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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
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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