   version = base + rc;
 
-    # modDirVersion needs to be x.y.z, will automatically add .0 if needed
-    modDirVersion = if (modDirVersionArg == null) then concatStringsSep "." (take 3 (splitVersion "${version}.0")) + rc else modDirVersionArg;
+  # modDirVersion needs to be x.y.z, where `z` could be `Z-rcN`
+  # nix kernel build will sanity check us if we get the modDirVersion wrong
+  modDirVersion = base + rc;
 
   # branchVersion needs to be x.y
-    extraMeta.branch = versions.majorMinor version;
+  extraMeta.branch = versions.majorMinor base;
 
   src = fetchFromGitHub {
     owner = "megous";
     repo = "linux";
     inherit rev hash;
   };
-  } // (args.argsOverride or { });
-  finalArgs = overridenArgs // {
-    kernelPatches = overridenArgs.kernelPatches or [] ++ extraKernelPatches;
-  };
-in buildLinux finalArgs
+
+  kernelPatches = (args.kernelPatches or []) ++ extraKernelPatches;
+
+  structuredExtraConfig = (args.structuredExtraConfig or {}) // kernelConfig;
+})