[PATCH 1/5] Fix xen_cpuid() inline asm to not clobber stack's red zone

[Olaf Hering]

Port xen-unstable changeset 24344:72f4e4cb7440 to kexec-tools:

  Pushing stuff onto the stack on x86-64 when we do not specify
  -mno-red-zone is unsafe. Since the complicated asm is due to register
  pressure on i386, we simply implement an all-new simpler alternative
  for x86-64.

Signed-off-by: Olaf Hering <olaf at aepfle.de>
---
 kexec/crashdump-xen.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kexec/crashdump-xen.c b/kexec/crashdump-xen.c
index 9dfabf8..d8bd0f4 100644
--- a/kexec/crashdump-xen.c
+++ b/kexec/crashdump-xen.c
@@ -41,18 +41,21 @@ void xen_sigill_handler(int sig)
 
 static void xen_cpuid(uint32_t idx, uint32_t *regs, int pv_context)
 {
-	asm volatile (
 #ifdef __i386__
-#define R(x) "%%e"#x"x"
+    /* 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" );
 #else
-#define R(x) "%%r"#x"x"
+    asm volatile (
+        "test %5,%5 ; 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) );
 #endif
-	"push "R(a)"; push "R(b)"; push "R(c)"; push "R(d)"\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 "R(d)"; pop "R(c)"; pop "R(b)"; pop "R(a)"\n\t"
-	: : "a" (idx), "c" (pv_context), "S" (regs) : "memory" );
 }
 
 static int check_for_xen(int pv_context)
-- 
1.8.0.1