[tip:x86/fpu] x86/fpu: Always allow FPU in interrupt if use_eager_fpu()

[tip-bot for Oleg Nesterov <tipbot@zytor.com>]

Commit-ID:  4b2e762e2e53c721458a83d547b222178bb72a34
Gitweb:     http://git.kernel.org/tip/4b2e762e2e53c721458a83d547b222178bb72a34
Author:     Oleg Nesterov <oleg@redhat.com>
AuthorDate: Mon, 19 Jan 2015 19:51:51 +0100
Committer:  Borislav Petkov <bp@suse.de>
CommitDate: Mon, 23 Feb 2015 15:50:41 +0100

x86/fpu: Always allow FPU in interrupt if use_eager_fpu()

The __thread_has_fpu() check in interrupted_kernel_fpu_idle() was needed
to prevent the nested kernel_fpu_begin(). Now that we have in_kernel_fpu
and !__thread_has_fpu() case in __kernel_fpu_begin() does not depend on
use_eager_fpu() (except clts) we can remove it.

__thread_has_fpu() can be false even if use_eager_fpu(), but this case
does not differ from !use_eager_fpu() case except we should not worry
about X86_CR0_TS, __kernel_fpu_begin()/end() will not touch this bit.

Note: I think we can kill all irq_fpu_usable() checks except in_kernel_fpu,
just we need to record the state of X86_CR0_TS in __kernel_fpu_begin() and
conditionalize stts() in __kernel_fpu_end(), but this needs another patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150119185151.GC16427@redhat.com
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kernel/i387.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index ad3a2a2..8416b5f 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -41,8 +41,8 @@ void kernel_fpu_enable(void)
  * 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().
+ * Except for the eagerfpu case when we return true; in the likely case
+ * the thread has FPU but we are not going to set/clear TS.
  */
 static inline bool interrupted_kernel_fpu_idle(void)
 {
@@ -50,7 +50,7 @@ static inline bool interrupted_kernel_fpu_idle(void)
 		return false;
 
 	if (use_eager_fpu())
-		return __thread_has_fpu(current);
+		return true;
 
 	return !__thread_has_fpu(current) &&
 		(read_cr0() & X86_CR0_TS);