resource - Address space rights and accounting
A resource is an immutable object that is used to validate access to syscalls that create objects backed by address space, or permit access to address space. These include vm objects, interrupts, and x86 ioports.
Resources are used to gate access to specific regions of address space and are required to create VMOs and IRQs, as well as accessing x86 ioports.
A resource object consists of a single resource kind, with base address and len parameters that define a range of address space the holder of the resource is granted access to. The range covers base up to but not including base + len. These objects are immutable after creation. Valid kind values are ZX_RSRC_KIND_ROOT, ZX_RSRC_KIND_HYPERVISOR, ZX_RSRC_KIND_MMIO, ZX_RSRC_KIND_IOPORT, ZX_RSRC_KIND_IRQ, ZX_RSRC_KIND_VMEX, and ZX_RSRC_KIND_SMC. New resources may be created with a root resource by calling zx_resource_create(). An initial root resource is created by the kernel during boot and handed off to the first userspace process started by userboot.
Resource allocations can be either shared or exclusive. A shared resource grants the permission to access the given address space, but does not reserve that address space exclusively for the owner of the resource. An exclusive resource grants access to the region to only the holder of the exclusive resource. Exclusive and shared resource ranges may not overlap.
Resources are lifecycle tracked and upon the last handle being closed will be freed. In the case of exclusive resources this means the given address range will be released back to the allocator for the given kind of resource. Objects created through a resource do not hold a reference to the resource and thus do not keep it alive.
Resources are typically private to the DDK and platform bus drivers. Presently, this means ACPI and platform bus hold the root resource respectively and hand out more fine-grained resources to other drivers.