With GNU make 4.4, the number of execution of the command present in
these $(shell ) increased greatly. This is probably because as of make
4.4, exported variable are also added to the environment of $(shell )
construct.

Also, `make -d` shows a lot of these:
    Makefile:15: not recursively expanding XEN_BUILD_DATE to export to shell function
    Makefile:16: not recursively expanding XEN_BUILD_TIME to export to shell function
    Makefile:17: not recursively expanding XEN_BUILD_HOST to export to shell function
    Makefile:14: not recursively expanding XEN_DOMAIN to export to shell function

So to avoid having these command been run more than necessary, we
will replace ?= by an equivalent but with immediate expansion.

Reported-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
---

Notes:
    v3:
    - replace evaluation on first use construct by immediate expansion which
      isn't likely to break and is clearer.

 xen/Makefile | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/xen/Makefile b/xen/Makefile
index e8aa663781..c1738dbbde 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -11,10 +11,18 @@ export XEN_FULLVERSION   = $(XEN_VERSION).$(XEN_SUBVERSION)$(XEN_EXTRAVERSION)
 -include xen-version
 
 export XEN_WHOAMI	?= $(USER)
-export XEN_DOMAIN	?= $(shell ([ -x /bin/dnsdomainname ] && /bin/dnsdomainname) || ([ -x /bin/domainname ] && /bin/domainname || echo [unknown]))
-export XEN_BUILD_DATE	?= $(shell LC_ALL=C date)
-export XEN_BUILD_TIME	?= $(shell LC_ALL=C date +%T)
-export XEN_BUILD_HOST	?= $(shell hostname)
+ifeq ($(origin XEN_DOMAIN), undefined)
+export XEN_DOMAIN	:= $(shell ([ -x /bin/dnsdomainname ] && /bin/dnsdomainname) || ([ -x /bin/domainname ] && /bin/domainname || echo [unknown]))
+endif
+ifeq ($(origin XEN_BUILD_DATE), undefined)
+export XEN_BUILD_DATE	:= $(shell LC_ALL=C date)
+endif
+ifeq ($(origin XEN_BUILD_TIME), undefined)
+export XEN_BUILD_TIME	:= $(shell LC_ALL=C date +%T)
+endif
+ifeq ($(origin XEN_BUILD_HOST), undefined)
+export XEN_BUILD_HOST	:= $(shell hostname)
+endif
 
 # Best effort attempt to find a python interpreter, defaulting to Python 3 if
 # available.  Fall back to just `python` if `which` is nowhere to be found.
-- 
Anthony PERARD

On 27.07.2023 16:54, Anthony PERARD wrote:
> With GNU make 4.4, the number of execution of the command present in
> these $(shell ) increased greatly. This is probably because as of make
> 4.4, exported variable are also added to the environment of $(shell )
> construct.
> 
> Also, `make -d` shows a lot of these:
>     Makefile:15: not recursively expanding XEN_BUILD_DATE to export to shell function
>     Makefile:16: not recursively expanding XEN_BUILD_TIME to export to shell function
>     Makefile:17: not recursively expanding XEN_BUILD_HOST to export to shell function
>     Makefile:14: not recursively expanding XEN_DOMAIN to export to shell function
> 
> So to avoid having these command been run more than necessary, we
> will replace ?= by an equivalent but with immediate expansion.
> 
> Reported-by: Jason Andryuk <jandryuk@gmail.com>
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>