README.md: Add design for interrupt abstractions

Add a brief description of the design of the Interrupt abstractions.

Signed-off-by: Alexandru-Cezar Sardan <alsardan@amazon.com>

Author: alsrdn

README.md

@@ -9,6 +9,8 @@ address range, IRQ number, etc)
 
 ## Design
 
+### I/O
+
 The virtual device model is built around four traits, `DevicePio` and
 `MutDevicePio` for
 [Programmed I/O](https://en.wikipedia.org/wiki/Programmed_input%E2%80%93output)
@@ -42,8 +44,38 @@ that there’s a virtual device registered on the bus for the requested address,
 and finally sends the request to that device.
 ![vm-device](https://user-images.githubusercontent.com/241037/143853115-b1526028-6836-4845-a311-71cf989c60ef.png)
 
+### Interrupts
+
+Interrupt delivery and configuration is built around the `Interrupt` and
+`InterruptSourceGroup` traits. These traits allow devices that are developped in
+separate crates from the VMM to trigger and configure interrupt delivery to the
+guest VM without having a dependency on the implementation of the interrupt
+mechanism.
+
+The `Interrupt` trait provides methods that are used by devices to trigger,
+enable and disable interrupts. Access to additional interrupt properties 
+is defined in new super-traits. `ConfigurableInterrupt` allows for devices to
+send or receive interrupt configuration parameters to/from the implementation
+inside the VMM. This is useful when devices need to specify custom data that
+the VMM will use when delivering the interrupt (e.g. MSI device id, PCI
+INTx pin etc).
+`MaskableInterrupt` is also defined as a supertrait for use with interrupts
+that can be masked/unmasked.
+Using super-traits for these properties allows further extension in the future.
+
+An `InterruptSourceGroup` stores a collection of interrupts of the same type and
+is the interface through which a device may request or release interrupts and
+perform group related actions like enabling or disabling all interrupts at once.
+Each device that generates interrupts can be assigned one or more
+`InterruptSourceGroup`s (depending on the types of interrupts it uses or logical
+grouping.) The following diagram depincts the interaction between the components
+that use the interrupt interface:
+
+![vm-device-interrupts](https://user-images.githubusercontent.com/86006646/148783015-fea49a7c-cff8-4ec7-8766-00b0baed41c5.png)
+
 ## Usage
 
+### I/O
 A device is usually attached to a particular bus and thus needs to implement a
 trait of only one type. For example, serial port on x86 is a PIO device, while
 VirtIO devices use MMIO. It’s also possible for a device to implement both. Once
@@ -67,6 +99,26 @@ address range to the device. The requests are dispatched by the client code, for
 example when handling VM exits, using `IoManager`'s methods `pio_read`,
 `pio_write`, `mmio_read` and `mmio_write`.
 
+### Interrupts
+
+To allow a device to use interrupts, a VMM must implement the `Interrupt` and
+`InterruptSourceGroup` traits for the interrupt mechanisms that the device requires.
+Implementation is machine or VMM speciffic and may depend on the types and number
+of IRQ chips that the machine has or interrupt delivery mechanisms (e.g. `EventFd`s).
+The device generally does not concern itself with the actual implementation of
+the interrupts and will be initialized with one or more `InterruptSourceGroup`s
+by the VMM.
+
+The device may define constraints for the types of interrupts that it needs
+(e.g. it needs a `ConfigurableInterrupt` that can receive the `LegacyIrqConfig`
+configuration struct).
+The device may also define constraints regarding the type of `NotifierType` that the
+`Interrupt` is using. This type defines the interrupt delivery mechanism and is
+specific to the Hypervisor (e.g. KVM irqfd, Xen evtchn etc).
+One example of this requirement is the development of a VFIO device. Since VFIO
+can trigger a KVM irqfd directly, the VFIO device would need to get access to the
+underlying irqfd in order to register it with VFIO.
+
 ## Examples
 
 ### Implementing a simple log PIO device
@@ -119,3 +171,4 @@ This project is licensed under either of:
 
 - [Apache License](http://www.apache.org/licenses/LICENSE-2.0), Version 2.0
 - [BSD-3-Clause License](https://opensource.org/licenses/BSD-3-Clause)
+