[Patch v7 09/24] x86/fpu: Ensure TIF_NEED_FPU_LOAD is set after saving FPU state

Dapeng Mi
2026-03-24T00:46:34Z

Following Peter and Dave's suggestion, Ensure that the TIF_NEED_FPU_LOAD
flag is always set after saving the FPU state. This guarantees that the
user space FPU state has been saved whenever the TIF_NEED_FPU_LOAD flag
is set.

A subsequent patch will verify if the user space FPU state can be
retrieved from the saved task FPU state in the NMI context by checking
the TIF_NEED_FPU_LOAD flag.

Please check the below link to get more background about the suggestion.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20251204154721.GB2619703@noisy.programming.kicks-ass.net/
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
---

V7: Add wrapper helper update_fpu_state_and_flag() and corresponding
comments.

arch/x86/include/asm/fpu/sched.h | 5 +++--
arch/x86/kernel/fpu/core.c | 27 ++++++++++++++++++++-------
2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/fpu/sched.h b/arch/x86/include/asm/fpu/sched.h
index 89004f4ca208..dcb2fa5f06d6 100644
--- a/arch/x86/include/asm/fpu/sched.h
+++ b/arch/x86/include/asm/fpu/sched.h
@@ -10,6 +10,8 @@
#include <asm/trace/fpu.h>

extern void save_fpregs_to_fpstate(struct fpu *fpu);
+extern void update_fpu_state_and_flag(struct fpu *fpu,
+ struct task_struct *task);
extern void fpu__drop(struct task_struct *tsk);
extern int fpu_clone(struct task_struct *dst, u64 clone_flags, bool minimal,
unsigned long shstk_addr);
@@ -36,8 +38,7 @@ static inline void switch_fpu(struct task_struct *old, int cpu)
!(old->flags & (PF_KTHREAD | PF_USER_WORKER))) {
struct fpu *old_fpu = x86_task_fpu(old);

- set_tsk_thread_flag(old, TIF_NEED_FPU_LOAD);
- save_fpregs_to_fpstate(old_fpu);
+ update_fpu_state_and_flag(old_fpu, old);
/*
* The save operation preserved register state, so the
* fpu_fpregs_owner_ctx is still @old_fpu. Store the
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 608983806fd7..48d1ab50a961 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -213,6 +213,19 @@ void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
}
}

+/*
+ * Save the FPU register state in fpu->fpstate->regs and set
+ * TIF_NEED_FPU_LOAD subsequently.
+ *
+ * Must be called with fpregs_lock() held, ensuring flag
+ * TIF_NEED_FPU_LOAD is set last.
+ */
+void update_fpu_state_and_flag(struct fpu *fpu, struct task_struct *task)
+{
+ save_fpregs_to_fpstate(fpu);
+ set_tsk_thread_flag(task, TIF_NEED_FPU_LOAD);
+}
+
void fpu_reset_from_exception_fixup(void)
{
restore_fpregs_from_fpstate(&init_fpstate, XFEATURE_MASK_FPSTATE);
@@ -379,17 +392,19 @@ int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)

fpregs_lock();
if (!cur_fps->is_confidential && !test_thread_flag(TIF_NEED_FPU_LOAD))
- save_fpregs_to_fpstate(fpu);
+ update_fpu_state_and_flag(fpu, current);

/* Swap fpstate */
if (enter_guest) {
- fpu->__task_fpstate = cur_fps;
+ WRITE_ONCE(fpu->__task_fpstate, cur_fps);
+ barrier();
fpu->fpstate = guest_fps;
guest_fps->in_use = true;
} else {
guest_fps->in_use = false;
fpu->fpstate = fpu->__task_fpstate;
- fpu->__task_fpstate = NULL;
+ barrier();
+ WRITE_ONCE(fpu->__task_fpstate, NULL);
}

cur_fps = fpu->fpstate;
@@ -481,10 +496,8 @@ void kernel_fpu_begin_mask(unsigned int kfpu_mask)
this_cpu_write(kernel_fpu_allowed, false);

if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
- !test_thread_flag(TIF_NEED_FPU_LOAD)) {
- set_thread_flag(TIF_NEED_FPU_LOAD);
- save_fpregs_to_fpstate(x86_task_fpu(current));
- }
+ !test_thread_flag(TIF_NEED_FPU_LOAD))
+ update_fpu_state_and_flag(x86_task_fpu(current), current);
__cpu_invalidate_fpregs_state();

/* Put sane initial values into the control registers. */
--
2.34.1