Re: [PATCH v5 26/26] KVM: arm/arm64: GICv4: Theory of operations

Marc Zyngier
2017-11-08T15:17:44Z

On 08/11/17 09:13, Auger Eric wrote:
> Hi Marc,
>
> On 27/10/2017 16:28, Marc Zyngier wrote:
>> Yet another braindump so I can free some cells...
>>
>> Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>> 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 d10e18eabd3b..e367d65a0ebe 100644
>> --- a/virt/kvm/arm/vgic/vgic-v4.c
>> +++ b/virt/kvm/arm/vgic/vgic-v4.c
>> @@ -23,6 +23,73 @@
>>
>> #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.
> We actually have a negotiation protocol between VFIO PCI (irq bypass
> producer) and KVM irqfd (IRQ bypass consumer). When both recognize they
> are tied together, handling an MSI tunneling, they initiate the
> forwarding setup.
>> + *
>> + * 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.
>
> Is it correct to say we replace the following injection chain:
>
> pEventID|
> (pITS) |-> pLPIID -> VFIO PCI IRQ handler -> KVM irqfd ...
> pDevID |
>
> vEventID|
> ... inject (vITS) | -> vLPIID
> vDevID |
>
> by
>
> pEventID|
> (pITS) | -> vLPIID
> pDevID |

That's indeed a pretty accurate description of what happens, with a side
messaging (doorbells).

Thanks,

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