[PATCH 3/5] KVM: arm64: Move double-checked lock to kvm_vgic_map_resources()

[Auger Eric]

Hi Alexandru,

On 12/1/20 4:01 PM, Alexandru Elisei wrote:
> kvm_vgic_map_resources() is called when a VCPU if first run and it maps all
> the VGIC MMIO regions. To prevent double-initialization, the VGIC uses the
> ready variable to keep track of the state of resources and the global KVM
> mutex to protect against concurrent accesses. After the lock is taken, the
> variable is checked again in case another VCPU took the lock between the
> current VCPU reading ready equals false and taking the lock.
> 
> The double-checked lock pattern is spread across four different functions:
> in kvm_vcpu_first_run_init(), in kvm_vgic_map_resource() and in
> vgic_{v2,v3}_map_resources(), which makes it hard to reason about and
> introduces minor code duplication. Consolidate the checks in
> kvm_vgic_map_resources(), where the lock is taken.
> 
> No functional change intended.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
> ---
>  arch/arm64/kvm/arm.c            | 8 +++-----
>  arch/arm64/kvm/vgic/vgic-init.c | 6 ++++++
>  arch/arm64/kvm/vgic/vgic-v2.c   | 3 ---
>  arch/arm64/kvm/vgic/vgic-v3.c   | 3 ---
>  4 files changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 9d69d2bf6943..65a5e89f5133 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -530,11 +530,9 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>  		 * Map the VGIC hardware resources before running a vcpu the
>  		 * first time on this VM.
>  		 */
> -		if (unlikely(!vgic_ready(kvm))) {
> -			ret = kvm_vgic_map_resources(kvm);
> -			if (ret)
> -				return ret;
> -		}
> +		ret = kvm_vgic_map_resources(kvm);
> +		if (ret)
> +			return ret;
>  	} else {
>  		/*
>  		 * Tell the rest of the code that there are userspace irqchip
> diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> index 32e32d67a127..a2f4d1c85f00 100644
> --- a/arch/arm64/kvm/vgic/vgic-init.c
> +++ b/arch/arm64/kvm/vgic/vgic-init.c
> @@ -428,7 +428,13 @@ int kvm_vgic_map_resources(struct kvm *kvm)
>  	struct vgic_dist *dist = &kvm->arch.vgic;
>  	int ret = 0;
>  
> +	if (likely(vgic_ready(kvm)))
> +		return 0;
> +
>  	mutex_lock(&kvm->lock);
> +	if (vgic_ready(kvm))
> +		goto out;
> +
While we are at it, the setting of dist->ready may be moved in
kvm_vgic_map_resources() too.

Thanks

Eric
>  	if (!irqchip_in_kernel(kvm))
>  		goto out;
>  
> diff --git a/arch/arm64/kvm/vgic/vgic-v2.c b/arch/arm64/kvm/vgic/vgic-v2.c
> index ebf53a4e1296..7f38c1a93639 100644
> --- a/arch/arm64/kvm/vgic/vgic-v2.c
> +++ b/arch/arm64/kvm/vgic/vgic-v2.c
> @@ -306,9 +306,6 @@ int vgic_v2_map_resources(struct kvm *kvm)
>  	struct vgic_dist *dist = &kvm->arch.vgic;
>  	int ret = 0;
>  
> -	if (vgic_ready(kvm))
> -		goto out;
> -
>  	if (IS_VGIC_ADDR_UNDEF(dist->vgic_dist_base) ||
>  	    IS_VGIC_ADDR_UNDEF(dist->vgic_cpu_base)) {
>  		kvm_err("Need to set vgic cpu and dist addresses first\n");
> diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
> index 9cdf39a94a63..35029c5cb0f1 100644
> --- a/arch/arm64/kvm/vgic/vgic-v3.c
> +++ b/arch/arm64/kvm/vgic/vgic-v3.c
> @@ -500,9 +500,6 @@ int vgic_v3_map_resources(struct kvm *kvm)
>  	int ret = 0;
>  	int c;
>  
> -	if (vgic_ready(kvm))
> -		goto out;
> -
>  	kvm_for_each_vcpu(c, vcpu, kvm) {
>  		struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  
>