SMP services

Introduction

The SMP services provide various abstractions related to SMP programming. It is defined inside shelter/lib/include/std/smp.h and implemented in shelter/lib/src/std/smp.c

GS register

Regarding the GS register, two functions are provided:

sh_smp_write_gs_base(sh_uint64 value): Write the IA32_GS_BASE MSR. with a value, return nothing
sh_smp_gs_base(): take no argument and return a sh_ap_CPU_STRUCT*

Spinlocks

The SMP services provide a basic implementation for a spinlock, which look like this:

typedef struct {
  volatile sh_uint32 spinlock;
  sh_uint32 lapic_id;
} sh_SPIN_LOCK;

Two macros are provided to initialize an unlocked spinlock:

SH_LOCK(): intended for spinlocks stored as global variables
SH_LOCK_LOCAL(): intended for spinlocks stored as local variables or into structs

This spinlock implementation requires sh_smp_gs_base() to return a valid pointer to a CPU struct. It uses the atomics primitives provided by compilers. Spinlock operations provide full memory ordering guarantees.

Four functions are provided for spinlock manipulation:

sh_spin_lock(sh_SPIN_LOCK *lock): lock a spinlock, block until the lock is acquired. Return nothing
sh_spin_unlock(sh_SPIN_LOCK *lock): unlock a spinlock, return nothing
sh_spin_trylock(sh_SPIN_LOCK *lock): Attempt to acquire the lock without blocking., return SH_TRUE if successfull, SH_FALSE otherwise.
sh_spin_wholock(sh_SPIN_LOCK *lock): return the LAPIC id of the CPU locking the spinlock. Return SH_UINT32_MAX if the spinlock isn't owned by any CPU

Current implementation doesn't disable interrupts while holding locks.

CPU count

SMP services can store the amount of CPU cores using the following functions:

sh_smp_set_cpu_count(sh_int16 cpu_count): set the count of CPU. It is only called one time by sh_ap_prepare_for_smp_launch()
sh_smp_get_cpu_count(): return the count of CPU, return -1 if value isn't initialized

Memory barrier

The SMP services provide the following primitives:

Function name	Role
`sh_mb()`	Full memory barrier
`sh_rmb()`	Read memory barrier
`sh_wmb()`	Write memory barrier

Current x86_64 implementation uses mfence/lfence/sfence.

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