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From: Oliver Upton <oliver.upton@linux.dev>
To: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Marc Zyngier <maz@kernel.org>,
	isaku.yamahata@intel.com, seanjc@google.com, pbonzini@redhat.com,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>
Subject: Re: KVM CPU hotplug notifier triggers BUG_ON on arm64
Date: Sat, 1 Jul 2023 10:42:28 -0700	[thread overview]
Message-ID: <ZKBlhJwl9YD5FHvs@linux.dev> (raw)
In-Reply-To: <aeab7562-2d39-e78e-93b1-4711f8cc3fa5@arm.com>

Hi Kristina,

Thanks for the bug report.

On Sat, Jul 01, 2023 at 01:50:52PM +0100, Kristina Martsenko wrote:
> Hi,
> 
> When I try to online a CPU on arm64 while a KVM guest is running, I hit a
> BUG_ON(preemptible()) (as well as a WARN_ON). See below for the full log.
> 
> This is on kvmarm/next, but seems to have been broken since 6.3. Bisecting it
> points at commit:
> 
>   0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock")

Makes sense. We were using a spinlock before, which implictly disables
preemption.

Well, one way to hack around the problem would be to just cram
preempt_{disable,enable}() into kvm_arch_hardware_disable(), but that's
kinda gross in the context of cpuhp which isn't migratable in the first
place. Let me have a look...

--
Thanks,
Oliver

WARNING: multiple messages have this Message-ID (diff)
From: Oliver Upton <oliver.upton@linux.dev>
To: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Marc Zyngier <maz@kernel.org>,
	isaku.yamahata@intel.com, seanjc@google.com, pbonzini@redhat.com,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>
Subject: Re: KVM CPU hotplug notifier triggers BUG_ON on arm64
Date: Sat, 1 Jul 2023 10:42:28 -0700	[thread overview]
Message-ID: <ZKBlhJwl9YD5FHvs@linux.dev> (raw)
In-Reply-To: <aeab7562-2d39-e78e-93b1-4711f8cc3fa5@arm.com>

Hi Kristina,

Thanks for the bug report.

On Sat, Jul 01, 2023 at 01:50:52PM +0100, Kristina Martsenko wrote:
> Hi,
> 
> When I try to online a CPU on arm64 while a KVM guest is running, I hit a
> BUG_ON(preemptible()) (as well as a WARN_ON). See below for the full log.
> 
> This is on kvmarm/next, but seems to have been broken since 6.3. Bisecting it
> points at commit:
> 
>   0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock")

Makes sense. We were using a spinlock before, which implictly disables
preemption.

Well, one way to hack around the problem would be to just cram
preempt_{disable,enable}() into kvm_arch_hardware_disable(), but that's
kinda gross in the context of cpuhp which isn't migratable in the first
place. Let me have a look...

--
Thanks,
Oliver

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http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2023-07-01 17:42 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-01 12:50 KVM CPU hotplug notifier triggers BUG_ON on arm64 Kristina Martsenko
2023-07-01 12:50 ` Kristina Martsenko
2023-07-01 17:42 ` Oliver Upton [this message]
2023-07-01 17:42   ` Oliver Upton
2023-07-03  9:45   ` Marc Zyngier
2023-07-03  9:45     ` Marc Zyngier
2023-07-03 10:36     ` Kristina Martsenko
2023-07-03 10:36       ` Kristina Martsenko
2023-07-03 16:02     ` Oliver Upton
2023-07-03 16:02       ` Oliver Upton
2023-07-03 16:38       ` Marc Zyngier
2023-07-03 16:38         ` Marc Zyngier

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