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Subject[tip:x86/urgent] x86: Allow FPU to be used at interrupt time even with eagerfpu
Commit-ID:  5187b28ff08249ab8a162e802209ed04e271ca02
Gitweb: http://git.kernel.org/tip/5187b28ff08249ab8a162e802209ed04e271ca02
Author: Pekka Riikonen <priikone@iki.fi>
AuthorDate: Mon, 13 May 2013 14:32:07 +0200
Committer: H. Peter Anvin <hpa@linux.intel.com>
CommitDate: Thu, 30 May 2013 16:36:42 -0700

x86: Allow FPU to be used at interrupt time even 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().

[ hpa: this is a performance regression, not a correctness regression,
but since it can be quite serious on CPUs which need encryption at
interrupt time I am marking this for urgent/stable. ]

Signed-off-by: Pekka Riikonen <priikone@iki.fi>
Link: http://lkml.kernel.org/r/alpine.GSO.2.00.1305131356320.18@git.silcnet.org
Cc: <stable@vger.kernel.org> v3.7+
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
---
arch/x86/kernel/i387.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

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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