i387: re-introduce FPU state preloading at context switch time
After all the FPU state cleanups and finally finding the problem that
caused all our FPU save/restore problems, this re-introduces the
preloading of FPU state that was removed in commit b3b0870ef3ff ("i387:
do not preload FPU state at task switch time").
However, instead of simply reverting the removal, this reimplements
preloading with several fixes, most notably
- properly abstracted as a true FPU state switch, rather than as
open-coded save and restore with various hacks.
In particular, implementing it as a proper FPU state switch allows us
to optimize the CR0.TS flag accesses: there is no reason to set the
TS bit only to then almost immediately clear it again. CR0 accesses
are quite slow and expensive, don't flip the bit back and forth for
no good reason.
- Make sure that the same model works for both x86-32 and x86-64, so
that there are no gratuitous differences between the two due to the
way they save and restore segment state differently due to
architectural differences that really don't matter to the FPU state.
- Avoid exposing the "preload" state to the context switch routines,
and in particular allow the concept of lazy state restore: if nothing
else has used the FPU in the meantime, and the process is still on
the same CPU, we can avoid restoring state from memory entirely, just
re-expose the state that is still in the FPU unit.
That optimized lazy restore isn't actually implemented here, but the
infrastructure is set up for it. Of course, older CPU's that use
'fnsave' to save the state cannot take advantage of this, since the
state saving also trashes the state.
In other words, there is now an actual _design_ to the FPU state saving,
rather than just random historical baggage. Hopefully it's easier to
follow as a result.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index ad25e51..77da5b4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -571,40 +571,16 @@
}
/*
- * 'math_state_restore()' saves the current math information in the
- * old math state array, and gets the new ones from the current task
- *
- * Careful.. There are problems with IBM-designed IRQ13 behaviour.
- * Don't touch unless you *really* know how it works.
- *
- * Must be called with kernel preemption disabled (eg with local
- * local interrupts as in the case of do_device_not_available).
+ * This gets called with the process already owning the
+ * FPU state, and with CR0.TS cleared. It just needs to
+ * restore the FPU register state.
*/
-void math_state_restore(void)
+void __math_state_restore(struct task_struct *tsk)
{
- struct task_struct *tsk = current;
-
/* We need a safe address that is cheap to find and that is already
- in L1. We're just bringing in "tsk->thread.has_fpu", so use that */
+ in L1. We've just brought in "tsk->thread.has_fpu", so use that */
#define safe_address (tsk->thread.has_fpu)
- if (!tsk_used_math(tsk)) {
- local_irq_enable();
- /*
- * does a slab alloc which can sleep
- */
- if (init_fpu(tsk)) {
- /*
- * ran out of memory!
- */
- do_group_exit(SIGKILL);
- return;
- }
- local_irq_disable();
- }
-
- __thread_fpu_begin(tsk);
-
/* AMD K7/K8 CPUs don't save/restore FDP/FIP/FOP unless an exception
is pending. Clear the x87 state here by setting it to fixed
values. safe_address is a random variable that should be in L1 */
@@ -623,6 +599,39 @@
force_sig(SIGSEGV, tsk);
return;
}
+}
+
+/*
+ * 'math_state_restore()' saves the current math information in the
+ * old math state array, and gets the new ones from the current task
+ *
+ * Careful.. There are problems with IBM-designed IRQ13 behaviour.
+ * Don't touch unless you *really* know how it works.
+ *
+ * Must be called with kernel preemption disabled (eg with local
+ * local interrupts as in the case of do_device_not_available).
+ */
+void math_state_restore(void)
+{
+ struct task_struct *tsk = current;
+
+ if (!tsk_used_math(tsk)) {
+ local_irq_enable();
+ /*
+ * does a slab alloc which can sleep
+ */
+ if (init_fpu(tsk)) {
+ /*
+ * ran out of memory!
+ */
+ do_group_exit(SIGKILL);
+ return;
+ }
+ local_irq_disable();
+ }
+
+ __thread_fpu_begin(tsk);
+ __math_state_restore(tsk);
tsk->fpu_counter++;
}