Re: [PATCH v1 1/2] KVM: arm64: Acquire mp_state_lock in kvm_arch_vcpu_ioctl_vcpu_init()

From: Marc Zyngier <maz@kernel.org>
To: Reiji Watanabe <reijiw@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH v1 1/2] KVM: arm64: Acquire mp_state_lock in kvm_arch_vcpu_ioctl_vcpu_init()
Date: Thu, 20 Apr 2023 09:16:43 +0100	[thread overview]
Message-ID: <86y1mnj7dg.wl-maz@kernel.org> (raw)
In-Reply-To: <20230420021302.iyl3pqo3lg6lpabv@google.com>

On Thu, 20 Apr 2023 03:13:02 +0100,
Reiji Watanabe <reijiw@google.com> wrote:
> 
> Hi Marc,
> 
> On Wed, Apr 19, 2023 at 08:12:45AM +0100, Marc Zyngier wrote:
> > On Wed, 19 Apr 2023 03:18:51 +0100,
> > Reiji Watanabe <reijiw@google.com> wrote:
> > > kvm_arch_vcpu_ioctl_vcpu_init() doesn't acquire mp_state_lock
> > > when setting the mp_state to KVM_MP_STATE_RUNNABLE. Fix the
> > > code to acquire the lock.
> > > 
> > > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > > ---
> > >  arch/arm64/kvm/arm.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index fbafcbbcc463..388aa4f18f21 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -1244,8 +1244,11 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
> > >  	 */
> > >  	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
> > >  		kvm_arm_vcpu_power_off(vcpu);
> > > -	else
> > > +	else {
> > > +		spin_lock(&vcpu->arch.mp_state_lock);
> > >  		WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
> > > +		spin_unlock(&vcpu->arch.mp_state_lock);
> > > +	}
> > >  
> > >  	return 0;
> > >  }
> > 
> > I'm not entirely convinced that this fixes anything. What does the
> > lock hazard against given that the write is atomic? But maybe a
> 
> It appears that kvm_psci_vcpu_on() expects the vCPU's mp_state
> to not be changed by holding the lock.  Although I don't think this
> code practically causes any real issues now, I am a little concerned
> about leaving one instance that updates mpstate without acquiring the
> lock, in terms of future maintenance, as holding the lock won't prevent
> mp_state from being updated.
> 
> What do you think ?

Right, fair enough. It is probably better to take the lock and not
have to think of this sort of things... I'm becoming more lazy by the
minute!

> 
> > slightly more readable of this would be to expand the critical section
> > this way:
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 4ec888fdd4f7..bb21d0c25de7 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1246,11 +1246,15 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
> >  	/*
> >  	 * Handle the "start in power-off" case.
> >  	 */
> > +	spin_lock(&vcpu->arch.mp_state_lock);
> > +
> >  	if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
> > -		kvm_arm_vcpu_power_off(vcpu);
> > +		__kvm_arm_vcpu_power_off(vcpu);
> >  	else
> >  		WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
> >  
> > +	spin_unlock(&vcpu->arch.mp_state_lock);
> > +
> >  	return 0;
> >  }
> > 
> > Thoughts?
> 
> Yes, it looks better!

Cool. I've applied this change to your patch, applied the series to
the lock inversion branch, and remerged the branch in -next.

We're getting there! ;-)

	M.

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