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

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 11, 2023, Sean Christopherson wrote:
> On Sat, Dec 09, 2023, James Gowans wrote:
> > 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.
> > 
> > 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?
> 
> I don't like (c).  VMX is the most sensitive/problematic, e.g. the whole blocking
> of INIT thing, but SVM can also run afoul of EFER.SVME being cleared, and KVM really 
> should leave virtualization enabled across kexec(), even if leaving virtualization

*shouldn't*

> enabled is relatively benign on other architectures.
> 
> One more option would be:
> 
>  d) Add another sycore hook, e.g. syscore_kexec() specifically for this path.
> 
> > My slight preference is towards adding syscore_shutdown() to kexec, but
> > I'm not sure that's feasible. Adding kexec maintainers for input.
> 
> In a vacuum, that'd be my preference too.  It's the obvious choice IMO, e.g. the
> kexec_image->preserve_context path does syscore_suspend() (and then resume(), so
> it's not completely uncharted territory.
> 
> However, there's a rather big wrinkle in that not all of the existing .shutdown()
> implementations are obviously ok to call during kexec.  Luckily, AFAICT there are
> very few users of the syscore .shutdown hook, so it's at least feasible to go that
> route.
> 
> x86's mce_syscore_shutdown() should be ok, and arguably is correct, e.g. I don't
> see how leaving #MC reporting enabled across kexec can work.
> 
> ledtrig_cpu_syscore_shutdown() is also likely ok and arguably correct.
> 
> The interrupt controllers though?  x86 disables IRQs at the very beginning of
> machine_kexec(), so it's likely fine.  But every other architecture?  No clue.
> E.g. PPC's default_machine_kexec() sends IPIs to shutdown other CPUs, though I
> have no idea if that can run afoul of any of the paths below.
> 
>         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,
> 
> The whole thing is a bit of a mess.  E.g. x86 treats machine_shutdown() from
> kexec pretty much the same as shutdown for reboot, but other architectures have
> what appear to be unique paths for handling kexec.
> 
> FWIW, if we want to go with option (b), syscore_shutdown() hooks could use
> kexec_in_progress to differentiate between "regular" shutdown/reboot and kexec.