• Al Viro's avatar
    alpha: lazy FPU switching · 05096666
    Al Viro authored
    	On each context switch we save the FPU registers on stack
    of old process and restore FPU registers from the stack of new one.
    That allows us to avoid doing that each time we enter/leave the
    kernel mode; however, that can get suboptimal in some cases.
    
    	For one thing, we don't need to bother saving anything
    for kernel threads.  For another, if between entering and leaving
    the kernel a thread gives CPU up more than once, it will do
    useless work, saving the same values every time, only to discard
    the saved copy as soon as it returns from switch_to().
    
    	Alternative solution:
    
    * 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.
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: default avatarMatt Turner <mattst88@gmail.com>
    05096666
fpu.h 2.13 KB