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Subject[PATCH RESEND] x86: irq_fpu_usable returns false negatives with eagerfpu
With the addition of eagerfpu the irq_fpu_usable() now returns false 
negatives especially in the case of ksoftirqd and interrupted idle task,
two common cases for FPU use for example in networking/crypto. With
eagerfpu=off FPU use is possible in those contexts. This is because of
the eagerfpu check in interrupted_kernel_fpu_idle():

...
* For now, with eagerfpu we will return interrupted kernel FPU
* state as not-idle. TBD: Ideally we can change the return value
* to something like __thread_has_fpu(current). But we need to
* be careful of doing __thread_clear_has_fpu() before saving
* the FPU etc for supporting nested uses etc. For now, take
* the simple route!
...
if (use_eager_fpu())
return 0;

As eagerfpu is automatically "on" on those CPUs that also have the
features like AES-NI this patch changes the eagerfpu check to return 1 in
case the kernel_fpu_begin() has not been said yet. Once it has been the
__thread_has_fpu() will start returning 0.

Notice that with eagerfpu the __thread_has_fpu is always true initially.
FPU use is thus always possible no matter what task is under us, unless
the state has already been saved with kernel_fpu_begin().

Signed-off-by: Pekka Riikonen <priikone@iki.fi>
---
diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 245a71d..cb33909 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -22,23 +22,19 @@
/*
* Were we in an interrupt that interrupted kernel mode?
*
- * For now, with eagerfpu we will return interrupted kernel FPU
- * state as not-idle. TBD: Ideally we can change the return value
- * to something like __thread_has_fpu(current). But we need to
- * be careful of doing __thread_clear_has_fpu() before saving
- * the FPU etc for supporting nested uses etc. For now, take
- * the simple route!
- *
* On others, we can do a kernel_fpu_begin/end() pair *ONLY* if that
* pair does nothing at all: the thread must not have fpu (so
* that we don't try to save the FPU state), and TS must
* be set (so that the clts/stts pair does nothing that is
* visible in the interrupted kernel thread).
+ *
+ * Except for the eagerfpu case when we return 1 unless we've already
+ * been eager and saved the state in kernel_fpu_begin().
*/
static inline bool interrupted_kernel_fpu_idle(void)
{
if (use_eager_fpu())
- return 0;
+ return __thread_has_fpu(current);

return !__thread_has_fpu(current) &&
(read_cr0() & X86_CR0_TS);
@@ -78,8 +74,8 @@ void __kernel_fpu_begin(void)
struct task_struct *me = current;

if (__thread_has_fpu(me)) {
- __save_init_fpu(me);
__thread_clear_has_fpu(me);
+ __save_init_fpu(me);
/* We do 'stts()' in __kernel_fpu_end() */
} else if (!use_eager_fpu()) {
this_cpu_write(fpu_owner_task, NULL);

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