[PATCH v2 1/2] KVM: Use syscore_ops instead of reboot_notifier to hook restart/shutdown

[Eric W. Biederman]

"Gowans, James" <jgowans at amazon.com> writes:

> Hi Sean,
>
> Blast from the past but I've just been bitten by this patch when
> rebasing across v6.4.
>
> On Fri, 2023-05-12 at 16:31 -0700, Sean Christopherson wrote:
>> Use syscore_ops.shutdown to disable hardware virtualization during a
>> reboot instead of using the dedicated reboot_notifier so that KVM disables
>> virtualization _after_ system_state has been updated.  This will allow
>> fixing a race in KVM's handling of a forced reboot where KVM can end up
>> enabling hardware virtualization between kernel_restart_prepare() and
>> machine_restart().
>
> The issue is that, AFAICT, the syscore_ops.shutdown are not called when
> doing a kexec. Reboot notifiers are called across kexec via:
>
> kernel_kexec
>   kernel_restart_prepare
>     blocking_notifier_call_chain
>       kvm_reboot
>
> So after this patch, KVM is not shutdown during kexec; if hardware virt
> mode is enabled then the kexec hangs in exactly the same manner as you
> describe with the reboot.

kernel_restart_prepare calls device_shutdown.  Which should call all
of the shutdown operations.  This has been the way the code has been
structured since December 2005.

> Some specific shutdown callbacks, for example IOMMU, HPET, IRQ, etc are
> called in native_machine_shutdown, but KVM is not one of these.
>
> Thoughts on possible ways to fix this:
> a) go back to reboot notifiers
> b) get kexec to call syscore_shutdown() to invoke all of these callbacks
> c) Add a KVM-specific callback to native_machine_shutdown(); we only
> need this for Intel x86, right?
>
> My slight preference is towards adding syscore_shutdown() to kexec, but
> I'm not sure that's feasible. Adding kexec maintainers for input.

Why isn't device_suthdown calling syscore_shutdown?

What problem are you running into with your rebase that worked with
reboot notifiers that is not working with syscore_shutdown?

Eric