wiki:boot_procedure

Boot procedure

A) General Principles

The ALMOS-MKH boot procedure can be decomposed in two phases:

  • The architecture dependent phase, implemented by an architecture specific boot_loader procedure.
  • The architecture independent phase, implemented by a generic (architecture independent) kernel-init procedure.

As the generic (i.e. architecture independent) kernel initialization procedure is executed in parallel by all kernel instances in all clusters containing at least one core and one memory bank, the main task of the boot-loader is to load - in each cluster - a local copy of the ALMOS-MKH kernel code, and a description of the hardware architecture, contained in a local boot_info_t data-structure.

This fixed size boot_info_t structure is build by the boot-loader, and stored at the beginning of the local copy of the kdata segment. As it contains both general and cluster specific information, the content depends on the cluster:

  • general hardware architecture features : number of clusters, topology, etc.
  • available external (shared) peripherals : types and features.
  • number of cores in cluster,
  • available internal (private) peripherals in cluster : types and features.
  • available physical memory in cluster.

This boot_info_t structure is defined in the boot_info.h file.

To build the various boot_info_t structures (one per cluster), the boot-loader uses the arch_info_t binary structure, that is described in section Hardware Platform Definition. This binary structure is contained in the arch_info.bin file, and must be stored in the file system root directory.

This method allows an intelligent boot_loader to check and - if required - reconfigure the hardware components, to guaranty that the generated boot_info_t structures contain only functionally tested hardware components.

We describe below the boot_loader for the TSAR architecture, the boot_loader for the I86 architecture, and the generic kernel initialization procedure.

B) Boot-loader for the TSAR architecture

The TSAR boot-loader uses an OS-independent pre-loader, stored in an addressable but non-volatile device, that load the TSAR boot-loader code from an external block-device to the cluster 0 physical memory. This preloader is specific for the TSAR architecture, but independent on the Operating System. It is used by ALMOS-MKH, but also by LINUX, NetBSD, ALMOS_MKH, or the GIET-VM.

The TSAR boot_loader allocates - in each cluster containing a physical memory bank - six fixed size memory zones, to store various binary files or data structures. The two first zones are permanently allocated: The PRE_LOADER zone is only defined in cluster 0, and contains the pre-loader code. The KERNEL_CODE zone containing the kcode and kdata sgments is directly used by the kernel when the boot_loader transfers control - in each cluster - to the kernel_init procedure. The BOOT_CODE, ARCH_INFO, KERNEL_ELF, and BOOT_STACK zones are temporary: they are only used - in each cluster - by the boot-loader code, and the corresponding physical memory can be freely re-allocated by the local kernel instance when it starts execution.

name description base address (physical) size
PRE_LOADER pre-loader code PRELOADER_BASE (0) PRELOADER_MAX_SIZE (16KB)
KERNEL_CODE kernel code and data KERNEL_CODE_BASE (16 KB) KERNEL_CODE_MAX_SIZE (2 MB - 16 KB)
BOOT_CODE boot-loader code and data BOOT_CODE_BASE (2 MB) BOOT_CODE_MAX_SIZE (1 MB)
ARCH_INFO arch_info.bin file copy ARCH_INFO_BASE (3 MB) ARCH_INFO_MAX_SIZE (1 MB)
KERNEL_ELF kernel.elf file copy KERNEL_ELF_BASE (4 MB) KERN_ELF_MAX_SIZE (2 MB)
BOOT_STACK boot stacks (one per core) BOOT_STACK_BASE (6 MB) BOOT_STACK_MAX_SIZE (1MB)

The values given in this array are indicative. The actual values are defined by configuration parameters in the boot_config.h file. The two main constraint are the following:

  • the kcode and kdata segments (in the KERNEL_CODE zone) must be entirely contained in one single big physical page (2 Mbytes), because it will be mapped as one single big page in all process virtual spaces.
  • the BOOT_CODE zone (containing the boot loader instructions and data) must be entirely contained in the

next big physical page, because it will be mapped in the boot-loader page table to allow the cores to access locally the boot code as soon as it has been copied in the local cluster.

