Physical device interrupt handling in XEN
In Xen, only the Hypervisor has an access to thehardware so that to achieve isolation (it is dangerous to share thehardware and let other domains access directly hardware devicessimultaneously).Let's take a little walkthrough dealing with Xen interrupts:
Handlinginterrupts in Xen is done by using event channels. Each domain can holdup to 1024 events. An event channel can have 2 flags associated with it: pending and mask. The mask flag can be updated only by guests. Thehypervisor cannot update it. These flags are not part of the eventchannel structure itself. (struct evtchn is defined in xen/include/xen/sched.h). There are 2 arrays in struct shared_info which contains these flags:evtchn_pending[] and evtchn_mask[] ; each holds 32 elements. (file xen/include/public/xen.h)
(The shared_info is a member in domain struct; it is the domainshared data area).
TBD: add info about event selectors (evtchn_pending_sel in vcpu_info).
Registration (or binding) of irqs in guest domains:
The guest OS calls init_IRQ() when it boots (start_kernel() method calls init_IRQ() ; file init/main.c).
(init_IRQ() is in file sparse/arch/xen/kernel/evtchn.c)
There can be 256 physical irqs; so there is an array calledirq_desc with 256 entries. (file sparse/include/linux/irq.h)
All elements in this array are initialized in init_IRQ() so that their status is disabled (IRQ_DISABLED).
Now, when a physical driver starts it usually calls request_irq().
This method eventually calls setup_irq() (both in sparse/kernel/irq/manage.c). which calls startup_pirq().
startup_pirq()send a hypercall to the hypervisor (HYPERVISOR_event_channel_op) inorder to bind the physical irq (pirq) . The hypercall is of typeEVTCHNOP_bind_pirq. See: startup_pirq() (file sparse/arch/xen/kernel/evtchn.c)
On the Hypervisor side, handling this hypervisor call is done in: evtchn_bind_pirq() method (file /common/event_channel.c) whichcalls pirq_guest_bind() (file arch/x86/irq.c).The pirq_guest_bind() changes the status of the corresponding irq_descarray element to be enabled (~IRQ_DISABLED). it also calls startup()method.
Nowwhen an interrupts arrives from the controller (the APIC), we arrive atdo_IRQ() method as is also in usual linux kernel (also inarch/x86/irq.c). The Hypervisor handles only timer and serialinterrupts. Other interrupts are passed to the domains by calling_do_IRQ_guest() (In fact, the IRQ_GUEST flag is set for all interruptsexcept for timer and serial interrupts). _do_IRQ_guest() send theinterrupt by calling send_guest_pirq() to all guests who are registeredon this IRQ. The send_guest_pirq() creates an event channel (aninstance of evtchn) and sets the pending flag of this event channel.(by calling evtchn_set_pending()) Then, asynchronously, Xen will notifythis domain regarding this interrupt (unless it is masked).
TBD: shared interrupts; avoiding problems with shared interrupts whenusing PCI express.
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