Re: [PATCH v4] xen: Add Darwin.mk for GNU toolchains

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.02.2026 11:02, Bertrand Marquis wrote:
> Hi Jan,
> 
>> On 9 Feb 2026, at 10:45, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 09.02.2026 10:11, Roger Pau Monné wrote:
>>> On Mon, Feb 09, 2026 at 07:30:30AM +0000, Bertrand Marquis wrote:
>>>> Hi Roger,
>>>>
>>>>> On 6 Feb 2026, at 18:25, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>>>>>
>>>>> On Fri, Feb 06, 2026 at 05:21:44PM +0100, 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=unknown, 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 toolchain from brew:
>>>>>> - brew install aarch64-elf-gcc aarch64-elf-binutils
>>>>>> - cd xen
>>>>>> - make XEN_TARGET_ARCH=arm64 CROSS_COMPILE=aarch64-elf- HOSTCC=clang
>>>>>
>>>>> Instead of `cd xen` I would use `make xen ...`.
>>>>
>>>> Ack the 'cd xen' line is useless and should be removed.
>>>>
>>>>>
>>>>>>
>>>>>> Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>>>>
>>>>> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>>>
>>>>> One comment below.
>>>>>
>>>>>> ---
>>>>>> Changes since v3:
>>>>>> - set XEN_COMPILE_ARCH to unknown instead of Darwin
>>>>>> - list binutils as a dependency to install in brew in commit message
>>>>>>
>>>>>> 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
>>>>>> ---
>>>>>> config/Darwin.mk | 7 +++++++
>>>>>> 1 file changed, 7 insertions(+)
>>>>>> create mode 100644 config/Darwin.mk
>>>>>>
>>>>>> diff --git a/config/Darwin.mk b/config/Darwin.mk
>>>>>> new file mode 100644
>>>>>> index 000000000000..176b27eac676
>>>>>> --- /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.
>>>>>
>>>>> Hm, is this actually fully true?  What's the Linux ABI exactly here?
>>>>>
>>>>> FreeBSD builds Xen natively, and it's not using the Linux ABI.
>>>>> FreeBSD uses no specific -target to the compiler invocations, and the
>>>>> linker emulation is set to elf_x86_64_fbsd.
>>>>>
>>>>> I think the point here is that the toolchain must support building ELF
>>>>> objects / images, because that's the binary format supported by Xen.
>>>>> Whether it's Linux ELF or FreeBSD ELF doesn't make a difference for
>>>>> standalone environments like the Xen kernel.
>>>>
>>>> This file is also use while compiling the tools and for those you 
>>>> definitely
>>>> to cross compile as they depend on Linux ABI.
>>>
>>> I'm not an expert on this at all, but IMO the tools don't depend on
>>> the Linux ABI at all.  Anyone can add the minimal OS-specific bindings
>>> required for the tools to work on environments different than Linux.
>>> We currently have {Net,Free}BSD and Linux.  In the past we also had
>>> Solaris, but that has bit rotten.
>>>
>>>> Maybe that works on BSD
>>>> but i doubt this would be the case on mac os.
>>>
>>> I doubt we would ever have any need to build an OSX native toolchain,
>>> as it's unlikely (?) that we will ever get Xen support in the Darwin
>>> kernel.
>>
>> If Darwin has some way to load "modules" (e.g. drivers) into the kernel, it
>> may in principle be possible to run OSX as a HVM-Dom0, with a Xen driver
>> loaded separately. That's far fetched, I know.
>>
>>>> Maybe we could simplify the sentence to not say anything we are unsure:
>>>>
>>>> # Cross compile on Mac OS
>>>>
>>>> Tell me if that works for you and i can submit a v5 or this might be 
>>>> possible to
>>>> fix on commit.
>>>
>>> I'm fine with it, I would like to hear Jan's opinion.
>>
>> As long as it's clear that all of this is only about building the hypervisor
>> (which doesn't look to be the case right now beyond the example given in the
>> commit message, which may be taken as really only an example), that's okay
>> [1]. I'd still prefer to mention the SVR4 ABI here, though, to at least
>> somewhat explain the "always cross" aspect.
>>
>> I can make edits while committing, but I'm unsure whether that's wanted in
>> this case.
> 
> I would suggest:
> 
> Cross compile on Mac OS, only hypervisor build has been tested, no tools
> 
> Please do the following edits on commit if that's ok for you :
> - remove 'cd xen' from commit message
> - use previous comment in Darwin.mk

There have been several "previous" comments there, so I think in this case
it may indeed be better ...

> Othewise happy to submit a v5

... if you do this. I'm sorry, it's quite a bit of bike-shedding on "merely"
commentary, yet I think we want to avoid misleading people.

Jan