Re: [PATCH v5 3/6] KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus

From: Marc Zyngier <maz@kernel.org>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Suzuki K Poulose <suzuki.poulose@arm.com>,
	Jing Zhang <jingzhangos@google.com>,
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	Paul Walmsley <paul.walmsley@sifive.com>,
	Albert Ou <aou@eecs.berkeley.edu>
Subject: Re: [PATCH v5 3/6] KVM: add kvm_lock_all_vcpus and kvm_trylock_all_vcpus
Date: Wed, 14 May 2025 10:33:50 +0100	[thread overview]
Message-ID: <86o6vvfse9.wl-maz@kernel.org> (raw)
In-Reply-To: <20250512180407.659015-4-mlevitsk@redhat.com>

On Mon, 12 May 2025 19:04:04 +0100,
Maxim Levitsky <mlevitsk@redhat.com> wrote:
> 
> In a few cases, usually in the initialization code, KVM locks all vCPUs
> of a VM to ensure that userspace doesn't do funny things while KVM performs
> an operation that affects the whole VM.
> 
> Until now, all these operations were implemented using custom code,
> and all of them share the same problem:
> 
> Lockdep can't cope with simultaneous locking of a large number of locks of
> the same class.
> 
> However if these locks are taken while another lock is already held,
> which is luckily the case, it is possible to take advantage of little known
> _nest_lock feature of lockdep which allows in this case to have an
> unlimited number of locks of same class to be taken.
> 
> To implement this, create two functions:
> kvm_lock_all_vcpus() and kvm_trylock_all_vcpus()
> 
> Both functions are needed because some code that will be replaced in
> the subsequent patches, uses mutex_trylock, instead of regular mutex_lock.
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.

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