Re: [PATCH v2 3/3] KVM: arm/arm64: Simplify active_change_prepare and plug race

[Marc Zyngier <marc.zyngier@arm.com>]

On 16/05/17 11:04, Christoffer Dall wrote:
> We don't need to stop a specific VCPU when changing the active state,
> because private IRQs can only be modified by a running VCPU for the
> VCPU itself and it is therefore already stopped.
> 
> However, it is also possible for two VCPUs to be modifying the active
> state of SPIs at the same time, which can cause the thread being stuck
> in the loop that checks other VCPU threads for a potentially very long
> time, or to modify the active state of a running VCPU.  Fix this by
> serializing all accesses to setting and clearing the active state of
> interrupts using the KVM mutex.
> 
> Reported-by: Andrew Jones <drjones@redhat.com>
> Signed-off-by: Christoffer Dall <cdall@linaro.org>
> ---
>  arch/arm/include/asm/kvm_host.h   |  2 --
>  arch/arm64/include/asm/kvm_host.h |  2 --
>  virt/kvm/arm/arm.c                | 20 ++++----------------
>  virt/kvm/arm/vgic/vgic-mmio.c     | 18 ++++++++++--------
>  virt/kvm/arm/vgic/vgic.c          | 11 ++++++-----
>  5 files changed, 20 insertions(+), 33 deletions(-)
> 
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index f0e6657..12274d4 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -233,8 +233,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
>  struct kvm_vcpu __percpu **kvm_get_running_vcpus(void);
>  void kvm_arm_halt_guest(struct kvm *kvm);
>  void kvm_arm_resume_guest(struct kvm *kvm);
> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
>  
>  int kvm_arm_copy_coproc_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
>  unsigned long kvm_arm_num_coproc_regs(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 5e19165..32cbe8a 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -333,8 +333,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
>  struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
>  void kvm_arm_halt_guest(struct kvm *kvm);
>  void kvm_arm_resume_guest(struct kvm *kvm);
> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
>  
>  u64 __kvm_call_hyp(void *hypfn, ...);
>  #define kvm_call_hyp(f, ...) __kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__)
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 3417e18..3c387fd 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -539,27 +539,15 @@ void kvm_arm_halt_guest(struct kvm *kvm)
>  	kvm_make_all_cpus_request(kvm, KVM_REQ_VCPU_EXIT);
>  }
>  
> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
> -{
> -	vcpu->arch.pause = true;
> -	kvm_vcpu_kick(vcpu);
> -}
> -
> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu)
> -{
> -	struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> -
> -	vcpu->arch.pause = false;
> -	swake_up(wq);
> -}
> -
>  void kvm_arm_resume_guest(struct kvm *kvm)
>  {
>  	int i;
>  	struct kvm_vcpu *vcpu;
>  
> -	kvm_for_each_vcpu(i, vcpu, kvm)
> -		kvm_arm_resume_vcpu(vcpu);
> +	kvm_for_each_vcpu(i, vcpu, kvm) {
> +		vcpu->arch.pause = false;
> +		swake_up(kvm_arch_vcpu_wq(vcpu));
> +	}
>  }
>  
>  static void vcpu_sleep(struct kvm_vcpu *vcpu)
> diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> index 64cbcb4..c1e4bdd 100644
> --- a/virt/kvm/arm/vgic/vgic-mmio.c
> +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> @@ -231,23 +231,21 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
>   * be migrated while we don't hold the IRQ locks and we don't want to be
>   * chasing moving targets.
>   *
> - * For private interrupts, we only have to make sure the single and only VCPU
> - * that can potentially queue the IRQ is stopped.
> + * For private interrupts we don't have to do anything because userspace
> + * accesses to the VGIC state already require all VCPUs to be stopped, and
> + * only the VCPU itself can modify its private interrupts active state, which
> + * guarantees that the VCPU is not running.
>   */
>  static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
>  {
> -	if (intid < VGIC_NR_PRIVATE_IRQS)
> -		kvm_arm_halt_vcpu(vcpu);
> -	else
> +	if (intid > VGIC_NR_PRIVATE_IRQS)
>  		kvm_arm_halt_guest(vcpu->kvm);
>  }
>  
>  /* See vgic_change_active_prepare */
>  static void vgic_change_active_finish(struct kvm_vcpu *vcpu, u32 intid)
>  {
> -	if (intid < VGIC_NR_PRIVATE_IRQS)
> -		kvm_arm_resume_vcpu(vcpu);
> -	else
> +	if (intid > VGIC_NR_PRIVATE_IRQS)
>  		kvm_arm_resume_guest(vcpu->kvm);
>  }
>  
> @@ -271,11 +269,13 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
>  {
>  	u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
>  
> +	mutex_lock(&vcpu->kvm->lock);
>  	vgic_change_active_prepare(vcpu, intid);
>  
>  	__vgic_mmio_write_cactive(vcpu, addr, len, val);
>  
>  	vgic_change_active_finish(vcpu, intid);
> +	mutex_unlock(&vcpu->kvm->lock);

Any reason not to move the lock/unlock calls to prepare/finish? Also, do
we need to take that mutex if intid is a PPI?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...