Re: [XEN PATCH v7 49/51] build: adding out-of-tree support to the xen build

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.10.2021 12:40, Juergen Gross wrote:
> On 18.10.21 12:36, Jan Beulich wrote:
>> On 18.10.2021 12:28, Juergen Gross wrote:
>>> On 18.10.21 11:51, Anthony PERARD wrote:
>>>> On Mon, Oct 18, 2021 at 11:02:20AM +0200, Jan Beulich wrote:
>>>>> On 15.10.2021 18:58, Anthony PERARD wrote:
>>>>>> I have to think about that. I've made some further progress in order to
>>>>>> be able to build the Xen pvhshim without a link farm and notice that
>>>>>> nearly every source file needs to use "$(srctree)/$(src)"
>>>>>
>>>>> Oh, now I'm curious as to the why here. I thought use of $(srctree)
>>>>> ought to be the exception.
>>>>
>>>> In Linux, the use of $(srctree) is indeed the exception. This is because
>>>> we have VPATH=$(srctree), so when `make` look for a prerequisite or a
>>>> target it will look first in the current directory and then in
>>>> $(srctree). That works fine as long as the source tree only have sources
>>>> and no built files.
>>>>
>>>> But if we want to be able to build the pv-shim without the linkfarm and
>>>> thus using out-of-tree build, we are going to need the ability to build
>>>> from a non-clean source tree. I don't think another way is possible.
>>>
>>> Is there any reason (apart from historical ones) to build the hypervisor
>>> in $(srctree)?
>>>
>>> I could see several advantages to build it in another directory as soon
>>> as the build system has this capability:
>>>
>>> - possibility to have a simple build target for building multiple archs
>>>     (assuming the cross-tools are available), leading to probably less
>>>     problems with breaking the build of "the other" architecture we are
>>>     normally not working with (and in future with e.g. Risc-V being added
>>>     this will be even more important)
>>>
>>> - possibility to have a debug and a non-debug build in parallel (in fact
>>>     at least at SUSE we are working around that by building those with an
>>>     intermediate "make clean" for being able to package both variants)
>>>
>>> - make clean for the hypervisor part would be just a "rm -r"
>>
>> I fully agree, yet ...
>>
>>> Yes, this would require us (the developers) to maybe change some habits,
>>> but I think this would be better than working around the issues by
>>> adding $(srctree) all over the build system.
>>
>> ... developers' habits would only be my second concern here (and if that
>> had been the only one, then I would not see this as a reason speaking
>> against the change, but as said I've never been building from the root,
>> and I've also been building sort of out-of-tree all the time). Yet while
>> writing this reply I came to realize that my primary concern was wrong:
>> People would not need to adjust their spec files (or alike), at least
>> not as long as they consume only files living under dist/.
>>
>> So, Anthony - thoughts about making the default in-tree Xen build
>> actually build into, say, build/xen/?
> 
> Or maybe even build-<arch>[-debug]/xen/?

I'd be okay with build-<arch>, but things would become questionable imo
when considering further elements recorded in .config: Where would you
draw the line?

Jan