kexec: do syscore_shutdown() in kernel_kexec

syscore_shutdown() runs driver and module callbacks to get the system into a state where it can be correctly shut down. In commit 6f389a8f1dd2 ("PM / reboot: call syscore_shutdown() after disable_nonboot_cpus()") syscore_shutdown() was removed from kernel_restart_prepare() and hence got (incorrectly?) removed from the kexec flow. This was innocuous until commit 6735150b6997 ("KVM: Use syscore_ops instead of reboot_notifier to hook restart/shutdown") changed the way that KVM registered its shutdown callbacks, switching from reboot notifiers to syscore_ops.shutdown. As syscore_shutdown() is missing from kexec, KVM's shutdown hook is not run and virtualisation is left enabled on the boot CPU which results in triple faults when switching to the new kernel on Intel x86 VT-x with VMXE enabled. Fix this by adding syscore_shutdown() to the kexec sequence. In terms of where to add it, it is being added after migrating the kexec task to the boot CPU, but before APs are shut down. It is not totally clear if this is the best place: in commit 6f389a8f1dd2 ("PM / reboot: call syscore_shutdown() after disable_nonboot_cpus()") it is stated that "syscore_ops operations should be carried with one CPU on-line and interrupts disabled." APs are only offlined later in machine_shutdown(), so this syscore_shutdown() is being run while APs are still online. This seems to be the correct place as it matches where syscore_shutdown() is run in the reboot and halt flows - they also run it before APs are shut down. The assumption is that the commit message in commit 6f389a8f1dd2 ("PM / reboot: call syscore_shutdown() after disable_nonboot_cpus()") is no longer valid. KVM has been discussed here as it is what broke loudly by not having syscore_shutdown() in kexec, but this change impacts more than just KVM; all drivers/modules which register a syscore_ops.shutdown callback will now be invoked in the kexec flow. Looking at some of them like x86 MCE it is probably more correct to also shut these down during kexec. Maintainers of all drivers which use syscore_ops.shutdown are added on CC for visibility. They are: arch/powerpc/platforms/cell/spu_base.c .shutdown = spu_shutdown, arch/x86/kernel/cpu/mce/core.c .shutdown = mce_syscore_shutdown, arch/x86/kernel/i8259.c .shutdown = i8259A_shutdown, drivers/irqchip/irq-i8259.c .shutdown = i8259A_shutdown, drivers/irqchip/irq-sun6i-r.c .shutdown = sun6i_r_intc_shutdown, drivers/leds/trigger/ledtrig-cpu.c .shutdown = ledtrig_cpu_syscore_shutdown, drivers/power/reset/sc27xx-poweroff.c .shutdown = sc27xx_poweroff_shutdown, kernel/irq/generic-chip.c .shutdown = irq_gc_shutdown, virt/kvm/kvm_main.c .shutdown = kvm_shutdown, This has been tested by doing a kexec on x86_64 and aarch64.

JGJames Gowans committed 2 years ago