x86: move I/O emulation stubs off the stack

This is needed as stacks are going to become non-executable.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2006,7 +2006,7 @@ static int emulate_privileged_op(struct 
                            ? (*(u32 *)&regs->reg = (val)) \
                            : (*(u16 *)&regs->reg = (val)))
     unsigned long code_base, code_limit;
-    char io_emul_stub[32];
+    char *io_emul_stub = NULL;
     void (*io_emul)(struct cpu_user_regs *) __attribute__((__regparm__(1)));
     uint64_t val;
 
@@ -2195,6 +2195,9 @@ static int emulate_privileged_op(struct 
      * GPR context. This is needed for some systems which (ab)use IN/OUT
      * to communicate with BIOS code in system-management mode.
      */
+    io_emul_stub = map_domain_page(this_cpu(stubs.mfn)) +
+                   (this_cpu(stubs.addr) & (PAGE_SIZE - 1)) +
+                   STUB_BUF_SIZE / 2;
     /* movq $host_to_guest_gpr_switch,%rcx */
     io_emul_stub[0] = 0x48;
     io_emul_stub[1] = 0xb9;
@@ -2212,7 +2215,7 @@ static int emulate_privileged_op(struct 
     io_emul_stub[15] = 0xc3;
 
     /* Handy function-typed pointer to the stub. */
-    io_emul = (void *)io_emul_stub;
+    io_emul = (void *)(this_cpu(stubs.addr) + STUB_BUF_SIZE / 2);
 
     if ( ioemul_handle_quirk )
         ioemul_handle_quirk(opcode, &io_emul_stub[12], regs);
@@ -2777,9 +2780,13 @@ static int emulate_privileged_op(struct 
  done:
     instruction_done(regs, eip, bpmatch);
  skip:
+    if ( io_emul_stub )
+        unmap_domain_page(io_emul_stub);
     return EXCRET_fault_fixed;
 
  fail:
+    if ( io_emul_stub )
+        unmap_domain_page(io_emul_stub);
     return 0;
 }