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Subject[PATCH 2/3] 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 <>
arch/x86/kernel/i387.c | 6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 26f0e80..4734865 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);

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