linux-kernel - Re: [PATCH RFC v3 4/9] KVM: arm/arm64: use locking helpers in kvm_vgic

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Message-ID: <20170829100016.GU24649@cbox>
Date:   Tue, 29 Aug 2017 12:00:16 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-mips@...ux-mips.org, kvm-ppc@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        James Hogan <james.hogan@...tec.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Graf <agraf@...e.com>
Subject: Re: [PATCH RFC v3 4/9] KVM: arm/arm64: use locking helpers in
 kvm_vgic_create()

On Mon, Aug 21, 2017 at 10:35:25PM +0200, Radim Krčmář wrote:
> No new VCPUs can be created because we are holding the kvm->lock.
> This means that if we successfuly lock all VCPUs, we'll be unlocking the
> same set and there is no need to do extra bookkeeping.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  virt/kvm/arm/vgic/vgic-init.c       | 24 +++++++++---------------
>  virt/kvm/arm/vgic/vgic-kvm-device.c |  6 +++++-
>  2 files changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-init.c b/virt/kvm/arm/vgic/vgic-init.c
> index 5801261f3add..feb766f74c34 100644
> --- a/virt/kvm/arm/vgic/vgic-init.c
> +++ b/virt/kvm/arm/vgic/vgic-init.c
> @@ -119,7 +119,7 @@ void kvm_vgic_vcpu_early_init(struct kvm_vcpu *vcpu)
>   */
>  int kvm_vgic_create(struct kvm *kvm, u32 type)
>  {
> -	int i, vcpu_lock_idx = -1, ret;
> +	int i, ret;
>  	struct kvm_vcpu *vcpu;
>  
>  	if (irqchip_in_kernel(kvm))
> @@ -140,18 +140,14 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
>  	 * vcpu->mutex.  By grabbing the vcpu->mutex of all VCPUs we ensure
>  	 * that no other VCPUs are run while we create the vgic.
>  	 */
> -	ret = -EBUSY;
> -	kvm_for_each_vcpu(i, vcpu, kvm) {
> -		if (!mutex_trylock(&vcpu->mutex))
> -			goto out_unlock;
> -		vcpu_lock_idx = i;
> -	}
> +	if (!lock_all_vcpus(kvm))
> +		return -EBUSY;
>  
> -	kvm_for_each_vcpu(i, vcpu, kvm) {
> -		if (vcpu->arch.has_run_once)
> +	kvm_for_each_vcpu(i, vcpu, kvm)
> +		if (vcpu->arch.has_run_once) {
> +			ret = -EBUSY;
>  			goto out_unlock;
> -	}
> -	ret = 0;
> +		}

I also prefer the additional brace here.

>  
>  	if (type == KVM_DEV_TYPE_ARM_VGIC_V2)
>  		kvm->arch.max_vcpus = VGIC_V2_MAX_CPUS;
> @@ -176,11 +172,9 @@ int kvm_vgic_create(struct kvm *kvm, u32 type)
>  	kvm->arch.vgic.vgic_cpu_base = VGIC_ADDR_UNDEF;
>  	kvm->arch.vgic.vgic_redist_base = VGIC_ADDR_UNDEF;
>  
> +	ret = 0;
>  out_unlock:
> -	for (; vcpu_lock_idx >= 0; vcpu_lock_idx--) {
> -		vcpu = kvm_get_vcpu(kvm, vcpu_lock_idx);
> -		mutex_unlock(&vcpu->mutex);
> -	}
> +	unlock_all_vcpus(kvm);
>  	return ret;
>  }
>  
> diff --git a/virt/kvm/arm/vgic/vgic-kvm-device.c b/virt/kvm/arm/vgic/vgic-kvm-device.c
> index 10ae6f394b71..c5124737c7fc 100644
> --- a/virt/kvm/arm/vgic/vgic-kvm-device.c
> +++ b/virt/kvm/arm/vgic/vgic-kvm-device.c
> @@ -270,7 +270,11 @@ static void unlock_vcpus(struct kvm *kvm, int vcpu_lock_idx)
>  
>  void unlock_all_vcpus(struct kvm *kvm)
>  {
> -	unlock_vcpus(kvm, atomic_read(&kvm->online_vcpus) - 1);
> +	int i;
> +	struct kvm_vcpu *tmp_vcpu;
> +
> +	kvm_for_each_vcpu(i, tmp_vcpu, kvm)
> +		mutex_unlock(&tmp_vcpu->mutex);
>  }
>  
>  /* Returns true if all vcpus were locked, false otherwise */
> -- 
> 2.13.3
> 

As noted on the other patch, it looks a bit strange to modify
unlock_all_vcpus() here without also doing something about the error
path in lock_all_vcpus().

Otherwise this patch looks fine to me.

Thanks,
-Christoffer