[PATCH] kbuild: try readelf first in gen_symversions

[Wentao Guan]

Use readelf to dig out if <file>.o contain a __export_symbol_*.

Instead of nm, readelf is more faster, and significantly improve speed
when enable CONFIG_MODVERSIONS.

Build x86_64_defconfigs in 2C4T cloud server with CONFIG_MODVERSIONS=y:
With patch:
real    17m21.019s
user    61m48.388s
sys     4m27.709s
Without patch:
real    17m39.435s
user    62m24.686s
sys     5m3.200s

Link: https://lore.kernel.org/all/tencent_2FA16E0A18D6D0C0703F5D49@xxxxxx/
Signed-off-by: Wentao Guan <guanwentao@xxxxxxxxxxxxx>
---
 scripts/Makefile.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 3498d25b15e85..54a91bc144cce 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -233,7 +233,7 @@ ifdef CONFIG_MODVERSIONS
 #   be compiled and linked to the kernel and/or modules.
 
 gen_symversions =								\
-	if $(NM) $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
+	if $(READELF) -sW $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
 		$(cmd_gensymtypes_$1) >> $(dot-target).cmd;			\
 	fi
 
-- 
2.30.2