s390/fpu: remove ARCH_WANTS_DYNAMIC_TASK_STRUCT support

s390 selects ARCH_WANTS_DYNAMIC_TASK_STRUCT in order to make the size of
the task structure dependent on the availability of the vector
facility. This doesn't make sense anymore because since many years all
machines provide the vector facility.

Therefore simplify the code a bit and remove s390 support for
ARCH_WANTS_DYNAMIC_TASK_STRUCT.
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/Kconfig

@@ -123,7 +123,6 @@ config S390
 	select ARCH_USE_BUILTIN_BSWAP
 	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_USE_SYM_ANNOTATIONS
-	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANTS_NO_INSTR
 	select ARCH_WANT_DEFAULT_BPF_JIT
 	select ARCH_WANT_IPC_PARSE_VERSION

arch/s390/include/asm/processor.h

@@ -184,11 +184,7 @@ struct thread_struct {
 	struct gs_cb *gs_cb;			/* Current guarded storage cb */
 	struct gs_cb *gs_bc_cb;			/* Broadcast guarded storage cb */
 	struct pgm_tdb trap_tdb;		/* Transaction abort diagnose block */
-	/*
-	 * Warning: 'fpu' is dynamically-sized. It *MUST* be at
-	 * the end.
-	 */
-	struct fpu fpu;			/* FP and VX register save area */
+	struct fpu fpu;				/* FP and VX register save area */
 };
 
 /* Flag to disable transactions. */

arch/s390/kernel/process.c

@@ -89,7 +89,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	 */
 	save_fpu_regs();
 
-	memcpy(dst, src, arch_task_struct_size);
+	*dst = *src;
 	dst->thread.fpu.regs = dst->thread.fpu.fprs;
 
 	/*

arch/s390/kernel/setup.c

@@ -819,22 +819,6 @@ static void __init setup_randomness(void)
 		static_branch_enable(&s390_arch_random_available);
 }
 
-/*
- * Find the correct size for the task_struct. This depends on
- * the size of the struct fpu at the end of the thread_struct
- * which is embedded in the task_struct.
- */
-static void __init setup_task_size(void)
-{
-	int task_size = sizeof(struct task_struct);
-
-	if (!MACHINE_HAS_VX) {
-		task_size -= sizeof(__vector128) * __NUM_VXRS;
-		task_size += sizeof(freg_t) * __NUM_FPRS;
-	}
-	arch_task_struct_size = task_size;
-}
-
 /*
  * Issue diagnose 318 to set the control program name and
  * version codes.
@@ -927,7 +911,6 @@ void __init setup_arch(char **cmdline_p)
 
 	os_info_init();
 	setup_ipl();
-	setup_task_size();
 	setup_control_program_code();
 
 	/* Do some memory reservations *before* memory is added to memblock */