[PATCH] tools/xen-detect: avoid possible pitfall with cpuid()

[Jan Beulich <jbeulich@xxxxxxxx>]

The 64-bit form forces %ecx to 0 while the 32-bit one so far didn't - it
only ended up that way when "pv_context" is zero. While presently no
leaf queried by callers has separate subleaves, let's avoid chancing it.

While there
- replace references to operands by number,
- relax constraints where possible,
- limit PUSH/POP to just the registers not also used as input,
all where applicable also for the 64-bit variant.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I'm pretty sure %edx also wouldn't need to be subject to PUSH/POP here,
but I didn't want to go more "against" the comment than obviously
justifiable by the input registers used. In fact I've observed gcc to
pick %edx for putting "pv_context" in.

--- a/tools/misc/xen-detect.c
+++ b/tools/misc/xen-detect.c
@@ -52,17 +52,19 @@ static void cpuid(uint32_t idx, uint32_t
 #ifdef __i386__
     /* Use the stack to avoid reg constraint failures with some gcc flags */
     asm volatile (
-        "push %%eax; push %%ebx; push %%ecx; push %%edx\n\t"
-        "test %1,%1 ; jz 1f ; ud2a ; .ascii \"xen\" ; 1: cpuid\n\t"
-        "mov %%eax,(%2); mov %%ebx,4(%2)\n\t"
-        "mov %%ecx,8(%2); mov %%edx,12(%2)\n\t"
-        "pop %%edx; pop %%ecx; pop %%ebx; pop %%eax\n\t"
-        : : "a" (idx), "c" (pv_context), "S" (regs) : "memory" );
+        "push %%ebx; push %%edx\n\t"
+        "test %[pv],%[pv] ; jz 1f ; ud2a ; .ascii \"xen\" ; 1: cpuid\n\t"
+        "mov %%eax,(%[regs]); mov %%ebx,4(%[regs])\n\t"
+        "mov %%ecx,8(%[regs]); mov %%edx,12(%[regs])\n\t"
+        "pop %%edx; pop %%ebx\n\t"
+        : "+a" (idx), "=c" (idx /* dummy */)
+        : "c" (0), [pv] "r" (pv_context), [regs] "SD" (regs)
+        : "memory" );
 #else
     asm volatile (
-        "test %5,%5 ; jz 1f ; ud2a ; .ascii \"xen\" ; 1: cpuid\n\t"
+        "test %[pv],%[pv] ; jz 1f ; ud2a ; .ascii \"xen\" ; 1: cpuid\n\t"
         : "=a" (regs[0]), "=b" (regs[1]), "=c" (regs[2]), "=d" (regs[3])
-        : "0" (idx), "1" (pv_context), "2" (0) );
+        : "0" (idx), "2" (0), [pv] "r" (pv_context) );
 #endif
 }