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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Jan,

> On 9 Feb 2026, at 11:17, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> 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.

ok.

Cheers
Bertrand

> 
> Jan