[Qemu-devel] [RFC PATCH] docs/booting.rst: start documenting the boot process

[Alex Bennée]

While working on some test cases I realised there was quite a lot of
assumed knowledge about how things boot up. I thought it would be
worth gathering this together in a user facing document where we could
pour in the details and background to the boot process. As it's quite
wordy I thought it should be a separate document to the manual (which
can obviously reference this). So far I've managed almost a thousand
words and haven't actually related anything to QEMU's options yet.

So going forward:

  - is this a useful document to have in docs?
  - are there any other areas to mention?
    - out auto-magic -bios selection seems like something worth covering
  - should we have some worked examples of command lines?
    - I was thinking for example of read-only and pflash examples
  - we should also describe why direct kernel boots exits
    - and also the fact they are not that direct (some code executes
      before a kernel - even if it's less than a full firmware)

Submitted for comment....

Signed-off-by: Alex Bennée <address@hidden>
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+=====================================
+Anatomy of a Boot, a QEMU perspective
+=====================================
+
+This document attempts to give an overview of how machines boot-up and
+how this matters to QEMU. We will discuss firmware and BIOSes and the
+things they do before the OS kernel is loaded and your usable system
+is finally ready.
+
+Firmware
+========
+
+When a CPU is powered up it knows nothing about it's environment. It's
+internal state, including the program counter (PC), will be reset to a
+defined set of values and it will attempt to fetch it's first
+instruction and execute it. It is the job of the firmware to bring a
+CPU up from it's first few instructions to running in a relatively
+sane execution environment. Firmware tends to be specific to the
+hardware in question and is stored on non-volatile memory (memory that
+survives a power off) usually a ROM or flash device on the computers
+main board.
+
+Some examples of what firmware does include:
+
+Early Hardware Setup
+--------------------
+
+Modern hardware often requires configuring before it is usable. For
+example most modern systems won't have working RAM until the memory
+controller has been programmed with the correct timings for whatever
+memory is installed on the system. Processors may boot with a very
+restricted view of the memory map until RAM and other key peripherals
+have been configured to appear in it's address space. Some hardware
+may not even appear until some sort of blob has been loaded into it so
+it can start responding to the CPU.
+
+Fortunately for QEMU we don't have to worry too much about this very
+low level configuration. The device model we present to the CPU at
+start-up will generally respond to IO access from processor straight
+away.
+
+BIOS or Firmware Services
+-------------------------
+
+In the early days of the PC era the BIOS or Basic Input/Output System
+provided an abstraction interface to the operating system which
+allowed them to do basic IO operations without having to directly
+drive the hardware. Since then the scope of these firmware services
+have grown as systems become more and more complex.
+
+Modern firmware often follows the Unified Extensible Firmware
+Interface (UEFI) which provides services like secure boot, persistent
+variables and external time-keeping.
+
+There can often be multiple levels of firmware service functions. For
+example systems which support secure execution enclaves generally have
+a firmware component that executes in this secure mode which the
+operating system can call in a defined secure manner to undertake
+security sensitive tasks on it's behalf.
+
+Hardware Enumeration
+--------------------
+
+It's easy to assume that modern hardware is built to be discover-able
+and all the operating system needs to do is enumerate the various
+buses on the system to find out what hardware exists. While buses like
+PCI and USB do support discovery there is usually much more on a
+modern system than just these two things.
+
+In the embedded world it used to be acceptable to have a custom
+compiled kernel which knew where everything is meant to be. However
+this was a brittle approach and not very flexible - most obviously if
+you try and use a kernel compiled for one piece of hardware on another
+piece of hardware that might nominally have the same processor.
+
+The more modern approach is to have a "generic" kernel that has a
+number of different drivers compiled in which are then enabled based
+on a hardware description provided by the firmware. This allows
+flexibility on both sides. The software distribution is less concerned
+about managing lots of different kernels for different pieces of
+hardware. The hardware manufacturer is also able to make small changes
+to the board over time to fix bugs or change minor components.
+
+The two main methods for this are the Advanced Configuration and Power
+Interface (ACPI) and Device Trees. ACPI originated from the PC world
+although is becoming increasingly common for "enterprise" hardware
+like servers. Device Trees of various forms have existed for a while
+with perhaps the most common being Flattened Device Trees (FDT).
+
+Boot Code
+=========
+
+The line between firmware and boot code is a very blurry one. However
+from a functionality point of view we have moved from ensuring the
+hardware is usable as a computing device to finding and loading a
+kernel which is then going to take over control of the system. Modern
+firmware often has the ability to boot a kernel directly and in some
+systems you might chain through several boot loaders before the final
+kernel takes control.
+
+The boot loader needs to do 3 things:
+
+  - find a kernel and load it into RAM
+  - ensure the CPU is in the correct mode for the kernel to boot
+  - pass any information the kernel may need to boot and can't find itself
+
+Once it has done these things it can jump to the kernel and let it get
+on with things.
+
+Kernel
+======
+
+The Kernel now takes over and will be in charge of the system from now
+on. It will enumerate all the devices on the system (again) and load
+drivers that can control them. It will then locate some sort of
+file-system and eventually start running programs that actually do
+work.
+
+------------------------
+How this relates to QEMU
+------------------------
+
+TODO:
+
+  - -bios and -drive flash
+  - dynamic and fixed hardware definitions
+  - direct kernel boots
-- 
2.20.1