[PATCH] eal/x86: fix LZCNT CPU feature bit

Yonghao Li cruselee1 at live.cn
Fri Aug 28 22:32:52 CEST 2026


LZCNT support is enumerated by CPUID.80000001H:ECX[5]. The current
definition checks ECX bit 4, which identifies CR8 legacy mode on AMD
processors and is reserved on Intel processors. As a result,
RTE_CPUFLAG_LZCNT can be reported incorrectly.

Check ECX bit 5 instead.

Fixes: af75078fece3 ("first public release")
Cc: stable at dpdk.org
Signed-off-by: Yonghao Li <cruselee1 at live.cn>
---
 .mailmap                   | 1 +
 lib/eal/x86/rte_cpuflags.c | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/.mailmap b/.mailmap
index fcb3d1bb3f..fad671fbe9 100644
--- a/.mailmap
+++ b/.mailmap
@@ -1933,6 +1933,7 @@ Yong Wang <wang.yong19 at zte.com.cn> <yongwang at vmware.com>
 Yong Xu <yong.xu at corigine.com>
 Yong Zhang <zhang.yong25 at zte.com.cn>
 Yongfeng Wang <mannywang at tencent.com>
+Yonghao Li <cruselee1 at live.cn>
 Yongji Xie <xieyongji at baidu.com>
 Yongjie Gu <yongjiex.gu at intel.com>
 Yongping Zhang <yongping.zhang at broadcom.com>
diff --git a/lib/eal/x86/rte_cpuflags.c b/lib/eal/x86/rte_cpuflags.c
index 5d1f352e04..80b71a3ce0 100644
--- a/lib/eal/x86/rte_cpuflags.c
+++ b/lib/eal/x86/rte_cpuflags.c
@@ -137,7 +137,7 @@ struct feature_entry rte_cpu_feature_table[] = {
 	FEAT_DEF(AVX512VP2INTERSECT, 0x00000007, 0, RTE_REG_EDX,  8)
 
 	FEAT_DEF(LAHF_SAHF, 0x80000001, 0, RTE_REG_ECX,  0)
-	FEAT_DEF(LZCNT, 0x80000001, 0, RTE_REG_ECX,  4)
+	FEAT_DEF(LZCNT, 0x80000001, 0, RTE_REG_ECX,  5)
 	FEAT_DEF(MONITORX, 0x80000001, 0, RTE_REG_ECX,  29)
 
 	FEAT_DEF(SYSCALL, 0x80000001, 0, RTE_REG_EDX, 11)
-- 
2.43.0



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