GIET-VM / Context Handler

The ctx_handler.c​ and ctx_handler.h​ files define the data structure and functions used to handle threads contexts. They are prefixed by "_" to remind that they can only be executed by a processor in kernel mode.

The GIET_VM threads try to implement the POSIX threads. A thread is statically allocated to a physical processor, as defined in the mapping, and the GIET-VM does not support thread migration. There is one scheduler per processor, and the max number of threads on one single processor cannot be larger than 13. A thread can be in three different states:

• RUNNING the thread is currently executed.
• RUNNABLE the thread is not running, but can be selected for execution at the next TICK.
• BLOCKED the thread is blocked, waiting on a condition defined in the NORUN context slot.

A thread context is an array of 64 words = 256 bytes. It contains copies of processor registers (when the thread is preempted):

• GPR[i] : generally stored in slot (i). $0, $26 & $27 are not saved.
• HI & LO registers : used by multiply/divide instructions.
• CP0 registers : EPC, SR, CR, BVAR
• CP2 registers : PTPR

It contains some general informations associated to the task:

• TTY : TTY channel allocated to the task
• NIC_RX : NIC_RX channel allocated to the task
• NIC_TX : NIC_TX channel allocated to the task
• CMA_RX : CMA channel for NIC_RX allocated to the task
• CMA_TX : CMA channel for NIC_TX allocated to the task
• CBA_FB : CMA channel for FBF display allocated to the task
• HBA : HBA channel local index
• TIM : TIM channel local index
• PTAB : page table virtual base address
• LTID : Task local index (in scheduler)
• VSID : Virtual space index
• N0RUN : bit-vector defining the task state ( blocked if non zero )
• TRDID : Thread index (in vspace)
• SIGNAL : bit-vector (inter-tasks signalisation)
• ENTRY : Virtual address of task entry point
• COPROC : cluster_xy defines the coordinates of coprocessor allocated to the thread

[0] *	[8] $8	[16] $16	[24] $24	[32] EPC	[40] TTY	[48] TRDID
[1] $1	[9] $9	[17] $17	[25] $25	[33] CR	[41] CMA_FB	[49] GTID
[2] $2	[10] $10	[18] $18	[26] LO	[34] SR	[42] CMA_RX	[50] NORUN
[3] $3	[11] $11	[19] $19	[27] HI	[35] BVAR	[43] CMA_TX	[51] COPROC
[4] $4	[12] $12	[20] $20	[28] $28	[36] PTAB	[44] NIC_RX	[52] ENTRY
[5] $5	[13] $13	[21] $21	[29] SP	[37] LTID	[45] NIC_TX	[53] SIGNAL
[6] $6	[14] $14	[22] $22	[30] $30	[38] VSID	[46] TIM	[54] *
[7] $7	[15] $15	[23] $23	[31] RA	[39] PTPR	[47] HBA	[55] *

void _ctx_switch()

This function performs a context switch between the running (calling) thread and another runnable thread, using a round-robin sheduling policy between all threads allocated to a given processor (static allocation). It selects the next runable thread to resume execution. If the only runnable task is the current task, return without context switch. If there is no runable task, the scheduler switch to the default "idle" thread. The return address contained in $31 is saved in the current thread context (in the ctx[31] slot), and the function actually returns to the address contained in the ctx[31] slot of the next thread context.

void _ctx_eret()

The address of this function is used to initialise the return address in the "idle" task context.

void _idle_thread()

This function is executed when no other thread can be executed.