i387: do not preload FPU state at task switch time

Yes, taking the trap to re-load the FPU/MMX state is expensive, but so
is spending several days looking for a bug in the state save/restore
code.  And the preload code has some rather subtle interactions with
both paravirtualization support and segment state restore, so it's not
nearly as simple as it should be.

Also, now that we no longer necessarily depend on a single bit (ie
TS_USEDFPU) for keeping track of the state of the FPU, we migth be able
to do better.  If we are really switching between two processes that
keep touching the FP state, save/restore is inevitable, but in the case
of having one process that does most of the FPU usage, we may actually
be able to do much better than the preloading.

In particular, we may be able to keep track of which CPU the process ran
on last, and also per CPU keep track of which process' FP state that CPU
has.  For modern CPU's that don't destroy the FPU contents on save time,
that would allow us to do a lazy restore by just re-enabling the
existing FPU state - with no restore cost at all!

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 24 deletions

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index fc676e44c77..5afe824c66e 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -571,28 +571,6 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
 }
 
 /*
- * __math_state_restore assumes that cr0.TS is already clear and the
- * fpu state is all ready for use.  Used during context switch.
- */
-void __math_state_restore(void)
-{
-        struct thread_info *thread = current_thread_info();
-        struct task_struct *tsk = thread->task;
-
-        /*
-         * Paranoid restore. send a SIGSEGV if we fail to restore the state.
-         */
-        if (unlikely(restore_fpu_checking(tsk))) {
-                stts();
-                force_sig(SIGSEGV, tsk);
-                return;
-        }
-
-        __thread_set_has_fpu(thread);   /* clts in caller! */
-        tsk->fpu_counter++;
-}
-
-/*
  * 'math_state_restore()' saves the current math information in the
  * old math state array, and gets the new ones from the current task
  *
@@ -622,9 +600,18 @@ void math_state_restore(void)
                 local_irq_disable();
         }
 
-        clts();                         /* Allow maths ops (or we recurse) */
+        __thread_fpu_begin(thread);
 
-        __math_state_restore();
+        /*
+         * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+         */
+        if (unlikely(restore_fpu_checking(tsk))) {
+                __thread_fpu_end(thread);
+                force_sig(SIGSEGV, tsk);
+                return;
+        }
+
+        tsk->fpu_counter++;
 }
 EXPORT_SYMBOL_GPL(math_state_restore);