• Christian Brauner's avatar
    nios2: enable HAVE_COPY_THREAD_TLS, switch to kernel_clone_args · 04bd52fb
    Christian Brauner authored
    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 nios2 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 nios2 is one of the few hat haven't yet. This also
    unblocks implementing the clone3() syscall on nios2. 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, nios2 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.)
    
    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 and ia64 is already
    done too. You can find a link to a booting qemu nios2 system with all the
    changes here at [1].
    
    [1]: https://asciinema.org/a/333353
    Cc: linux-kernel@vger.kernel.org
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Acked-by: default avatarLey Foon Tan <ley.foon.tan@intel.com>
    Signed-off-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
    04bd52fb
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