[Xen-changelog] [xen staging] libx86: Work around GCC being unable to spill the PIC hard register

[patchbot@xxxxxxx]

commit 73ee5574084b9bfa6194c1096cb1e4ce76fedb3f
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Nov 19 13:03:02 2018 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Nov 19 16:58:35 2018 +0000

    libx86: Work around GCC being unable to spill the PIC hard register
    
    Versions of GCC before 5 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 â??x86_cpuid_policy_fill_nativeâ??:
      include/xen/lib/x86/cpuid.h:25:5: error: inconsistent operand constraints 
in an â??asmâ??
           asm ( "cpuid"
           ^
    
    See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54232 for more details.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/include/xen/lib/x86/cpuid.h | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/xen/include/xen/lib/x86/cpuid.h b/xen/include/xen/lib/x86/cpuid.h
index 266c910fd3..22d43ef7ba 100644
--- a/xen/include/xen/lib/x86/cpuid.h
+++ b/xen/include/xen/lib/x86/cpuid.h
@@ -20,21 +20,51 @@ struct cpuid_leaf
     uint32_t a, b, c, d;
 };
 
+/*
+ * Versions of GCC before 5 unconditionally reserve %rBX as the PIC hard
+ * register, and are unable to cope with spilling it.  This results in a
+ * rather cryptic error:
+ *    error: inconsistent operand constraints in an â??asmâ??
+ *
+ * In affected situations, work around the issue by using a separate register
+ * to hold the the %rBX output, and xchg twice to leave %rBX preserved around
+ * the asm() statement.
+ */
+#if defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) && 
defined(__i386__)
+# define XCHG_BX "xchg %%ebx, %[bx];"
+# define BX_CON [bx] "=&r"
+#elif defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) && \
+    defined(__x86_64__) && (defined(__code_model_medium__) || \
+                            defined(__code_model_large__))
+# define XCHG_BX "xchg %%rbx, %q[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)
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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