Re: [PATCH v3 04/10] KVM: arm/arm64: use vcpu request in kvm_arm_halt_vcpu

[Christoffer Dall <cdall@linaro.org>]

On Wed, May 03, 2017 at 06:06:29PM +0200, Andrew Jones wrote:
> VCPU halting/resuming is partially implemented with VCPU requests.
> When kvm_arm_halt_guest() is called all VCPUs get the EXIT request,
> telling them to exit guest mode and look at the state of 'pause',
> which will be true, telling them to sleep.  As ARM's VCPU RUN
> implements the memory barrier pattern described in "Ensuring Requests
> Are Seen" of Documentation/virtual/kvm/vcpu-requests.rst, there's
> no way for a VCPU halted by kvm_arm_halt_guest() to miss the pause
> state change.  However, before this patch, a single VCPU halted with
> kvm_arm_halt_vcpu() did not get a request, opening a tiny race window.
> This patch adds the request, closing the race window and also allowing
> us to remove the final check of pause in VCPU RUN, as the final check
> for requests is sufficient.
> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> 
> ---
> 
> I have two questions about the halting/resuming.
> 
> Question 1:
> 
> Do we even need kvm_arm_halt_vcpu()/kvm_arm_resume_vcpu()? It should
> only be necessary if one VCPU can activate or inactivate the private
> IRQs of another VCPU, right?  That doesn't seem like something that
> should be possible, but I'm GIC-illiterate...

True, it shouldn't be possible.  I wonder if we were thinking of
userspace access to the CPU-specific data, but we already ensure that no
VCPUs are running at that time, so I don't think it should be necessary.

> 
> Question 2:
> 
> It's not clear to me if we have another problem with halting/resuming
> or not.  If it's possible for VCPU1 and VCPU2 to race in
> vgic_mmio_write_s/cactive(), then the following scenario could occur,
> leading to VCPU3 being in guest mode when it should not be.  Does the
> hardware prohibit more than one VCPU entering trap handlers that lead
> to these functions at the same time?  If not, then I guess pause needs
> to be a counter instead of a boolean.
> 
>  VCPU1                 VCPU2                  VCPU3
>  -----                 -----                  -----
>                        VCPU3->pause = true;
>                        halt(VCPU3);
>                                               if (pause)
>                                                 sleep();
>  VCPU3->pause = true;
>  halt(VCPU3);
>                        VCPU3->pause = false;
>                        resume(VCPU3);
>                                               ...wake up...
>                                               if (!pause)
>                                                 Enter guest mode. Bad!
>  VCPU3->pause = false;
>  resume(VCPU3);
> 
> (Yes, the "Bad!" is there to both identify something we don't want
>  occurring and to make fun of Trump's tweeting style.)

I think it's bad, and it might be even worse, because it could lead to a
CPU looping forever in the host kernel, since there's no guarantee to
exit from the VM in the other VCPU thread.

But I think simply taking the kvm->lock mutex to serialize the mmio
active change operations should be sufficient.

If we agree on this I can send a patch with your reported by that fixes
that issue, which gets rid of kvm_arm_halt_vcpu and requires you to
modify your first patch to clear the KVM_REQ_VCPU_EXIT flag for each
vcpu in kvm_arm_halt_guest instead and you can fold the remaining change
from this patch into a patch that completely gets rid of the pause flag.

See untested patch draft at the end of this mail.

Thanks,
-Christoffer

> ---
>  arch/arm/kvm/arm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 47f6c7fdca96..9174ed13135a 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -545,6 +545,7 @@ void kvm_arm_halt_guest(struct kvm *kvm)
>  void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
>  {
>  	vcpu->arch.pause = true;
> +	kvm_make_request(KVM_REQ_VCPU_EXIT, vcpu);
>  	kvm_vcpu_kick(vcpu);
>  }
>  
> @@ -664,7 +665,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  
>  		if (ret <= 0 || need_new_vmid_gen(vcpu->kvm) ||
>  		    kvm_request_pending(vcpu) ||
> -		    vcpu->arch.power_off || vcpu->arch.pause) {
> +		    vcpu->arch.power_off) {
>  			vcpu->mode = OUTSIDE_GUEST_MODE;
>  			local_irq_enable();
>  			kvm_pmu_sync_hwstate(vcpu);
> -- 
> 2.9.3
> 


Untested draft patch:

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index d488b88..b77a3af 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -234,8 +234,6 @@ 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 578df18..7a38d5a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -334,8 +334,6 @@ 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 7941699..932788a 100644
--- a/virt/kvm/arm/arm.c
+++ b/virt/kvm/arm/arm.c
@@ -542,27 +542,15 @@ 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 2a5db13..c143add 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);
 }
 
@@ -258,6 +256,7 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
 	u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
 	int i;
 
+	mutex_lock(&vcpu->kvm->lock);
 	vgic_change_active_prepare(vcpu, intid);
 	for_each_set_bit(i, &val, len * 8) {
 		struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
@@ -265,6 +264,7 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
 		vgic_put_irq(vcpu->kvm, irq);
 	}
 	vgic_change_active_finish(vcpu, intid);
+	mutex_unlock(&vcpu->kvm->lock);
 }
 
 void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
@@ -274,6 +274,7 @@ void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
 	u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
 	int i;
 
+	mutex_lock(&vcpu->kvm->lock);
 	vgic_change_active_prepare(vcpu, intid);
 	for_each_set_bit(i, &val, len * 8) {
 		struct vgic_irq *irq = vgic_get_irq(vcpu->kvm, vcpu, intid + i);
@@ -281,6 +282,7 @@ void vgic_mmio_write_sactive(struct kvm_vcpu *vcpu,
 		vgic_put_irq(vcpu->kvm, irq);
 	}
 	vgic_change_active_finish(vcpu, intid);
+	mutex_unlock(&vcpu->kvm->lock);
 }
 
 unsigned long vgic_mmio_read_priority(struct kvm_vcpu *vcpu,