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Date:   Thu, 1 Sep 2022 14:18:09 +0800
From:   Chao Gao <chao.gao@intel.com>
To:     <isaku.yamahata@intel.com>
CC:     <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <isaku.yamahata@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>,
        "Sean Christopherson" <seanjc@google.com>,
        Kai Huang <kai.huang@intel.com>,
        "Will Deacon" <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 05/19] KVM: Rename and move CPUHP_AP_KVM_STARTING to
 ONLINE section
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On Tue, Aug 30, 2022 at 05:01:20AM -0700, isaku.yamahata@intel.com wrote:
>From: Chao Gao <chao.gao@intel.com>
>
>The CPU STARTING section doesn't allow callbacks to fail. Move KVM's
>hotplug callback to ONLINE section so that it can abort onlining a CPU in
>certain cases to avoid potentially breaking VMs running on existing CPUs.
>For example, when kvm fails to enable hardware virtualization on the
>hotplugged CPU.
>
>Place KVM's hotplug state before CPUHP_AP_SCHED_WAIT_EMPTY as it ensures
>when offlining a CPU, all user tasks and non-pinned kernel tasks have left
>the CPU, i.e. there cannot be a vCPU task around. So, it is safe for KVM's
>CPU offline callback to disable hardware virtualization at that point.
>Likewise, KVM's online callback can enable hardware virtualization before
>any vCPU task gets a chance to run on hotplugged CPUs.
>
>KVM's CPU hotplug callbacks are renamed as well.
>
>Suggested-by: Thomas Gleixner <tglx@linutronix.de>
>Signed-off-by: Chao Gao <chao.gao@intel.com>

Isaku, your signed-off-by is missing.

>Link: https://lore.kernel.org/r/20220216031528.92558-6-chao.gao@intel.com
>---
> include/linux/cpuhotplug.h |  2 +-
> virt/kvm/kvm_main.c        | 30 ++++++++++++++++++++++--------
> 2 files changed, 23 insertions(+), 9 deletions(-)
>
>diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
>index f61447913db9..7972bd63e0cb 100644
>--- a/include/linux/cpuhotplug.h
>+++ b/include/linux/cpuhotplug.h
>@@ -185,7 +185,6 @@ enum cpuhp_state {
> 	CPUHP_AP_CSKY_TIMER_STARTING,
> 	CPUHP_AP_TI_GP_TIMER_STARTING,
> 	CPUHP_AP_HYPERV_TIMER_STARTING,
>-	CPUHP_AP_KVM_STARTING,
> 	CPUHP_AP_KVM_ARM_VGIC_INIT_STARTING,
> 	CPUHP_AP_KVM_ARM_VGIC_STARTING,
> 	CPUHP_AP_KVM_ARM_TIMER_STARTING,

The movement of CPUHP_AP_KVM_STARTING changes the ordering between
CPUHP_AP_KVM_STARTING and CPUHP_AP_KVM_ARM_* above [1]. We need
the patch [2] from Marc to avoid breaking ARM.

[1] https://lore.kernel.org/lkml/87sfsq4xy8.wl-maz@kernel.org/
[2] https://lore.kernel.org/lkml/20220216031528.92558-5-chao.gao@intel.com/