Re: [PATCH v2] x86/shutdown: change default reboot method preference

[Jan Beulich <jbeulich@suse.com>]

On 15.09.2023 09:43, Roger Pau Monne wrote:
> 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 method seems to be widely untested, 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.
> 
> Add a special case for one Acer model that does require being rebooted using
> ResetSystem().  See Linux commit 0082517fa4bce for rationale.
> 
> I'm not aware of using ACPI reboot causing issues on boxes that do have
> properly implemented ResetSystem() methods.

A data point from a new system I'm still in the process of setting up: The
ACPI reboot method, as used by Linux, unconditionally means a warm reboot.
The EFI method, otoh, properly distinguishes "reboot=warm" from our default
of explicitly requesting cold reboot. (Without taking the EFI path, I
assume our write to the relevant BDA location simply has no effect, for
this being a legacy BIOS thing, and the system apparently defaults to warm
reboot when using the ACPI method.)

Clearly, as a secondary effect, this system adds to my personal experience
of so far EFI reboot consistently working on all x86 hardware I have (had)
direct access to. (That said, this is the first non-Intel system, which
likely biases my overall experience.)

Jan