Build failure: linux (powerpc)

[alyssais]

Building powerpc kernels (I only tested pkgsCross.powernv, but examination of the kernel source code suggests to me that 32-bit powerpc, and well as big-endian 64-bit will be the same) is broken since #221707. This happens because on powerpc, the kernel target we install is vmlinux, rather than any derivative of it like zImage. The kernel build system will strip when creating such a derivative image, but because we install vmlinux directly, we get an unstripped image. This means the kernel contains references to its build directory, which since that PR will be the $dev output.

AIUI, 64-bit POWER boots via kexec, and a zImage is a self-extracting kernel ELF image, so hopefully the fix would just be to switch the kernel target for powerpc to zImage, and things will work again, and POWER users will get smaller kernel images as a bonus.

I'm not sure about 32-bit PowerPC, but this kernel documentation might be helpful.

If we can't use an *Image target for any powerpc targets, there is also a target called vmlinux.strip that we could fall back to, but let's take the opportunity to evaluate whether anything else is more suitable first, when we're changing this stuff anyway.

Apart from powerpc, the only target we have that uses vmlinux as its kernel target is a MIPS machine called the Fuloong 2.0. I don't know how it boots.

[ghost]

I'll handle this.

I don't have big-endian POWER hardware, but @alyssais pointed out that qemu can probably boot a BE kernel under emulation at full hardware speeds. So if that works I can take care of the 32-bit POWER (which is mostly BE) too.

[vkleen]

For reference, I managed to get a working kernel derivation for Power9 with

kernelPackages = pkgs.linuxKernel.packageAliases.linux_latest.extend
    (final: prev: lib.optionalAttrs pkgs.stdenv.hostPlatform.isPower64 {
      kernel = lib.makeOverridable
        (args:
          let
            kernel = prev.kernel.override args;
          in
          kernel.overrideAttrs (o: {
            buildFlags = o.buildFlags ++ [ "modules" "zImage" ];
            installFlags = o.installFlags ++ [ "KBUILD_IMAGE=$(boot)/zImage" ];
            installTargets = [ "install" ];
            postFixup = (o.postFixup or "") + ''
              strip -s $out/zImage
            '';
            inherit (kernel) passthru;
          }))
        { };
    });

Just setting building a zImage without fully stripping it afterwards wasn't enough. There still was a reference to $dev in the debug info somewhere.

[alyssais]

Yep, very plausible that there are other references as well. We had some of those for x86 and RISC-V, although that specific issue didn't apply to other architectures.

[ghost]

I'll dig into this this evening. @vkleen thank you for the tip!

Also putting this link here so I don't lose it: https://lore.kernel.org/linux-kbuild/[email protected]/

[ghost]

Verified (after fixing a bunch of other things) that mips64el is affected too. Thanks again for the heads up @alyssais. Working on a fix.

[ghost]

#228018

[alyssais]

Edit: I meant to post this here.

[ghost]

Fixed by

• linuxManualConfig: revert #221707 #245449