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

[cdall@linaro.org (Christoffer Dall)]

On Tue, May 23, 2017 at 10:05:13AM +0100, Marc Zyngier wrote:
> 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.

So assuming Drew's patches will go on top of these, should we merge this
as fixes to -rcX, or queue them for v4.13 ?

I'm leaning towards the latter because I don't think we've seen these
races do something bad in the wild, and they're probably not going to be
backportable to stable anyway.  Thoughts?

-Christoffer