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authorLinus Torvalds <torvalds@linux-foundation.org>2012-02-16 15:45:23 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-16 15:45:23 -0800
commitb3b0870ef3ffed72b92415423da864f440f57ad6 (patch)
treeb3e128019581669d44e6634d3b1bfb169c73598d /arch/x86/kernel/traps.c
parent6d59d7a9f5b723a7ac1925c136e93ec83c0c3043 (diff)
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>
Diffstat (limited to 'arch/x86/kernel/traps.c')
-rw-r--r--arch/x86/kernel/traps.c35
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);