This is part of a larger series that aims at getting rid of the
copy_thread()/copy_thread_tls() split that makes the process creation
codepaths in the kernel more convoluted and error-prone than they need
to be.
I'm converting all the remaining arches that haven't yet switched and
am collecting individual acks. Once I have them, I'll send the whole series
removing the copy_thread()/copy_thread_tls() split, the
HAVE_COPY_THREAD_TLS define and the legacy do_fork() helper. The only
kernel-wide process creation entry point for anything not going directly
through the syscall path will then be based on struct kernel_clone_args.
No more danger of weird process creation abi quirks between architectures
hopefully, and easier to maintain overall.
It also unblocks implementing clone3() on architectures not support
copy_thread_tls(). Any architecture that wants to implement clone3()
will need to select HAVE_COPY_THREAD_TLS and thus need to implement
copy_thread_tls(). So both goals are connected but independently
beneficial.

HAVE_COPY_THREAD_TLS means that a given architecture supports
CLONE_SETTLS and not setting it should usually mean that the
architectures doesn't implement it but that's not how things are. In
fact all architectures support CLONE_TLS it's just that they don't
follow the calling convention that HAVE_COPY_THREAD_TLS implies. That
means all architectures can be switched over to select
HAVE_COPY_THREAD_TLS. Once that is done we can remove that macro (yay,
less code), the unnecessary do_fork() export in kernel/fork.c, and also
rename copy_thread_tls() back to copy_thread(). At this point
copy_thread() becomes the main architecture specific part of process
creation but it will be the same layout and calling convention for all
architectures. (Once that is done we can probably cleanup each
copy_thread() function even more but that's for the future.)

Since ia64 does support CLONE_SETTLS there's no reason to not select
HAVE_COPY_THREAD_TLS. This brings us one step closer to getting rid of
the copy_thread()/copy_thread_tls() split we still have and ultimately
the HAVE_COPY_THREAD_TLS define in general. A lot of architectures have
already converted and ia64 is one of the few hat haven't yet. This also
unblocks implementing the clone3() syscall on ia64. Once that is done we
can get of another ARCH_WANTS_* macro.

Once Any architecture that supports HAVE_COPY_THREAD_TLS cannot call the
do_fork() helper anymore. This is fine and intended since it should be
removed in favor of the new, cleaner _do_fork() calling convention based
on struct kernel_clone_args. In fact, most architectures have already
switched.  With this patch, ia64 joins the other arches which can't use
the fork(), vfork(), clone(), clone3() syscalls directly and who follow
the new process creation calling convention that is based on struct
kernel_clone_args which we introduced a while back. This means less
custom assembly in the architectures entry path to set up the registers
before calling into the process creation helper and it is easier to to
support new features without having to adapt calling conventions. It
also unifies all process creation paths between fork(), vfork(),
clone(), and clone3(). (We can't fix the ABI nightmare that legacy
clone() is but we can prevent stuff like this happening in the future.)

Well, the first version I nothing to test this with. I don't know how to
reasonably explain what happened but thanks to Adrian I'm now sitting at
home next to a HP Integrity RX2600. I've done some testing and my initial
version had a bug that became obvious when I took a closer look. The switch
stack logic assumes that ar.pfs is stored in r16 and I changed that to r2.
So with that fixed the following test program runs without any problems:

 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE 1
 #endif
 #include <errno.h>
 #include <fcntl.h>
 #include <linux/sched.h>
 #include <sched.h>
 #include <signal.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <syscall.h>
 #include <unistd.h>

 #define IA64_SYSCALL_OFFSET 1024
 #ifndef __NR_clone
 #define __NR_clone (104 + IA64_SYSCALL_OFFSET)
 #endif

 #ifndef __NR_clone2
 #define __NR_clone2 (189 + IA64_SYSCALL_OFFSET)
 #endif

 /*
  * sys_clone(unsigned long flags,
  *	     unsigned long stack,
  *	     int *parent_tidptr,
  *	     int *child_tidptr,
  *	     unsigned long tls)
  */
 static pid_t ia64_raw_clone(void)
 {
 	return syscall(__NR_clone, SIGCHLD, 0, NULL, NULL, 0);
 }

