Re: [PATCH v3] xen: Add Darwin.mk for GNU toolchains

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 06, 2026 at 01:34:23PM +0000, Bertrand Marquis wrote:
> 
> 
> > On 6 Feb 2026, at 14:29, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
> > 
> > On Fri, Feb 06, 2026 at 11:38:02AM +0100, Jan Beulich wrote:
> >> On 06.02.2026 09:17, Bertrand Marquis wrote:
> >>> Xen does not provide a Darwin build configuration for selecting
> >>> GNU tool definitions. On macOS, the tools we use are either GNU
> >>> compatible or we only rely on features supported by Mac OS, so
> >>> using the GNU tool definitions is appropriate.
> >>> 
> >>> Add config/Darwin.mk to include StdGNU.mk and force
> >>> XEN_COMPILE_ARCH=Darwin, ensuring Darwin builds always follow
> >>> the cross-compile path as we depend on the Linux ABI so compiling
> >>> on Mac OS is always a cross compilation case.
> >>> 
> >>> An example of how to build the hypervisor for arm64 on Mac OS
> >>> (tools cannot be build for now) using a compiler from brew:
> >>> - brew install aarch64-elf-gcc
> >>> - cd xen
> >>> - make XEN_TARGET_ARCH=arm64 CROSS_COMPILE=aarch64-elf- HOSTCC=gcc

I've just noticed: it's a bit misleading to use HOSTCC=gcc here, as
(under a normal OSX system) gcc is a plain wrapped around clang:

% gcc -v
Apple clang version 17.0.0 (clang-1700.6.3.2)
Target: arm64-apple-darwin24.6.0
Thread model: posix

You might as well use HOSTCC=clang and make it explicit the host
compiler is clang and not gcc.

> >>> 
> >>> Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> >>> ---
> >>> Changes since v2:
> >>> - Subject was "xen: Add macOS hypervisor build configuration"
> >>> - Update Darwin.mk comments to more accurate versions (Jan)
> >>> - Remove the build-on-macos help as we have no dependency on anything
> >>>  coming from brew anymore and the toolchain can be retrieved by lots of
> >>>  other solutions than brew on mac os. Switch to a simple doc in the
> >>>  commit message instead
> >> 
> >> Did you see Roger's notice on Matrix about objcopy?
> > 
> > I think Bertrand considers objcopy to be part of the toolchain, hence
> > "retrieving a toolchain" is meant to include objcopy (either the GNU,
> > LLVM or elftoolchain one)
> 
> Sorry i only saw your message in matrix.
> 
> I checked and i installed both gcc and binutils in homebrew.
> 
> So i think the commit message needs modifying to stay:
> 
> brew install aarch64-elf-gcc aarch64-elf-binutils
> 
> to be fully complete.

Yes, I didn't notice that in the commit message you explicitly
mentioned the brew install dependencies.  There's also bison and flex
needed for Kconfig, but AFAICT those are part of command line tools.
I think python is also part of the command line tools, and not sure
it's required for arm64, as it's required for x86 to generate the
cpuid headers (but I don't know if arm64 has anything equivalent).

> > 
> >>> --- /dev/null
> >>> +++ b/config/Darwin.mk
> >>> @@ -0,0 +1,7 @@
> >>> +# Use GNU tool definitions as the tools we are using are either GNU 
> >>> compatible
> >>> +# or we only use features which are supported on Mac OS.
> >>> +include $(XEN_ROOT)/config/StdGNU.mk
> >>> +
> >>> +# Xen uses Linux'es ABI so we are cross compiling on Mac OS.
> >>> +# Force COMPILE_ARCH to a fake value to make sure it is always the case.
> >>> +XEN_COMPILE_ARCH = Darwin
> >> 
> >> I first wondered why you say "fake", seeing the file is named Darwin.mk. 
> >> But
> >> in Config.mk's cross-compile check the build host OS doesn't even matter. 
> >> So
> >> yes, it needs faking here for the time being.
> > 
> > Hm, setting it to "Darwin" seems weird to me.  I understand the
> > purpose of this is to force the user to set XEN_TARGET_ARCH
> > explicitly.  I however wouldn't see it as fully uncorrect to not set
> > this.  It will then execute `uname -m` and get `arm64` back for Apple
> > silicon macs (which is kind of OK?).  Other I suggest we use a non-OSX
> > specific value here, so that if required we could distinguish this
> > case where the user is expected to provide XEN_COMPILE_ARCH.
> > 
> > Maybe XEN_COMPILE_ARCH = { unknown | undefined }?
> 
> I am ok to change this with either but maybe unsupported could be
> a third choice?

No strong opinion for either naming, as long as we don't explicitly
use "Darwin".

Thanks, Roger.