On 20/11/2018 14:18, Christoffer Dall wrote:
> 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.
Ah, I now remember that one. I guess it is a matter of skipping the
requester vcpu in the kvm_for_each_vcpu loop.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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