[RISC-V] secret behind 'tp'(X4) register in Linux kernel

'current' macro is mainly used to read 'task_struct' address of current process.

arch/riscv/include/asm/current.h
register struct task_struct *riscv_current_is_tp __asm__("tp");

static __always_inline struct task_struct *get_current(void)
{
return riscv_current_is_tp;
}

#define current get_current()

Body of current is 'get_current()' macro. We can deduce that "tp"(X4) holds start address of current.

Core routine of context switching in RISC-V Linux kernel is below.

ENTRY(__switch_to)
/* Save context into prev->thread */
li    a4,  TASK_THREAD_RA
add   a3, a0, a4
add   a4, a1, a4
REG_S ra,  TASK_THREAD_RA_RA(a3)
REG_S sp,  TASK_THREAD_SP_RA(a3)
REG_S s0,  TASK_THREAD_S0_RA(a3)
REG_S s1,  TASK_THREAD_S1_RA(a3)
REG_S s2,  TASK_THREAD_S2_RA(a3)
REG_S s3,  TASK_THREAD_S3_RA(a3)
REG_S s4,  TASK_THREAD_S4_RA(a3)
REG_S s5,  TASK_THREAD_S5_RA(a3)
REG_S s6,  TASK_THREAD_S6_RA(a3)
REG_S s7,  TASK_THREAD_S7_RA(a3)
REG_S s8,  TASK_THREAD_S8_RA(a3)
REG_S s9,  TASK_THREAD_S9_RA(a3)
REG_S s10, TASK_THREAD_S10_RA(a3)
REG_S s11, TASK_THREAD_S11_RA(a3)
/* Restore context from next->thread */
REG_L ra,  TASK_THREAD_RA_RA(a4)
REG_L sp,  TASK_THREAD_SP_RA(a4)
REG_L s0,  TASK_THREAD_S0_RA(a4)
REG_L s1,  TASK_THREAD_S1_RA(a4)
REG_L s2,  TASK_THREAD_S2_RA(a4)
REG_L s3,  TASK_THREAD_S3_RA(a4)
REG_L s4,  TASK_THREAD_S4_RA(a4)
REG_L s5,  TASK_THREAD_S5_RA(a4)
REG_L s6,  TASK_THREAD_S6_RA(a4)
REG_L s7,  TASK_THREAD_S7_RA(a4)
REG_L s8,  TASK_THREAD_S8_RA(a4)
REG_L s9,  TASK_THREAD_S9_RA(a4)
REG_L s10, TASK_THREAD_S10_RA(a4)
REG_L s11, TASK_THREAD_S11_RA(a4)
/* Swap the CPU entry around. */
lw a3, TASK_TI_CPU(a0)
lw a4, TASK_TI_CPU(a1)
sw a3, TASK_TI_CPU(a1)
sw a4, TASK_TI_CPU(a0)
/* The offset of thread_info in task_struct is zero. */
move tp, a1
ret
ENDPROC(__switch_to)
   

'move tp, a1' instruction corresponding to 'move X4, X11' is added to save the starting address of 'task_struct.

Body of 'alloc_fd' function.

fs/file.c
static int alloc_fd(unsigned start, unsigned end, unsigned flags)
{
struct files_struct *files = current->files;

The 'current' macro is used as a way to read starting address of 'task_struct'.

SP:FFFFFFFF8014E5B0|711D      alloc_fd: c.addi16sp -0x60         ; -96
SP:FFFFFFFF8014E5B2|E8A2                c.sdsp     x8,0x50(x2)   ; x8,80(x2)
SP:FFFFFFFF8014E5B4|F852                c.sdsp     x20,0x30(x2)   ; x20,48(x2)
SP:FFFFFFFF8014E5B6|F456                c.sdsp     x21,0x28(x2)   ; x21,40(x2)
SP:FFFFFFFF8014E5B8|F05A                c.sdsp     x22,0x20(x2)   ; x22,32(x2)
SP:FFFFFFFF8014E5BA|EC5E                c.sdsp     x23,0x18(x2)   ; x23,24(x2)
SP:FFFFFFFF8014E5BC|E862                c.sdsp     x24,0x10(x2)   ; x24,16(x2)
SP:FFFFFFFF8014E5BE|EC86                c.sdsp     x1,0x58(x2)   ; x1,88(x2)
SP:FFFFFFFF8014E5C0|E4A6                c.sdsp     x9,0x48(x2)   ; x9,72(x2)
SP:FFFFFFFF8014E5C2|E0CA                c.sdsp     x18,0x40(x2)   ; x18,64(x2)
SP:FFFFFFFF8014E5C4|FC4E                c.sdsp     x19,0x38(x2)   ; x19,56(x2)
SP:FFFFFFFF8014E5C6|E466                c.sdsp     x25,0x8(x2)   ; x25,8(x2)
SP:FFFFFFFF8014E5C8|1080                c.addi4spn x8,0x60       ; x8,96
SP:FFFFFFFF8014E5CA|67823903            ld         x18,0x678(x4)   ; x18,1656(x4)

After 'ld x18,0x678(x4)' instruction is executed, x18 is holding the &(current->files) where 0x678 is offset of 'struct task_struct.files.
This is true because x4 holds the entry address of 'task_struct'.

fs/file.c
int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
{
struct task_struct *me = current;
struct files_struct *cur_fds = me->files, *fds = NULL;
 
      SP:FFFFFFFF8014EC14|__close_range:    c.addi16sp -0x60         ; -96
      SP:FFFFFFFF8014EC16|                  c.sdsp     x8,0x50(x2)   ; x8,80(x2)
      SP:FFFFFFFF8014EC18|                  c.sdsp     x1,0x58(x2)   ; x1,88(x2)
      SP:FFFFFFFF8014EC1A|                  c.sdsp     x21,0x28(x2)   ; x21,40(x2)
      SP:FFFFFFFF8014EC1C|                  c.addi4spn x8,0x60       ; x8,96
      SP:FFFFFFFF8014EC1E|                  ld         x15,0x460(x4)   ; x15,1120(x4)
      SP:FFFFFFFF8014EC22|                  sd         x15,-0x58(x8)   ; x15,-88(x8)
      SP:FFFFFFFF8014EC26|                  c.li       x15,0x0       ; x15,0