From 7954907bedafd0f8e81633803945cb304793b29d Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Wed, 21 Dec 2016 17:40:16 +0000 Subject: [PATCH] irqchip/gic-v4: Add some basic documentation Do a braindump of the way things are supposed to work. Reviewed-by: Thomas Gleixner Reviewed-by: Eric Auger Signed-off-by: Marc Zyngier --- 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 6d33828..8eb2ad3 100644 --- a/drivers/irqchip/irq-gic-v4.c +++ b/drivers/irqchip/irq-gic-v4.c @@ -23,6 +23,77 @@ #include +/* + * 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; -- 2.7.4