1Steps for adding TLS support for a new platform: 2 3(1) Declare TLS variant in machine/types.h by defining either 4__HAVE_TLS_VARIANT_I or __HAVE_TLS_VARIANT_II. 5 6(2) _lwp_makecontext has to set the reserved register or kernel transfer 7variable in uc_mcontext to the provided value of 'private'. See 8src/lib/libc/arch/$PLATFORM/gen/_lwp.c. 9 10This is not possible on the VAX as there is no free space in ucontext_t. 11This requires either a special version of _lwp_create or versioning 12everything using ucontext_t. Debug support depends on getting the data from 13ucontext_t, so the second option is possibly required. 14 15(3) _lwp_setprivate(2) has to update the same register as 16_lwp_makecontext uses for the private area pointer. Normally 17cpu_lwp_setprivate is provided by MD to reflect the kernel view and 18enabled by defining __HAVE_CPU_LWP_SETPRIVATE in machine/types.h. 19cpu_setmcontext is responsible for keeping the MI l_private field 20synchronised by calling lwp_setprivate as needed. 21 22cpu_switchto has to update the mapping. 23 24_lwp_setprivate is used for the initial thread, all other threads 25created by libpthread use _lwp_makecontext for this purpose. 26 27(4) Provide __tls_get_addr and possible other MD functions for dynamic 28TLS offset computation. If such alternative entry points exist (currently 29only i386), also add a weak reference to 0 in src/lib/libc/tls/tls.c. 30 31The generic implementation can be found in tls.c and is used with 32__HAVE_COMMON___TLS_GET_ADDR. It depends on ___lwp_getprivate_fast 33(see below). 34 35(5) Implement the necessary relocation records in mdreloc.c. There are 36typically three relocation types found in dynamic binaries: 37 38(a) R_TYPE(TLS_DTPOFF): Offset inside the module. The common TLS code 39ensures that the DTV vector points to offset 0 inside the module TLS block. 40This is normally def->st_value + rela->r_addend. 41 42(b) R_TYPE(TLS_DTPMOD): Module index. 43 44(c) R_TYPE(TLS_TPOFF): Static TLS offset. The code has to check whether 45the static TLS offset for this module has been allocated 46(defobj->tls_done) and otherwise call _rtld_tls_offset_allocate(). This 47may fail if no static space is available and the object has been pulled 48in via dlopen(3). 49 50For TLS Variant I, this is typically: 51 52def->st_value + rela->r_addend + defobj->tlsoffset + sizeof(struct tls_tcb) 53 54e.g. the relocation doesn't include the fixed TCB. 55 56For TLS Variant II, this is typically: 57 58def->st_value - defobj->tlsoffset + rela->r_addend 59 60e.g. starting offset is counting down from the TCB. 61 62(6) Implement _lwp_getprivate_fast() in machine/mcontext.h and set 63__HAVE___LWP_GETPRIVATE_FAST in machine/types.h. 64 65(7) Test using src/tests/lib/libc/tls. Make sure with "objdump -R" that 66t_tls_dynamic has two TPOFF relocations and h_tls_dlopen.so.1 and 67libh_tls_dynamic.so.1 have both two DTPMOD and DTPOFF relocations. 68