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

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

On 23/05/17 09:43, Christoffer Dall wrote:
> On Mon, May 22, 2017 at 04:30:22PM +0100, Marc Zyngier wrote:
>> 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?
> 
> I guess we strictly don't need to take the mutex if it's a PPI, no.
> 
> But I actually preferred this symmetry because you can easily tell we
> don't have a bug (famous last words) by locking and unlocking the mutex
> in the same function.
> 
> I don't feel strongly about it though, so I can move it if you prefer
> it.

No, that's fine, I just wanted to check whether my understanding was
correct.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>

Thanks,

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