On Mon, Nov 19, 2018 at 05:07:56PM +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 running a vcpu cannot get rescheduled.
"running a vcpu cannot get rescheduled" ?
>
> Solve this by waiting for all vcpus to be halted after emmiting the halt
> request.
>
> Fixes commit 6c1b7521f4a07cc63bbe2dfe290efed47cdb780a ("KVM: arm/arm64:
> Factor out functionality to get vgic mmio requester_vcpu")
>
> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
> Suggested-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> virt/kvm/arm/vgic/vgic-mmio.c | 33 +++++++++++----------------------
> 1 file changed, 11 insertions(+), 22 deletions(-)
>
> diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> index f56ff1c..eefd877 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 +347,18 @@ 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) {
> + while (tmp->cpu != -1)
> + cond_resched();
We used to have something like this which Andre then found out it could
deadlock the system, because the VCPU making this request wouldn't have
called kvm_arch_vcpu_put, and its cpu value would still have a value.
That's why we have the vcpu && vcpu != requester check.
Thanks,
Christoffer
