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

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, Oct 18, 2021 at 12:36:45PM +0200, 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:
> >>> 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/?

I don't think I should be the one answering this question. It would
probably be fairly easy to do.

But, I've already got some unfinished patches which add the ability to
do in-tree and out-of-tree at the same time, so I'll propose them and we
can decide whether they are worth it or if they are too much. That is,
after we have at least the ability to do out-of-tree build.

Cheers,

-- 
Anthony PERARD