On 11/12/2018 10:20, Christoffer Dall wrote:

On Mon, Nov 26, 2018 at 06:26:44PM +0000, Julien Thierry wrote:

...

To change the active state of an MMIO, halt is requested for all vcpus of the affected guest before modifying the IRQ state. This is done by calling cond_resched_lock() in vgic_mmio_change_active(). However interrupts are disabled at this point and we cannot reschedule a vcpu.

Solve this by waiting for all vcpus to be halted after emmiting the halt request.

Signed-off-by: Julien Thierry julien.thierry@arm.com Suggested-by: Marc Zyngier marc.zyngier@arm.com Cc: Christoffer Dall christoffer.dall@arm.com Cc: Marc Zyngier marc.zyngier@arm.com Cc: stable@vger.kernel.org

---

virt/kvm/arm/vgic/vgic-mmio.c | 36 ++++++++++++++---------------------- 1 file changed, 14 insertions(+), 22 deletions(-)

diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c index f56ff1c..5c76a92 100644 --- a/virt/kvm/arm/vgic/vgic-mmio.c +++ b/virt/kvm/arm/vgic/vgic-mmio.c @@ -313,27 +313,6 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq, spin_lock_irqsave(&irq->irq_lock, flags);

• /*

• * If this virtual IRQ was written into a list register, we
  

• * have to make sure the CPU that runs the VCPU thread has
  

• * synced back the LR state to the struct vgic_irq.
  

• *
  

• * As long as the conditions below are true, we know the VCPU thread
  

• * may be on its way back from the guest (we kicked the VCPU thread in
  

• * vgic_change_active_prepare)  and still has to sync back this IRQ,
  

• * so we release and re-acquire the spin_lock to let the other thread
  

• * sync back the IRQ.
  

• *
  

• * When accessing VGIC state from user space, requester_vcpu is
  

• * NULL, which is fine, because we guarantee that no VCPUs are running
  

• * when accessing VGIC state from user space so irq->vcpu->cpu is
  

• * always -1.
  

• */
  

• while (irq->vcpu && /* IRQ may have state in an LR somewhere */

•       irq->vcpu != requester_vcpu && /* Current thread is not the VCPU thread */
  

•       irq->vcpu->cpu != -1) /* VCPU thread is running */
  

• cond_resched_lock(&irq->irq_lock);
  

• if (irq->hw) { vgic_hw_irq_change_active(vcpu, irq, active, !requester_vcpu); } else {

@@ -368,8 +347,21 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq, */ static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid) {

• if (intid > VGIC_NR_PRIVATE_IRQS)

• if (intid > VGIC_NR_PRIVATE_IRQS) {

• struct kvm_vcpu *tmp;
  

• int i;
  

• kvm_arm_halt_guest(vcpu->kvm);

• /* Wait for each vcpu to be halted */
  

• kvm_for_each_vcpu(i, tmp, vcpu->kvm) {
  

• 	if (tmp == vcpu)
  

• 		continue;
  

• 	while (tmp->cpu != -1)
  

• 		cond_resched();
  

• }
  

I'm actually thinking we don't need this loop at all after the requet rework which causes:

1. kvm_arm_halt_guest() to use kvm_make_all_cpus_request(kvm, KVM_REQ_SLEEP), and
2. KVM_REQ_SLEEP uses REQ_WAIT, and
3. REQ_WAIT requires the VCPU to respond to IPIs before returning, and
4. a VCPU thread can only respond when it enables interrupt, and
5. enabling interrupts when running a VCPU only happens after syncing the VGIC hwstate.

Does that make sense?

I'm not super familiar with what goes on with the vgic hwstate syncing, but looking at kvm_arm_halt_guest() and kvm_arch_vcpu_ioctl_run(), I agree with the reasoning.

It would be good if someone can validate this, but if it holds this patch just becomes a nice deletion of the logic in vgic-mmio_change_active.

As long as running kvm_vgic_sync_hwstate() on each vcpu is all that is needed before we can modify the active state, I think your solution is definitely the way to go.

Thanks,