 /*
  * sys_clone2(unsigned long flags,
  *	      unsigned long stack,
  *	      unsigned long stack_size,
  *	      int *parent_tidptr,
  *	      int *child_tidptr,
  *	      unsigned long tls)
  */
 static pid_t ia64_raw_clone2(void)
 {
 	return syscall(__NR_clone2, SIGCHLD, 0, 0, NULL, NULL, 0);
 }

 /*
  * Let's use the "standard stack limit" (i.e. glibc thread size default) for
  * stack sizes: 8MB.
  */
 #define __STACK_SIZE (8 * 1024 * 1024)

 /* This is not always defined in sched.h. */
 extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
                      size_t __child_stack_size, int __flags, void *__arg, ...);

 pid_t libc_clone2(int (*fn)(void *), void *arg)
 {
 	pid_t ret;
 	void *stack;

 	stack = malloc(__STACK_SIZE);
 	if (!stack)
 		return -ENOMEM;

 	 return __clone2(fn, stack, __STACK_SIZE, SIGCHLD, arg, NULL, NULL, NULL);
 }

 static int libc_clone2_child(void *data)
 {
 	fprintf(stderr, "I'll just see myself out\n");
 	_exit(EXIT_SUCCESS);
 }

 int main(void)
 {
 	for (int i = 0; i < 1000; i++) {
 		pid_t pid = ia64_raw_clone();
 		if (pid < 0)
 			_exit(EXIT_FAILURE);

 		if (pid == 0)
 			_exit(EXIT_SUCCESS);

 		if (wait(NULL) != pid)
 			_exit(EXIT_FAILURE);
 		fprintf(stderr, "ia64_raw_clone() passed\n");

 		pid = ia64_raw_clone2();
 		if (pid < 0)
 			_exit(EXIT_FAILURE);

 		if (pid == 0)
 			_exit(EXIT_SUCCESS);

 		if (wait(NULL) != pid)
 			_exit(EXIT_FAILURE);
 		fprintf(stderr, "ia64_raw_clone2() passed\n");

 		pid = libc_clone2(libc_clone2_child, NULL);
 		if (pid < 0)
 			_exit(EXIT_FAILURE);

 		if (wait(NULL) != pid)
 			_exit(EXIT_FAILURE);
 		fprintf(stderr, "libc_clone2() passed\n");
 	}

 	_exit(EXIT_SUCCESS);
 }

For some more context, please see:
commit 606e9ad2
Merge: ac61145a 457677c7
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat Jan 11 15:33:48 2020 -0800

    Merge tag 'clone3-tls-v5.5-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

    Pull thread fixes from Christian Brauner:
     "This contains a series of patches to fix CLONE_SETTLS when used with
      clone3().

      The clone3() syscall passes the tls argument through struct clone_args
      instead of a register. This means, all architectures that do not
      implement copy_thread_tls() but still support CLONE_SETTLS via
      copy_thread() expecting the tls to be located in a register argument
      based on clone() are currently unfortunately broken. Their tls value
      will be garbage.

      The patch series fixes this on all architectures that currently define
      __ARCH_WANT_SYS_CLONE3. It also adds a compile-time check to ensure
      that any architecture that enables clone3() in the future is forced to
      also implement copy_thread_tls().

      My ultimate goal is to get rid of the copy_thread()/copy_thread_tls()
      split and just have copy_thread_tls() at some point in the not too
      distant future (Maybe even renaming copy_thread_tls() back to simply
      copy_thread() once the old function is ripped from all arches). This
      is dependent now on all arches supporting clone3().

      While all relevant arches do that now there are still four missing:
      ia64, m68k, sh and sparc. They have the system call reserved, but not
      implemented. Once they all implement clone3() we can get rid of
      ARCH_WANT_SYS_CLONE3 and HAVE_COPY_THREAD_TLS.

Note that in the meantime, m68k has already switched to the new calling
convention. And I've got sparc patches acked by Dave, too.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Qais Yousef <qais.yousef@arm.com>
Cc: linux-ia64@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20200517151635.3085756-1-christian.brauner@ubuntu.com