[dpdk-dev] [PATCH v2] eal: fix up bad asm in rte_cpu_get_features

Neil Horman nhorman at tuxdriver.com
Wed Mar 19 15:48:44 CET 2014

The recent conversion to build dpdk as a DSO has an error in
rte_cpu_get_features.  When being build with -fpie, %ebx gets clobbered by the
cpuid instruction which is also the pic register.  Therefore the inline asm
tries to save off %ebx, but does so incorrectly.  It starts by loading
params.ebx to "D" which is %edi, but then the first instruction moves %ebx to
%edi, clobbering the input value. Then after the operation is complete, "D"
(%edi) is stored to the local ebx variable, but only after the xchgl instruction
has happened, which means ebx holds only the PIC pointer.  This behavior was
causing strange segfults for me when running the cpuid instruction.

The fix is pretty easy, split the asm into two separate directives, the first
saving ebx, and using it to grab the appropriate cpuid info (and correctly
listing %edi as a clobbered register in the process, and then a subsequent asm
directive preforming the reverse exchange (again, listing %edi as being

Signed-off-by: Neil Horman <nhorman at tuxdriver.com>

Change notes

v2) Fix constraints to ensure that ebx isn't overwritten before asm starts
 lib/librte_eal/common/eal_common_cpuflags.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/lib/librte_eal/common/eal_common_cpuflags.c b/lib/librte_eal/common/eal_common_cpuflags.c
index 1ebf78c..75b505f 100644
--- a/lib/librte_eal/common/eal_common_cpuflags.c
+++ b/lib/librte_eal/common/eal_common_cpuflags.c
@@ -190,7 +190,7 @@ static const struct feature_entry cpu_feature_table[] = {
 static inline int
 rte_cpu_get_features(struct cpuid_parameters_t params)
-	int eax, ebx, ecx, edx;            /* registers */
+	int eax, ebx, ecx, edx, oldebx;            /* registers */
 #ifndef __PIC__
    asm volatile ("cpuid"
@@ -206,18 +206,21 @@ rte_cpu_get_features(struct cpuid_parameters_t params)
                    "d" (params.edx));
 	asm volatile ( 
-            "mov %%ebx, %%edi\n"
+            "xchgl %%ebx, %%edi\n"
-            "xchgl %%ebx, %%edi;\n"
             : "=a" (eax),
-              "=D" (ebx),
+              "=b" (ebx),
               "=c" (ecx),
-              "=d" (edx)
+              "=d" (edx),
+	      "=D" (oldebx)
             /* input */
             : "a" (params.eax),
               "D" (params.ebx),
               "c" (params.ecx),
               "d" (params.edx));
+	asm volatile ("xchgl %%ebx, %%edi;\n"
+		      : : "D" (oldebx));
 	switch (params.return_register) {

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