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    Subject[PATCH v3 33/59] irqchip/gic-v4: Add some basic documentation
    Date
    Do a braindump of the way things are supposed to work.

    Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Eric Auger <eric.auger@redhat.com>
    Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
    ---
    drivers/irqchip/irq-gic-v4.c | 71 ++++++++++++++++++++++++++++++++++++++++++++
    1 file changed, 71 insertions(+)

    diff --git a/drivers/irqchip/irq-gic-v4.c b/drivers/irqchip/irq-gic-v4.c
    index 6fe2e09172ed..d10d73477d01 100644
    --- a/drivers/irqchip/irq-gic-v4.c
    +++ b/drivers/irqchip/irq-gic-v4.c
    @@ -22,6 +22,77 @@

    #include <linux/irqchip/arm-gic-v4.h>

    +/*
    + * WARNING: The blurb below assumes that you understand the
    + * intricacies of GICv3, GICv4, and how a guest's view of a GICv3 gets
    + * translated into GICv4 commands. So it effectively targets at most
    + * two individuals. You know who you are.
    + *
    + * The core GICv4 code is designed to *avoid* exposing too much of the
    + * core GIC code (that would in turn leak into the hypervisor code),
    + * and instead provide a hypervisor agnostic interface to the HW (of
    + * course, the astute reader will quickly realize that hypervisor
    + * agnostic actually means KVM-specific - what were you thinking?).
    + *
    + * In order to achieve a modicum of isolation, we try to hide most of
    + * the GICv4 "stuff" behind normal irqchip operations:
    + *
    + * - Any guest-visible VLPI is backed by a Linux interrupt (and a
    + * physical LPI which gets unmapped when the guest maps the
    + * VLPI). This allows the same DevID/EventID pair to be either
    + * mapped to the LPI (host) or the VLPI (guest). Note that this is
    + * exclusive, and you cannot have both.
    + *
    + * - Enabling/disabling a VLPI is done by issuing mask/unmask calls.
    + *
    + * - Guest INT/CLEAR commands are implemented through
    + * irq_set_irqchip_state().
    + *
    + * - The *bizarre* stuff (mapping/unmapping an interrupt to a VLPI, or
    + * issuing an INV after changing a priority) gets shoved into the
    + * irq_set_vcpu_affinity() method. While this is quite horrible
    + * (let's face it, this is the irqchip version of an ioctl), it
    + * confines the crap to a single location. And map/unmap really is
    + * about setting the affinity of a VLPI to a vcpu, so only INV is
    + * majorly out of place. So there.
    + *
    + * A number of commands are simply not provided by this interface, as
    + * they do not make direct sense. For example, MAPD is purely local to
    + * the virtual ITS (because it references a virtual device, and the
    + * physical ITS is still very much in charge of the physical
    + * device). Same goes for things like MAPC (the physical ITS deals
    + * with the actual vPE affinity, and not the braindead concept of
    + * collection). SYNC is not provided either, as each and every command
    + * is followed by a VSYNC. This could be relaxed in the future, should
    + * this be seen as a bottleneck (yes, this means *never*).
    + *
    + * But handling VLPIs is only one side of the job of the GICv4
    + * code. The other (darker) side is to take care of the doorbell
    + * interrupts which are delivered when a VLPI targeting a non-running
    + * vcpu is being made pending.
    + *
    + * The choice made here is that each vcpu (VPE in old northern GICv4
    + * dialect) gets a single doorbell LPI, no matter how many interrupts
    + * are targeting it. This has a nice property, which is that the
    + * interrupt becomes a handle for the VPE, and that the hypervisor
    + * code can manipulate it through the normal interrupt API:
    + *
    + * - VMs (or rather the VM abstraction that matters to the GIC)
    + * contain an irq domain where each interrupt maps to a VPE. In
    + * turn, this domain sits on top of the normal LPI allocator, and a
    + * specially crafted irq_chip implementation.
    + *
    + * - mask/unmask do what is expected on the doorbell interrupt.
    + *
    + * - irq_set_affinity is used to move a VPE from one redistributor to
    + * another.
    + *
    + * - irq_set_vcpu_affinity once again gets hijacked for the purpose of
    + * creating a new sub-API, namely scheduling/descheduling a VPE
    + * (which involves programming GICR_V{PROP,PEND}BASER) and
    + * performing INVALL operations.
    + */
    +
    static struct irq_domain *gic_domain;
    static const struct irq_domain_ops *vpe_domain_ops;

    --
    2.11.0
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