GIET_VM / Mapping

The GIET_VM is a fully static operating system supporting clusterized, shared address space, NUMA (Non Uniform memory Acces), many-cores architectures, contening MIPS32 processors.

All software objects (user applications code and data, but also kernel code and critical kernel structures such as the page tables or the processors schedulers) are statically build and loaded from disk into physical memory by the bootloader in the boot phase.

The main advantage of this static approach is to provide the system designed a full control on the placement of tasks on processors but also of the data (software objects) on the distributed physical memory banks. It allows optional replication of critical objects such as kernel code, or page tables.

To control this mapping, the system designer must define a mapping, as described below.

C mapping data structure

A dedicated C binary data structure, is used by the GIET_VM bootloader to map one or several multi-threaded user applications on a generic, shared address space multi-processors, multi-clusters architecture. The next section describes how this this C binary structure can be generated by the genmap tool from a source description using the PYTHON language. We describe in this section this C structure.

The C mapping data structure contains the following informations:

1. It contains a description of the target, clusterized, hardware architecture, with the following constraints: The clusters are organised in a 2D mesh topology, and the number of clusters is variable (can be one). The number of processors per cluster is variable (can be one). The number of physical memory banks is variable (up to one physical memory bank per cluster. Most peripherals are external and localized in one specific I/O cluster. The physical address width is between 32 and 48 bits, and is the concatenation of 3 fields: the LSB field (32 bits) define a 4 Gbits physical address space inside a single cluster. The X and Y fields (up to 8 bits for each field) define the cluster coordinate.

2. It contains a description of the user applications to be launched on the platform. An user application is characterized by a a virtual address space, called a vspace. An user application can be multi-threaded, and the number of parallel tasks sharing the same address space in a given application is variable (can be one). The GIET_VM provide a specific Multi-Writer/Multi?-Reader communication middleware for send/receive inter-tasks communication. Each vspace contains a variable number of virtual segments, called vsegs. The number of simultaneously mapped vspaces on a given architecture is variable (can be one).

3. It contains the mapping directives: The tasks are statically allocated to processors. The various software objects (user and kernel code segments, tasks stacks, tasks heaps, communication channels, etc.) are called vobjs, and are statically placed on the distributed physical memory banks (called psegs), using the page tables (one page table per vspace) that define the mapping of the vsegs on the psegs.

The C binary mapping data structure is defined in the mapping_info.h​ file, and is organised as the concatenation of a fixed size header, and 11 variable size arrays:

• mapping_cluster_t cluster[]
• mapping_pseg_t pseg[]
• mapping_vspace_t vspace[]
• mapping_vseg_t vseg[]
• mapping_vobj_t vobj[]
• mapping_task_t task[]
• mapping_proc_t proc[]
• mapping_irq_t irq[]
• mapping_coproc_t coproc[]
• mapping_cp_port_t cp_port[]
• mapping_periph_t periph[]

The map.bin file must be stored on disk and will be loaded in memory by the GIET_VM bootloader in the seg_boot_mapping segment.

Python mapping introduction

A specific mapping requires at least two python files:

• The arch.py file is attached to a given hardware architecture. It describes both the (possibly generic) hardware architectures, and the mapping of the kernel software objects on this hardware architecture.
• The appli.py file is attached to a given user application. It describes both the application structure (tasks and communication channels), and the mapping of the application tasks and software objects on the architecture.

The various Python Classes used by these these files are defined in the mapping.py​ file.

Python architecture description

To define an architecture, you must use the following constructors:

Construct mapping

The Mapping constructor build a mapping object and define the target architecture general parameters:

name	mapping name == architecture name
x_size	number of clusters in a row of the 2D mesh
y_size	number of clusters in a column of the 2D mesh
nprocs	number of processors per cluster
x_width	number of bits to encode X coordinate in paddr
y_width	number of bits to encode Y coordinate in paddr
p_width	number of bits to encode local processor index
paddr_width	number of bits in physical address
coherence	Boolean true if hardware cache coherence
irq_per_proc	number of IRQ lines between XCU and one proc (GIET_VM use only one)
use_ramdisk	Boolean true if the architecture contains a RamDisk?
x_io	io_cluster X coordinate
y_io	io_cluster Y coordinate
peri_increment	virtual address increment for peripherals replicated in all clusters
reset_address	physical base address of the ROM containing the preloader code
ram_base	physical memory bank base address in cluster [0,0]
ram_size	physical memory bank size in one cluster (bytes)

Construct external peripherals

The mapping.addPeriph( ) construct adds one peripheral, and the associated physical segment. It has the following arguments:

name	peripheral name
base	peripheral segment physical base address
size	peripheral segment size
ptype	Peripheral type
subtype	Peripheral subtype
channels	number of channels for multi-channels peripherals
arg	optionnal argument depending on peripheral type

The supported peripheral types and subtypes are defined in the mapping_info.h​ and mapping.py​ files.

Construct physical memory banks

The mapping.addRam( ) construct adds one physical memory bank, and the associated physical segment. It has the following arguments:

Python application description