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

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Message-ID: <39032d1d-3371-1751-6981-f30811db19a8@arm.com>
Date:   Wed, 8 Nov 2017 15:19:36 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Auger Eric <eric.auger@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Mark Rutland <mark.rutland@....com>,
        Andre Przywara <Andre.Przywara@....com>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Shanker Donthineni <shankerd@...eaurora.org>
Subject: Re: [PATCH v5 26/26] KVM: arm/arm64: GICv4: Theory of operations

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@...aro.org>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....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...