Re: [PATCH v4 2/5] arm64: KVM: use mutex_trylock_nest_lock when locking all vCPUs

From: Sean Christopherson <seanjc@google.com>
To: Marc Zyngier <maz@kernel.org>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
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Subject: Re: [PATCH v4 2/5] arm64: KVM: use mutex_trylock_nest_lock when locking all vCPUs
Date: Fri, 2 May 2025 13:58:51 -0700	[thread overview]
Message-ID: <aBUyC5kgTipXud-7@google.com> (raw)
In-Reply-To: <864iy4ivro.wl-maz@kernel.org>

On Thu, May 01, 2025, Marc Zyngier wrote:
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 69782df3617f..834f08dfa24c 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1368,6 +1368,40 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
> >  	return 0;
> >  }
> >  
> > +/*
> > + * Try to lock all of the VM's vCPUs.
> > + * Assumes that the kvm->lock is held.
> 
> Assuming is not enough. These assertions have caught a number of bugs,
> and I'm not prepared to drop them.
> 
> > + */
> > +int kvm_trylock_all_vcpus(struct kvm *kvm)
> > +{
> > +	struct kvm_vcpu *vcpu;
> > +	unsigned long i, j;
> > +
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		if (!mutex_trylock_nest_lock(&vcpu->mutex, &kvm->lock))
> > +			goto out_unlock;
> > +	return 0;
> > +
> > +out_unlock:
> > +	kvm_for_each_vcpu(j, vcpu, kvm) {
> > +		if (i == j)
> > +			break;
> > +		mutex_unlock(&vcpu->mutex);
> > +	}
> > +	return -EINTR;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_trylock_all_vcpus);
> > +
> > +void kvm_unlock_all_vcpus(struct kvm *kvm)
> > +{
> > +	struct kvm_vcpu *vcpu;
> > +	unsigned long i;
> > +
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		mutex_unlock(&vcpu->mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_unlock_all_vcpus);
> 
> I don't mind you not including the assertions in these helpers, 

I do :-)  I see no reason not to add assertions here, if locking all vCPUs is
a hot path, we've probably got bigger problems.

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