[PATCH v4 26/26] KVM: arm/arm64: GICv4: Theory of operations

[Christoffer Dall]

On Fri, Oct 06, 2017 at 04:34:01PM +0100, Marc Zyngier wrote:
> Yet another braindump so I can free some cells...
> 
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>

Acked-by: Christoffer Dall <christoffer.dall at linaro.org>

> ---
>  virt/kvm/arm/vgic/vgic-v4.c | 67 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
> 
> diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
> index 68a2ef6df161..b87806fea554 100644
> --- a/virt/kvm/arm/vgic/vgic-v4.c
> +++ b/virt/kvm/arm/vgic/vgic-v4.c
> @@ -22,6 +22,73 @@
>  
>  #include "vgic.h"
>  
> +/*
> + * How KVM uses GICv4 (insert rude comments here):
> + *
> + * The vgic-v4 layer acts as a bridge between several entities:
> + * - The GICv4 ITS representation offered by the ITS driver
> + * - VFIO, which is in charge of the PCI endpoint
> + * - The virtual ITS, which is the only thing the guest sees
> + *
> + * The configuration of VLPIs is triggered by a callback from VFIO,
> + * instructing KVM that a PCI device has been configured to deliver
> + * MSIs to a vITS.
> + *
> + * kvm_vgic_v4_set_forwarding() is thus called with the routing entry,
> + * and this is used to find the corresponding vITS data structures
> + * (ITS instance, device, event and irq) using a process that is
> + * extremely similar to the injection of an MSI.
> + *
> + * At this stage, we can link the guest's view of an LPI (uniquely
> + * identified by the routing entry) and the host irq, using the GICv4
> + * driver mapping operation. Should the mapping succeed, we've then
> + * successfully upgraded the guest's LPI to a VLPI. We can then start
> + * with updating GICv4's view of the property table and generating an
> + * INValidation in order to kickstart the delivery of this VLPI to the
> + * guest directly, without software intervention. Well, almost.
> + *
> + * When the PCI endpoint is deconfigured, this operation is reversed
> + * with VFIO calling kvm_vgic_v4_unset_forwarding().
> + *
> + * Once the VLPI has been mapped, it needs to follow any change the
> + * guest performs on its LPI through the vITS. For that, a number of
> + * command handlers have hooks to communicate these changes to the HW:
> + * - Any invalidation triggers a call to its_prop_update_vlpi()
> + * - The INT command results in a irq_set_irqchip_state(), which
> + *   generates an INT on the corresponding VLPI.
> + * - The CLEAR command results in a irq_set_irqchip_state(), which
> + *   generates an CLEAR on the corresponding VLPI.
> + * - DISCARD translates into an unmap, similar to a call to
> + *   kvm_vgic_v4_unset_forwarding().
> + * - MOVI is translated by an update of the existing mapping, changing
> + *   the target vcpu, resulting in a VMOVI being generated.
> + * - MOVALL is translated by a string of mapping updates (similar to
> + *   the handling of MOVI). MOVALL is horrible.
> + *
> + * Note that a DISCARD/MAPTI sequence emitted from the guest without
> + * reprogramming the PCI endpoint after MAPTI does not result in a
> + * VLPI being mapped, as there is no callback from VFIO (the guest
> + * will get the interrupt via the normal SW injection). Fixing this is
> + * not trivial, and requires some horrible messing with the VFIO
> + * internals. Not fun. Don't do that.
> + *
> + * Then there is the scheduling. Each time a vcpu is about to run on a
> + * physical CPU, KVM must tell the corresponding redistributor about
> + * it. And if we've migrated our vcpu from one CPU to another, we must
> + * tell the ITS (so that the messages reach the right redistributor).
> + * This is done in two steps: first issue a irq_set_affinity() on the
> + * irq corresponding to the vcpu, then call its_schedule_vpe(). You
> + * must be in a non-preemptible context. On exit, another call to
> + * its_schedule_vpe() tells the redistributor that we're done with the
> + * vcpu.
> + *
> + * Finally, the doorbell handling: Each vcpu is allocated an interrupt
> + * which will fire each time a VLPI is made pending whilst the vcpu is
> + * not running. Each time the vcpu gets blocked, the doorbell
> + * interrupt gets enabled. When the vcpu is unblocked (for whatever
> + * reason), the doorbell interrupt is disabled.
> + */
> +
>  static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info)
>  {
>  	struct kvm_vcpu *vcpu = info;
> -- 
> 2.14.1
>