linux-kernel - Re: [PATCH v4 5/5] x86: KVM: SEV: implement kvm_lock_all

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Message-ID: <20250503100820.GF4198@noisy.programming.kicks-ass.net>
Date: Sat, 3 May 2025 12:08:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
	linux-riscv@...ts.infradead.org,
	Kunkun Jiang <jiangkunkun@...wei.com>,
	Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Boqun Feng <boqun.feng@...il.com>, Borislav Petkov <bp@...en8.de>,
	Albert Ou <aou@...s.berkeley.edu>, Anup Patel <anup@...infault.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Alexandre Ghiti <alex@...ti.fr>,
	Alexander Potapenko <glider@...gle.com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Andre Przywara <andre.przywara@....com>, x86@...nel.org,
	Joey Gouly <joey.gouly@....com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm-riscv@...ts.infradead.org,
	Atish Patra <atishp@...shpatra.org>, Ingo Molnar <mingo@...hat.com>,
	Jing Zhang <jingzhangos@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, kvmarm@...ts.linux.dev,
	Will Deacon <will@...nel.org>,
	Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
	Sebastian Ott <sebott@...hat.com>, Shusen Li <lishusen2@...wei.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Randy Dunlap <rdunlap@...radead.org>, Marc Zyngier <maz@...nel.org>,
	Zenghui Yu <yuzenghui@...wei.com>
Subject: Re: [PATCH v4 5/5] x86: KVM: SEV: implement kvm_lock_all_vcpus and
 use it

On Fri, May 02, 2025 at 01:57:13PM -0700, Sean Christopherson wrote:

> int kvm_lock_all_vcpus(struct kvm *kvm)
> {
> 	struct kvm_vcpu *vcpu;
> 	unsigned long i, j;
> 	int r;
> 
> 	lockdep_assert_held(&kvm->lock);

So I agree that having this assertion here is probably good from a
read-code pov, however, strictly speaking, it is redundant in that:

> 	kvm_for_each_vcpu(i, vcpu, kvm) {
> 		r = mutex_lock_killable_nest_lock(&vcpu->mutex, &kvm->lock);

will implicitly assert kvm->lock is held. If you try to use an unheld
lock as nest lock, it will complain loudly :-)

(my inner pendant had to reply, ignore at will :-)

> 		if (r)
> 			goto out_unlock;
> 	}
> 	return 0;
> 
> out_unlock:
> 	kvm_for_each_vcpu(j, vcpu, kvm) {
> 		if (i == j)
> 			break;
> 		mutex_unlock(&vcpu->mutex);
> 	}
> 	return r;
> }
> EXPORT_SYMBOL_GPL(kvm_lock_all_vcpus);