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Date:   Mon, 11 Dec 2023 09:51:07 -0800
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Subject: Re: [PATCH v2 1/2] KVM: Use syscore_ops instead of reboot_notifier to
 hook restart/shutdown
From:   Sean Christopherson <seanjc@google.com>
To:     James Gowans <jgowans@amazon.com>
Cc:     "pbonzini@redhat.com" <pbonzini@redhat.com>,
        Alexander Graf <graf@amazon.de>,
        "Jan =?utf-8?Q?Sch=C3=B6nherr?=" <jschoenh@amazon.de>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        "yuzenghui@huawei.com" <yuzenghui@huawei.com>,
        "atishp@atishpatra.org" <atishp@atishpatra.org>,
        "kvm-riscv@lists.infradead.org" <kvm-riscv@lists.infradead.org>,
        "james.morse@arm.com" <james.morse@arm.com>,
        "suzuki.poulose@arm.com" <suzuki.poulose@arm.com>,
        "oliver.upton@linux.dev" <oliver.upton@linux.dev>,
        "chenhuacai@kernel.org" <chenhuacai@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvmarm@lists.linux.dev" <kvmarm@lists.linux.dev>,
        "maz@kernel.org" <maz@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "aleksandar.qemu.devel@gmail.com" <aleksandar.qemu.devel@gmail.com>,
        "anup@brainfault.org" <anup@brainfault.org>,
        "kexec@lists.infradead.org" <kexec@lists.infradead.org>
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On Mon, Dec 11, 2023, Sean Christopherson wrote:
> On Sat, Dec 09, 2023, James Gowans wrote:
> > Hi Sean,
> >=20
> > Blast from the past but I've just been bitten by this patch when
> > rebasing across v6.4.
> >=20
> > 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 dis=
ables
> > > virtualization _after_ system_state has been updated.=C2=A0 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().
> >=20
> > The issue is that, AFAICT, the syscore_ops.shutdown are not called when
> > doing a kexec. Reboot notifiers are called across kexec via:
> >=20
> > kernel_kexec
> >   kernel_restart_prepare
> >     blocking_notifier_call_chain
> >       kvm_reboot
> >=20
> > 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.
> >=20
> > Some specific shutdown callbacks, for example IOMMU, HPET, IRQ, etc are
> > called in native_machine_shutdown, but KVM is not one of these.
> >=20
> > 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 callback=
s
> > c) Add a KVM-specific callback to native_machine_shutdown(); we only
> > need this for Intel x86, right?
>=20
> 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=20
> should leave virtualization enabled across kexec(), even if leaving virtu=
alization

*shouldn't*

> enabled is relatively benign on other architectures.
>=20
> One more option would be:
>=20
>  d) Add another sycore hook, e.g. syscore_kexec() specifically for this p=
ath.
>=20
> > My slight preference is towards adding syscore_shutdown() to kexec, but
> > I'm not sure that's feasible. Adding kexec maintainers for input.
>=20
> 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 resum=
e(), so
> it's not completely uncharted territory.
>=20
> However, there's a rather big wrinkle in that not all of the existing .sh=
utdown()
> implementations are obviously ok to call during kexec.  Luckily, AFAICT t=
here are
> very few users of the syscore .shutdown hook, so it's at least feasible t=
o go that
> route.
>=20
> 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.
>=20
> ledtrig_cpu_syscore_shutdown() is also likely ok and arguably correct.
>=20
> The interrupt controllers though?  x86 disables IRQs at the very beginnin=
g 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, tho=
ugh I
> have no idea if that can run afoul of any of the paths below.
>=20
>         arch/powerpc/platforms/cell/spu_base.c  .shutdown =3D spu_shutdow=
n,
>         arch/x86/kernel/cpu/mce/core.c	        .shutdown =3D mce_syscore_=
shutdown,
>         arch/x86/kernel/i8259.c                 .shutdown =3D i8259A_shut=
down,
>         drivers/irqchip/irq-i8259.c	        .shutdown =3D i8259A_shutdown=
,
>         drivers/irqchip/irq-sun6i-r.c	        .shutdown =3D sun6i_r_intc_=
shutdown,
>         drivers/leds/trigger/ledtrig-cpu.c	.shutdown =3D ledtrig_cpu_sysc=
ore_shutdown,
>         drivers/power/reset/sc27xx-poweroff.c	.shutdown =3D sc27xx_powero=
ff_shutdown,
>         kernel/irq/generic-chip.c	        .shutdown =3D irq_gc_shutdown,
>         virt/kvm/kvm_main.c	                .shutdown =3D kvm_shutdown,
>=20
> The whole thing is a bit of a mess.  E.g. x86 treats machine_shutdown() f=
rom
> kexec pretty much the same as shutdown for reboot, but other architecture=
s have
> what appear to be unique paths for handling kexec.
>=20
> FWIW, if we want to go with option (b), syscore_shutdown() hooks could us=
e
> kexec_in_progress to differentiate between "regular" shutdown/reboot and =
kexec.