[patch 08/31] x86/fpu: Do not inherit FPU context for kernel and IO worker threads

Thomas Gleixner
2021-10-12T00:06:49Z

There is no reason why kernel and IO worker threads need a full clone of
the parent's FPU state. Both are kernel threads which are not supposed to
use FPU. So copying a large state or doing XSAVE() is pointless. Just clean
out the minimaly required state for those tasks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/fpu/core.c | 37 ++++++++++++++++++++++++++-----------
1 file changed, 26 insertions(+), 11 deletions(-)

--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -212,6 +212,15 @@ static inline void fpstate_init_xstate(s
xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT | xfeatures_mask_all;
}

+static inline unsigned int init_fpstate_copy_size(void)
+{
+ if (!use_xsave())
+ return fpu_kernel_xstate_size;
+
+ /* XSAVE(S) just needs the legacy and the xstate header part */
+ return sizeof(init_fpstate.xsave);
+}
+
static inline void fpstate_init_fxstate(struct fxregs_state *fx)
{
fx->cwd = 0x37f;
@@ -260,6 +269,23 @@ int fpu_clone(struct task_struct *dst)
return 0;

/*
+ * Enforce reload for user space tasks and prevent kernel threads
+ * from trying to save the FPU registers on context switch.
+ */
+ set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);
+
+ /*
+ * No FPU state inheritance for kernel threads and IO
+ * worker threads.
+ */
+ if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
+ /* Clear out the minimal state */
+ memcpy(&dst_fpu->state, &init_fpstate,
+ init_fpstate_copy_size());
+ return 0;
+ }
+
+ /*
* If the FPU registers are not owned by current just memcpy() the
* state. Otherwise save the FPU registers directly into the
* child's FPU context, without any memory-to-memory copying.
@@ -272,8 +298,6 @@ int fpu_clone(struct task_struct *dst)
save_fpregs_to_fpstate(dst_fpu);
fpregs_unlock();

- set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);
-
trace_x86_fpu_copy_src(src_fpu);
trace_x86_fpu_copy_dst(dst_fpu);

@@ -322,15 +346,6 @@ static inline void restore_fpregs_from_i
pkru_write_default();
}

-static inline unsigned int init_fpstate_copy_size(void)
-{
- if (!use_xsave())
- return fpu_kernel_xstate_size;
-
- /* XSAVE(S) just needs the legacy and the xstate header part */
- return sizeof(init_fpstate.xsave);
-}
-
/*
* Reset current->fpu memory state to the init values.
*/