sdk/fidl/zbi/overview.fidl

// Copyright 2023 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/// This library contains the definitions for the Zircon Boot Image (ZBI) format.
///
/// A Zircon Boot Image consists of a container header followed by boot items.
/// Each boot item has a header (zbi_header_t) and then a payload of
/// `zbi_header_t.length bytes`, which can be any size.  `The zbi_header_t.type`
/// field indicates how to interpret the payload.  Many types specify an additional
/// type-specific header that begins a variable-sized payload.
/// `zbi_header_t.length` does not include the `zbi_header_t` itself, but does
/// include any type-specific headers as part of the payload.  All fields in all
/// header formats are little-endian.
///
/// Padding bytes appear after each item as needed to align the payload size up to
/// a `ZBI_ALIGNMENT` (8-byte) boundary.  This padding is not reflected in the
/// `zbi_header_t.length` value.
///
/// A bootable ZBI can be booted by a Zircon-compatible boot loader. It contains one
/// `ZBI_TYPE_KERNEL_{ARCH}` boot item that must come first, followed by any number
/// of additional boot items whose number, types, and details much comport with that
/// kernel's expectations.
///
/// A partial ZBI cannot be booted, and is only used during the build process.  It
/// contains one or more boot items and can be combined with a kernel and other
/// ZBIs to make a bootable ZBI.
///
/// ZBI_TYPE_STORAGE_* types represent an image that might otherwise appear on some
/// block storage device, i.e. a RAM disk of some sort.  All zbi_header_t fields
/// have the same meanings for all these types.  The interpretation of the payload
/// (after possible decompression) is  indicated by the specific `zbi_header_t.type`
/// value.
///
/// **Note:** The ZBI_TYPE_STORAGE_* types are not a long-term stable ABI.
///  - Items of these types are always packed for a specific version of the
///    kernel and userland boot services, often in the same build that compiles
///    the kernel.
///  - These item types are **not** expected to be synthesized or
///    examined by boot loaders.
///  - New versions of the `zbi` tool will usually retain the ability to
///    read old formats and non-default switches to write old formats, for
///    diagnostic use.
///
/// ## Maintenance & Evolution
///
/// ### ABI Stability
///
/// The ZBI definitions form the contract between a bootloader and the kernel so
/// must remain ABI backwards-compatible. Failure to do so would result in undefined
/// behavior when devices update their kernel to the new ABI but their bootloader
/// still uses the old ABI (or vice-versa).
///
/// It is generally infeasible to update all bootloaders and kernels to a new ABI in
/// lockstep, not only due to the proliferation of boards running Fuchsia, but also
/// because many boards do not support firmware A/B/R without which it is impossible
/// to guarantee atomic updates to both the bootloader and kernel.
///
/// ### API Stability
///
/// API stability is not required. These definitions are not exported to Fuchsia
/// application developers so will not break any out-of-tree builds if e.g. a
/// constant name is updated.
///
/// They are exported in the Firmware SDK for firmware developers, but firmware
/// does not expect to automatically roll new SDK builds; any uprev to a new SDK
/// is an intentional action that expects some porting work.
library zbi;