A core is identified by two indexes: cxy is the cluster identifier, an lid is the core local index in cluster. The CP0 register containing the core gid (global hardware identifier) has a fixed format: gid = (cxy << 2) + lid

All cores contribute to the boot procedure, but all cores are not simultaneously active:

  • in the first phase - fully sequencial - only core[0][0] is running (core 0 in cluster 0).
  • In the second phase - partially parallel - only core[cxy][0] is running in each cluster.
  • in the last phase - fully parallel - all core[cxy][lid] are running.

We describe below the four phases of the TSAR boot-loader:

B1. Pre-loader phase

  • In the TSAR_LETI architecture, the pre-loader code is stored in the first 16 kbytes of the physical address space in cluster 0.
  • In the TSAR_IOB architecture, the preloader is stored in an external ROM, that is accessed throug the IO_bridge located in cluster 0.

At reset, the MMU is de-activated (for both data and instructions), and the extension address registers supporting direct access to remote memory banks (for data only) contain the 0 value. Therefore, all cores can only access the physical address space of cluster 0.

All cores execute the same pre-loader code, but the work done depends on the core identifier:

  • The core[0][0] load in the BOOT_CODE zone of cluster 0, the boot-loader code stored on disk.
  • All other cores do only one task before going to sleep (i.e. low-power state): each core activates its private WTI channel in the local ICU (Interrupt Controller Unit) to be later activated by an IPI (Inter Processor Interrupt).

B2. Boot-loader sequencial phase

In this phase, only core [0][0] is running, while all other cores are blocked in the preloaded, waiting to be activated by an IPI.

The first instructions of the boot-loader are defined in the boot_entry.S file. This assembly code is executed by all cores entering the boot-loader, but not at the same time.

Each core running this assembly code makes the 3 following actions:

  • It initializes the core stack pointer depending on the lid value extracted from the gid, using the BOOT_STACK_BASE and BOOT_STACK_SIZE parameters defined in the boot_config.h file,
  • It changes the value of the DATA address extension CP2 register, using the cxy value extracted from the gid, to force all cores to use the local stack segments.
  • It jumps to the boot_loader() C function defined in the boot.c file, passing the two (cxy , lid) arguments.

In this sequencial phase, the core[0][0] executing this C function makes the following actions:

  • The core[0][0] initializes 2 peripherals: The TTY terminal (channel 0) to display log messages, and the IOC peripheral to access the disk file system.
  • The core[0][0] initializes the boot-loader FAT32, allowing the boot loader to access files stored in the FAT32 file system on disk.
  • The core[0][0] load in the KERNEL_ELF zone the kernel.elf file from the disk file system..
  • Then it copies in the KERNEL_CORE zone the kcode and kdata segments, using the addresses contained in the .elf file (identity mapping).
  • The core[0][0] load in the ARCH_INFO zone the arch_info.bin file from the disk file system.
  • Then it builds from this arch_info.t structure the specific boot_info_t structure for cluster 0, and stores it in the kdata segment.
  • The core[0][0] send IPIs to activate all cores [cxy][0] in all other clusters.

B3. Boot-loader partially parallel phase

In this phase all core[cxy][0], other than the core[0][0] are running.

At this point, all DATA extension registers point already on the local cluster( to use the local stack).

