Re: [PATCHES] alpha asm glue cleanups and fixes

[Al Viro <viro@zeniv.linux.org.uk>]

Resurrecting old series:

 	Fallout from asm glue review on alpha:

1) TIF_NOTIFY_SIGNAL support is broken; do_work_pending() handles
it, but the logics *calling* do_work_pending() ignores that flag
completely.  If it's called for other reasons - fine, but
TIF_NOTIFY_SIGNAL alone will not suffice for that.  Bug from the
last cycle.  5.11 bug.

2) _TIF_ALLWORK_MASK is junk - never had been used.
 
3) !AUDIT_SYSCALL configs have buggered logics for going into
straced syscall path.  Any thread flag (including TIF_SIGNAL_PENDING)
will suffice to send us there.  3.14 bug.
 
4) on straced syscalls we have force_successful_syscall_return() broken -
it ends up with a3 *not* set to 0.

5) on non-straced syscalls force_successful_syscall_return() handling is
suboptimal - it duplicates code from the normal syscall return path for
no good reason; instead of branching to the copy, it might branch to the
original just fine.

6) ret_from_fork could just as well go to ret_from_user - it's not going
to be hit when returning to kernel mode.

7) lazy FPU switching.  We save/restore all FPU registers a lot more than
we have to; the following reduces the amount quite a bit:
	* move the array we save into from switch_stack to thread_info
	* have a (thread-synchronous) flag set when we save them
	* have another flag set when they should be restored on return to
userland.
	* do *NOT* save/restore them in do_switch_stack()/undo_switch_stack().
	* restore on the exit to user mode if the restore flag had been set.
Clear both flags.
	* on context switch, entry to fork/clone/vfork, before entry into
do_signal() and on entry into straced syscall save the registers and set
the 'saved' flag unless it had been already set.
	* on context switch set the 'restore' flag as well.
	* have copy_thread() set both flags for child, so the registers would
be restored once the child returns to userland.
	* use the saved data in setup_sigcontext(); have restore_sigcontext()
set both flags and copy from sigframe to save area.
	* teach ptrace to look for FPU registers in thread_info instead of
switch_stack.
	* teach isolated accesses to FPU registers (rdfpcr, wrfpcr, etc.)
to check the 'saved' flag (under preempt_disable()) and work with the save area
if it's been set; if 'saved' flag is found upon write access, set 'restore' flag
as well.  NOTE: it's tempting to just force register saving in those - it
would simplify the code quite a bit.  Unfortunately, it would also force the
full FPU save/restore in situations where we really don't want the overhead
of that ;-/

Tested on qemu and on real hw (older one - the only EV67 box I have is not
in good condition).  Seems to work; benefits depend upon the load.

Patchset lives in git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git
#next.alpha; individual patches in followups.