linux-kernel - Re: [PATCH v2 2/4] KVM: x86: move sev_lock/unlock_vcpus_for_migration to kvm

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Message-ID: <20250410081640.GX9833@noisy.programming.kicks-ass.net>
Date: Thu, 10 Apr 2025 10:16:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	kvm-riscv@...ts.infradead.org,
	Oliver Upton <oliver.upton@...ux.dev>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jing Zhang <jingzhangos@...gle.com>,
	Waiman Long <longman@...hat.com>, x86@...nel.org,
	Kunkun Jiang <jiangkunkun@...wei.com>,
	Boqun Feng <boqun.feng@...il.com>, Anup Patel <anup@...infault.org>,
	Albert Ou <aou@...s.berkeley.edu>, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Zenghui Yu <yuzenghui@...wei.com>,
	Borislav Petkov <bp@...en8.de>, Alexandre Ghiti <alex@...ti.fr>,
	Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
	Sebastian Ott <sebott@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Atish Patra <atishp@...shpatra.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Randy Dunlap <rdunlap@...radead.org>, Will Deacon <will@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	linux-riscv@...ts.infradead.org, Marc Zyngier <maz@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Joey Gouly <joey.gouly@....com>, Ingo Molnar <mingo@...hat.com>,
	Andre Przywara <andre.przywara@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sean Christopherson <seanjc@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 2/4] KVM: x86: move
 sev_lock/unlock_vcpus_for_migration to kvm_main.c

On Tue, Apr 08, 2025 at 09:41:34PM -0400, Maxim Levitsky wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 69782df3617f..71c0d8c35b4b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1368,6 +1368,77 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
>  	return 0;
>  }
>  
> +
> +/*
> + * Lock all VM vCPUs.
> + * Can be used nested (to lock vCPUS of two VMs for example)
> + */
> +int kvm_lock_all_vcpus_nested(struct kvm *kvm, bool trylock, unsigned int role)
> +{
> +	struct kvm_vcpu *vcpu;
> +	unsigned long i, j;
> +
> +	lockdep_assert_held(&kvm->lock);
> +
> +	kvm_for_each_vcpu(i, vcpu, kvm) {
> +
> +		if (trylock && !mutex_trylock_nested(&vcpu->mutex, role))
> +			goto out_unlock;
> +		else if (!trylock && mutex_lock_killable_nested(&vcpu->mutex, role))
> +			goto out_unlock;
> +
> +#ifdef CONFIG_PROVE_LOCKING
> +		if (!i)
> +			/*
> +			 * Reset the role to one that avoids colliding with
> +			 * the role used for the first vcpu mutex.
> +			 */
> +			role = MAX_LOCK_DEPTH - 1;
> +		else
> +			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
> +#endif
> +	}

This code is all sorts of terrible.

Per the lockdep_assert_held() above, you serialize all these locks by
holding that lock, this means you can be using the _nest_lock()
annotation.

Also, the original code didn't have this trylock nonsense, and the
Changelog doesn't mention this -- in fact the Changelog claims no
change, which is patently false.

Anyway, please write like:

	kvm_for_each_vcpu(i, vcpu, kvm) {
		if (mutex_lock_killable_nest_lock(&vcpu->mutex, &kvm->lock))
			goto unlock;
	}

	return 0;

unlock:

	kvm_for_each_vcpu(j, vcpu, kvm) {
		if (j == i)
			break;

		mutex_unlock(&vcpu->mutex);
	}
	return -EINTR;

And yes, you'll have to add mutex_lock_killable_nest_lock(), but that
should be trivial.