qemu/target/riscv/cpu_user.h

#define xRA 1   /* return address (aka link register) */
#define xSP 2   /* stack pointer */
#define xGP 3   /* global pointer */
#define xTP 4   /* thread pointer */

#define xA0 10  /* gpr[10-17] are syscall arguments */
#define xA1 11
#define xA2 12
#define xA3 13
#define xA4 14
#define xA5 15
#define xA6 16
#define xA7 17  /* syscall number for RVI ABI */
#define xT0 5   /* syscall number for RVE ABI */