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



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:
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.

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/?


Juergen

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