[PATCH] x86/shutdown: change default reboot method preference

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The current logic to chose the preferred reboot method is based on the mode Xen
has been booted into, so if the box is booted from UEFI, the preferred reboot
method will be to use the ResetSystem() run time service call.

However, that call seems to be widely untested once the firmware has exited
boot services, and quite often leads to a result similar to:

Hardware Dom0 shutdown: rebooting machine
----[ Xen-4.18-unstable  x86_64  debug=y  Tainted:   C    ]----
CPU:    0
RIP:    e008:[<0000000000000017>] 0000000000000017
RFLAGS: 0000000000010202   CONTEXT: hypervisor
[...]
Xen call trace:
   [<0000000000000017>] R 0000000000000017
   [<ffff83207eff7b50>] S ffff83207eff7b50
   [<ffff82d0403525aa>] F machine_restart+0x1da/0x261
   [<ffff82d04035263c>] F apic_wait_icr_idle+0/0x37
   [<ffff82d040233689>] F smp_call_function_interrupt+0xc7/0xcb
   [<ffff82d040352f05>] F call_function_interrupt+0x20/0x34
   [<ffff82d04033b0d5>] F do_IRQ+0x150/0x6f3
   [<ffff82d0402018c2>] F common_interrupt+0x132/0x140
   [<ffff82d040283d33>] F 
arch/x86/acpi/cpu_idle.c#acpi_idle_do_entry+0x113/0x129
   [<ffff82d04028436c>] F 
arch/x86/acpi/cpu_idle.c#acpi_processor_idle+0x3eb/0x5f7
   [<ffff82d04032a549>] F arch/x86/domain.c#idle_loop+0xec/0xee

****************************************
Panic on CPU 0:
FATAL TRAP: vector = 6 (invalid opcode)
****************************************

Which in most cases does lead to a reboot, however that's unreliable.

Change the default reboot preference to prefer ACPI over UEFI if available and
not in reduced hardware mode.

This is in line to what Linux does, so it's unlikely to cause issues on current
and future hardware, since there's a much higher chance of vendors testing
hardware with Linux rather than Xen.

I'm not aware of using ACPI reboot causing issues on boxes that do have
properly implemented ResetSystem() methods.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
I haven't found anything in the UEFI spec that mandates to use ResetSystem()
when booted from UEFI, I've only found:

"One method of resetting the platform is through the EFI Runtime Service
ResetSystem()."

But that's vaguely a recommendation.

I've found a lot of boxes that don't reboot properly using ResetSystem(), and I
think our default should attempt to make sure Xen reliably reboots on as much
hardware as possible.
---
 xen/arch/x86/shutdown.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/shutdown.c b/xen/arch/x86/shutdown.c
index 7619544d14da..81d8eda64a41 100644
--- a/xen/arch/x86/shutdown.c
+++ b/xen/arch/x86/shutdown.c
@@ -150,12 +150,12 @@ static void default_reboot_type(void)
 
     if ( xen_guest )
         reboot_type = BOOT_XEN;
+    else if ( !acpi_disabled && !acpi_gbl_reduced_hardware )
+        reboot_type = BOOT_ACPI;
     else if ( efi_enabled(EFI_RS) )
         reboot_type = BOOT_EFI;
-    else if ( acpi_disabled )
-        reboot_type = BOOT_KBD;
     else
-        reboot_type = BOOT_ACPI;
+        reboot_type = BOOT_KBD;
 }
 
 static int __init cf_check override_reboot(const struct dmi_system_id *d)
-- 
2.42.0