x86/EFI: allow reboot= overrides when running under EFI

By default we will always use EFI reboot mechanism when
running under EFI platforms. However some EFI platforms
are buggy and need to use the ACPI mechanism to
reboot (such as Lenovo ThinkCentre M57). As such
respect the 'reboot=' override and DMI overrides
for EFI platforms.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

- BOOT_INVALID is just zero
- also consider acpi_disabled in BOOT_INVALID resolution
- duplicate BOOT_INVALID resolution in machine_restart()
- don't fall back from BOOT_ACPI to BOOT_EFI (if it was overridden, it
  surely was for a reason)
- adjust doc change formatting

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
v2: Update reboot_type prior to calling efi_reset_system(), as
    requested by Andrew.

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1145,7 +1145,7 @@ The following resources are available:
   Technology.
 
 ### reboot
-> `= t[riple] | k[bd] | a[cpi] | p[ci] | n[o] [, [w]arm | [c]old]`
+> `= t[riple] | k[bd] | a[cpi] | p[ci] | e[fi] | n[o] [, [w]arm | [c]old]`
 
 > Default: `0`
 
@@ -1165,6 +1165,9 @@ Specify the host reboot method.
 
 `pci` instructs Xen to reboot the host using PCI reset register (port CF9).
 
+'efi' instructs Xen to reboot using the EFI reboot call (in EFI mode by
+ default it will use that method first).
+
 ### ro-hpet
 > `= <boolean>`
 
--- a/xen/arch/x86/shutdown.c
+++ b/xen/arch/x86/shutdown.c
@@ -28,16 +28,18 @@
 #include <asm/apic.h>
 
 enum reboot_type {
+        BOOT_INVALID,
         BOOT_TRIPLE = 't',
         BOOT_KBD = 'k',
         BOOT_ACPI = 'a',
         BOOT_CF9 = 'p',
+        BOOT_EFI = 'e',
 };
 
 static int reboot_mode;
 
 /*
- * reboot=t[riple] | k[bd] | a[cpi] | p[ci] | n[o] [, [w]arm | [c]old]
+ * reboot=t[riple] | k[bd] | a[cpi] | p[ci] | n[o] | [e]fi [, [w]arm | [c]old]
  * warm   Don't set the cold reboot flag
  * cold   Set the cold reboot flag
  * no     Suppress automatic reboot after panics or crashes
@@ -45,8 +47,9 @@ static int reboot_mode;
  * kbd    Use the keyboard controller. cold reset (default)
  * acpi   Use the RESET_REG in the FADT
  * pci    Use the so-called "PCI reset register", CF9
+ * efi    Use the EFI reboot (if running under EFI)
  */
-static enum reboot_type reboot_type = BOOT_ACPI;
+static enum reboot_type reboot_type = BOOT_INVALID;
 static void __init set_reboot_type(char *str)
 {
     for ( ; ; )
@@ -63,6 +66,7 @@ static void __init set_reboot_type(char 
             reboot_mode = 0x0;
             break;
         case 'a':
+        case 'e':
         case 'k':
         case 't':
         case 'p':
@@ -106,6 +110,14 @@ void machine_halt(void)
     __machine_halt(NULL);
 }
 
+static void default_reboot_type(void)
+{
+    if ( reboot_type == BOOT_INVALID )
+        reboot_type = efi_enabled ? BOOT_EFI
+                                  : acpi_disabled ? BOOT_KBD
+                                                  : BOOT_ACPI;
+}
+
 static int __init override_reboot(struct dmi_system_id *d)
 {
     enum reboot_type type = (long)d->driver_data;
@@ -452,6 +464,7 @@ static struct dmi_system_id __initdata r
 
 static int __init reboot_init(void)
 {
+    default_reboot_type();
     dmi_check_system(reboot_dmi_table);
     return 0;
 }
@@ -465,7 +478,7 @@ static void noreturn __machine_restart(v
 void machine_restart(unsigned int delay_millisecs)
 {
     unsigned int i, attempt;
-    enum reboot_type orig_reboot_type = reboot_type;
+    enum reboot_type orig_reboot_type;
     const struct desc_ptr no_idt = { 0 };
 
     watchdog_disable();
@@ -504,15 +517,20 @@ void machine_restart(unsigned int delay_
         tboot_shutdown(TB_SHUTDOWN_REBOOT);
     }
 
-    efi_reset_system(reboot_mode != 0);
+    /* Just in case reboot_init() didn't run yet. */
+    default_reboot_type();
+    orig_reboot_type = reboot_type;
 
     /* Rebooting needs to touch the page at absolute address 0. */
-    *((unsigned short *)__va(0x472)) = reboot_mode;
+    if ( reboot_type != BOOT_EFI )
+        *((unsigned short *)__va(0x472)) = reboot_mode;
 
     for ( attempt = 0; ; attempt++ )
     {
         switch ( reboot_type )
         {
+        case BOOT_INVALID:
+            ASSERT_UNREACHABLE();
         case BOOT_KBD:
             /* Pulse the keyboard reset line. */
             for ( i = 0; i < 100; i++ )
@@ -532,6 +550,11 @@ void machine_restart(unsigned int delay_
             reboot_type = (((attempt == 1) && (orig_reboot_type == BOOT_ACPI))
                            ? BOOT_ACPI : BOOT_TRIPLE);
             break;
+        case BOOT_EFI:
+            reboot_type = acpi_disabled ? BOOT_KBD : BOOT_ACPI;
+            efi_reset_system(reboot_mode != 0);
+            *((unsigned short *)__va(0x472)) = reboot_mode;
+            break;
         case BOOT_TRIPLE:
             asm volatile ("lidt %0; int3" : : "m" (no_idt));
             reboot_type = BOOT_KBD;