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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.10.2021 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:
>>> On Thu, Oct 14, 2021 at 12:14:29PM +0200, Jan Beulich wrote:
>>>> On 24.08.2021 12:50, Anthony PERARD wrote:
>>>>> --- a/xen/Rules.mk
>>>>> +++ b/xen/Rules.mk
>>>>> @@ -38,7 +38,7 @@ SPECIAL_DATA_SECTIONS := rodata $(foreach a,1 2 4 8 16, 
>>>>> \
>>>>>                           $(foreach r,rel rel.ro,data.$(r).local)
>>>>>  
>>>>>  # The filename build.mk has precedence over Makefile
>>>>> -mk-dir := $(src)
>>>>> +mk-dir := $(srctree)/$(src)
>>>>>  include $(if $(wildcard 
>>>>> $(mk-dir)/build.mk),$(mk-dir)/build.mk,$(mk-dir)/Makefile)
>>>>
>>>> Perhaps already when it was changed to $(src) the name has become
>>>> slightly misleading, at least imo: I would rather expect a variable
>>>> with this name to refer to the build dir/tree. Maybe "srcdir" or
>>>> even shorted "sd" right from the start? (Reaching here I can finally
>>>> see why having a shorthand is helpful.)
>>>
>>> I have to think about that. I've made some further progress in order to
>>> be able to build the Xen pvhshim without a link farm and notice that
>>> nearly every source file needs to use "$(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.
> 
> This can be done by teaching make to only look for source file in
> $(srctree), and to look for generated/built file only in $(objtree).
> That's mostly done by not using VPATH and making few other adjustments.

Oh, I see. That's ugly. Just like in Linux I was expecting that we'd
do/support only in-tree or only out-of-tree builds, but not a mix of
both within the same tree. But I can see how this would get in the
way of a build of everything from tree root (something I personally
have never made use of).

Jan