Re: [PATCH v3 57/59] KVM: arm/arm64: GICv4: Theory of operations

[Marc Zyngier]

On 28/08/17 19:18, Christoffer Dall wrote:
> On Mon, Jul 31, 2017 at 06:26:35PM +0100, Marc Zyngier wrote:
>> Yet another braindump so I can free some cells...
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>> ---
>>  virt/kvm/arm/vgic/vgic-v4.c | 68 +++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 68 insertions(+)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c
>> index 0a8deefbcf1c..0c002d2be620 100644
>> --- a/virt/kvm/arm/vgic/vgic-v4.c
>> +++ b/virt/kvm/arm/vgic/vgic-v4.c
>> @@ -22,6 +22,74 @@
>>  
>>  #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().
> 
> So is VFIO notified of this or does it still think the IRQ is
> forwarded?  Or does it not care, and it's state maintained by the irq
> subsystem?

VFIO shouldn't care. The whole forward/bypass looks pretty stateless,
and VFIO will happily inject the interrupt if it gets remapped, as its
own interrupt handlers are still live.

>> + * - 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.
> 
> Is there not a quick way to check with VFIO or the irq subsystem if this
> interrupt can be forwarded and attempt that when handling the MAPTI in
> the vTIS, or does this break in horrible ways?

The problem we have here is that we need to map a purely virtual
interrupt to a Linux IRQ. VFIO does that job by using the offset of the
guest write into the MSI-X table and finding which MSI descriptor is
associated with this entry, giving us the corresponding interrupt.

We could keep track of the previous mappings we've been given, use that
as a hint for the new mapping, and be able to revert it should the guest
update the MSI on the endpoint. It feels pretty involved for something
that is pretty theoretical right now, but I'm happy to try it...

>> + *
>> + * 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. This behaviour is
>> + * pretty similar to that of the backgroud timer.
> 
> *background
> 
> However, I think you can probably drop the last sentence in interest of
> clarity.

Fair enough.

> 
>> + */
>> +
>>  static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info)
>>  {
>>  	struct kvm_vcpu *vcpu = info;
>> -- 
>> 2.11.0
>>
> 
> Thanks for an otherwise excellent (and of course entertaining) writeup!
> -Christoffer
> 

Thanks!

	M.
-- 
Jazz is not dead. It just smells funny...