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    SubjectRe: [PATCH v2 1/4] KVM: arm/arm64: vgic: Do not cond_resched_lock() with IRQs disabled
    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?

    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.


    Thanks,

    Christoffer

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