[Xen-devel] [PATCH] libx86: Work around GCC bug with ebx output constrants

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Some versions of GCC can't compile cpuid.c, and fail with the rather cryptic:

  In file included from lib/x86/cpuid.c:3:0:
  lib/x86/cpuid.c: In function ‘x86_cpuid_policy_fill_native’:
  include/xen/lib/x86/cpuid.h:25:5: error: inconsistent operand constraints in 
an ‘asm’
       asm ( "cpuid"
       ^

In practice, this is a collision between the output constraint and the GOT
which is held in %ebx when compiling with -fPIC for libraries.

This affects at least GCC 4.9 as shipped in Debian Jessie, but experimentally
is fixed in GCC 6 and later.  Curiously, it only affects 32-bit builds.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/include/xen/lib/x86/cpuid.h | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/xen/include/xen/lib/x86/cpuid.h b/xen/include/xen/lib/x86/cpuid.h
index 266c910..77f63d5 100644
--- a/xen/include/xen/lib/x86/cpuid.h
+++ b/xen/include/xen/lib/x86/cpuid.h
@@ -20,21 +20,48 @@ struct cpuid_leaf
     uint32_t a, b, c, d;
 };
 
+/*
+ * Some versions of GCC are unable to cope with preserving the GOT (held in
+ * %ebx) around an asm() with an %ebx output constraint, and produce a rather
+ * cryptic error:
+ *    error: inconsistent operand constraints in an ‘asm’
+ *
+ * Experimentally, 64-bit builds work correctly, and 32-bit builds on GCC 6 or
+ * later work correctly.
+ *
+ * To work around the issue, use a separate register to hold the the ebx
+ * output, and xchg twice to leave %ebx preserved around the asm() statement.
+ */
+#if __PIC__ && __i386__ && __GNUC__ < 6 && !__clang__
+#define XCHG_BX "xchg %%ebx, %k[bx];"
+#define BX_CON [bx] "=&r"
+#else
+#define XCHG_BX ""
+#define BX_CON "=b"
+#endif
+
 static inline void cpuid_leaf(uint32_t leaf, struct cpuid_leaf *l)
 {
-    asm ( "cpuid"
-          : "=a" (l->a), "=b" (l->b), "=c" (l->c), "=d" (l->d)
+    asm ( XCHG_BX
+          "cpuid;"
+          XCHG_BX
+          : "=a" (l->a), BX_CON (l->b), "=c" (l->c), "=d" (l->d)
           : "a" (leaf) );
 }
 
 static inline void cpuid_count_leaf(
     uint32_t leaf, uint32_t subleaf, struct cpuid_leaf *l)
 {
-    asm ( "cpuid"
-          : "=a" (l->a), "=b" (l->b), "=c" (l->c), "=d" (l->d)
+    asm ( XCHG_BX
+          "cpuid;"
+          XCHG_BX
+          : "=a" (l->a), BX_CON (l->b), "=c" (l->c), "=d" (l->d)
           : "a" (leaf), "c" (subleaf) );
 }
 
+#undef BX_CON
+#undef XCHG
+
 #define CPUID_GUEST_NR_BASIC      (0xdu + 1)
 #define CPUID_GUEST_NR_FEAT       (0u + 1)
 #define CPUID_GUEST_NR_CACHE      (5u + 1)
-- 
2.1.4


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