The core[cxy][0] exécute the following tasks:

  • To access the global data stored in cluster cxy, the core[cxy][0] copies the boot-loader code from BOOT_CODE zone in cluster 0 to BOOT_CORE zone in cluster cxy.
  • To access the instructions stored in cluster cxy, the core[cxy][0] creates a minimal page table containing two big pages mapping respectively the local BOOT_CORE zone, and the local KERNEL_CODE zone, and activates the instruction MMU. [TO BE DONE]
  • The core[cxy][0] copies the arch_info.bin structure from ARCH_INFO zone in cluster 0 to ARCH_INFO zone in cluster cxy.
  • The core[cxy][0] copies the kcode and kdata segments from KERNEL_CODE zone in cluster 0 to KERNEL_CODE zone in cluster cxy.
  • The core[cxy][0] builds from the arch_info.t the specific boot_info_t structure for cluster cxy, and stores it in the local kdata segment.
  • All core[cxy][0], including core[0][0], synchronize using a global barrier.
  • In each cluster cxy, the core[cxy][0] activates the other cores that are blocked in the pre-loader.

B4. Boot-loader fully parallel phase

In this phase all core[cxy][lid] are running.

Each core must initialise few registers, as described below, and jump to the kernel_entry address. This address is defined in the kernel.elf file, and registered in the kernel_entry global variable.

  • argument : the kernel_init() function unique argument is a pointer on the boot_info_t structure, that is the first variable in the data segment.
  • stack pointer : In each cluster an array of idle thread descriptors, indexed by the local core index, is defined in the kdatasegment, on top of the boot_info_t structure. For any thread, the thread descriptor contains the kernel stack, and this is used to initialize the stack pointer.
  • base register : in each core, the cp0_ebase register, defines the kernel entry point in case of interrupt, exception, or syscall, and must be initialized. [TO BE MOVED to kernel_init()]
  • status register : in each core, the cp0_sr register defines the core state, and must be initialized (UM bit reset / IE bit reset / BEV bit reset ).

At this point, the boot-loader completed its job:

  • The kernel code kcode and kdata segments are loaded - in all clusters - in the first offset physical pages.
  • The hardware architecture described by the arch_info.binfile has been analyzed, and copied - in each cluster - in the boot_info_t structure, stored in the kdata segment.
  • Each local kernel instance can use all the physical memory that is not used to store the kernel kcode and kdata segments themselves.

C) Boot-loader for the I86 architecture

TODO

D) Generic kernel initialization procedure

The kernel_init( boot_info_t * info ) function is the kernel entry point when the boot_loader transfers control to the kernel. The argument is a pointer on the fixed size boot_info_t structure, stored in the local kdata segment.

When a core enters this function, the MMU status depends on the target architecture:

  • For the TSAR architectures, the instruction MMU has been activated and uses the Page Table defined by the boot-loader. The data MMU is de-activated, and the DATA address extension register points on the local physical memory.
  • For the I86 architectures, both the instruction and the data MMUs have been activated, an use the Page Table defined by the boot-loader.

In both cases, a new GPT (Generic Page Table), and a new VSL (Virtual Segments List) must be created in each cluster. These structures will be used by all kernel threads (all threads directly attached to the local kernel process) and are stored in the process_zero VMM. The kcode and (possibly) kdata segments are copied in all user process descriptors.

In each cluster, all local cores execute this procedure in parallel, but most tasks are only executed by core[0]. This procedure uses two synchronisation barriers, defined as global variables in the kdata segment:

  • the global_barrier variable is used to synchronize all core[0] in all clusters containing a kernel instance.
  • the local_barrier variable is used to synchronize all cores in a given cluster.

The kernel initialization procedure execute sequentially the following steps:.

D1) Core and cluster identification

Each core has an unique hardware identifier, called gid, that is hard-wired in a read-only register. From the kernel point of view a core is identified by a composite index (cxy,lid), where cxy is the cluster identifier, and lid is a local (continuous) index in the cluster. The association between the gid hardware index and the (cxy,lid) composite index is defined in the boot_info_t structure. In this first step, each core makes an associative search in the boot_info_t structure to obtain the (cxy,lid) indexes from the gid index.

The core[0] initialize the global variable local_cxy defining the local cluster identifier, and initialises the local cluster descriptor from informations found in the boot_info_t structure. All cores makes a first initialization of their private kernel idle_thread. Finally, the core[0] in cluster[0] initialise the kernel TXT0. This terminal is used by any kernel instance running on any core to display log or debug messages. This terminal is configured in non-descheduling mode : the calling thread call directly the relevant TXT driver, without using a server thread.

A synchronization barrier is used to avoid other cores to use the TXT0 terminal before initialization.

D2) Cluster manager initialization

In each cluster, the core[0] makes the cluster manager initialization, namely the cores descriptors array, the DQDT, and the local physical memory allocators.

A synchonization barrier is used to avoid access to cluster manager before initialization.

D3) Kernel entry point and process_zero initialization

All cores initialise the registers, defining the kernel entry point(s) in case of interrupt, exception or system call. This must be done here because the VFS initialization uses RPCs requiring Inter Processor Interrupts. All core initialise their (currently running) IDLE thread descriptor. In each cluster the core[0] initializes the local process_zero descriptor, containing al kernel threads in a given cluster. This include the creation of the local kernel GPT and VSL.

A synchronization barrier is used to avoid access to VSL/GPT before initialization.

D4) MMU activation

In each cluster, all cores activate their private MMU, as required by the architecture. For TSAR, only the instruction MMU is activated, but the data MMU is de-activated. Moreover, the core[0] in cluster[0] initializes the external IOPIC device

A synchronization barrier is used to avoid access to IOPIC before initialization.

D5) Internal & external devices initialization

In each cluster, the core[0] makes the devices initialization. For multi-channels devices, there is one channel device (called chdev_t) per channel. For internal (replicated) devices, the chdev descriptors are allocated in the local cluster. For external (shared) devices, the chdev descriptors are regularly distributed on all clusters. These external chdev are indexed by a global index, and the host cluster is computed from this index by a modulo.

The internal devices descriptors are created first( ICU, then MMC, then DMA ), because the ICU device is used by all other devices. Then the WTI mailboxes used for IPIs (Inter Processor Interrupt) are allocated in local ICU : one WTI mailbox per core. Then each external chdev descriptor is created in the cluster where it must be created.

A synchronization barrier is used to avoid access to devices before initialization.

D6) IPI, Idle thread, and VFS root initialization

Each core enable its private input IPI, and completes initialization of its (currently running) idle thread descriptor. Then core[0] in cluster[0] creates the root VFS in cluster[0]. This requires to access the file system on disk.

A synchronization barrier is used to avoid access to VFS root before initialization.

D7) VFS root initialisation in all clusters

In each cluster other than cluster[0], the core[0] initializes the VFS and FS contexts in the local cluster, from values registered in cluster[0].

A synchronization barrier is used to avoid access to VFS before initialization.

D8) DEVFS global initialization

The core[0] in cluster[0] makes the DEVFS global initialization: It initializes the DEVFS context, and creates the DEVFSbdev and external directory inodes in cluster[0].

A synchronization barrier is used to avoid access to DEVFS root before initialization.

D9) DEVFS local initialization

In each cluster[cxy], the core[0] completes in parallel the DEVFS initialization. Each core[0] get the extended pointers on the dev and external directories from values stored in cluster[0]. Then each core[0] creates the DEVFS internal directory, and creates the pseudo-files for all chdevs in cluster[cxy].

A synchronization barrier is used to avoid access to DEVFS before initialization.

D10) Process init creation

The core[0] in cluster[0] creates (i.e. allocates memory, and initializes) the process descriptor for the first user process. This includes the VMM initialization : the user process GPT and VSL inherits relevant informations from the kernel process GPT and VSL.

The core[0] in cluster[0] displays the ALMOS-MK banner.

A last synchronization barrier is used before jumping to the idle_thread() function.

D11) Scheduler activation

Finally, all cores make the three following actions:

  • set the TICK timer, and unmask interrupts to activate the scheduler.
  • jump to the idle_thread() function, and wait for an useful thread to be scheduled.
Last modified 7 months ago Last modified on Mar 10, 2019, 9:06:44 PM

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