arm64/fpsimd: Add fpsimd_save_and_flush_current_state()

When the current task's FPSIMD/SVE/SME state may be live on *any* CPU in
the system, special care must be taken when manipulating that state, as
this manipulation can race with preemption and/or asynchronous usage of
FPSIMD/SVE/SME (e.g. kernel-mode NEON in softirq handlers).

Even when manipulation is is protected with get_cpu_fpsimd_context() and
get_cpu_fpsimd_context(), the logic necessary when the state is live on
the current CPU can be wildly different from the logic necessary when
the state is not live on the current CPU. A number of historical and
extant issues result from failing to handle these cases consistetntly
and/or correctly.

To make it easier to get such manipulation correct, add a new
fpsimd_save_and_flush_current_state() helper function, which ensures
that the current task's state has been saved to memory and any stale
state on any CPU has been "flushed" such that is not live on any CPU in
the system. This will allow code to safely manipulate the saved state
without risk of races.

Subsequent patches will use the new function.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-11-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/include/asm/fpsimd.h

@@ -96,6 +96,7 @@ struct cpu_fp_state {
 extern void fpsimd_bind_state_to_cpu(struct cpu_fp_state *fp_state);
 
 extern void fpsimd_flush_task_state(struct task_struct *target);
+extern void fpsimd_save_and_flush_current_state(void);
 extern void fpsimd_save_and_flush_cpu_state(void);
 
 static inline bool thread_sm_enabled(struct thread_struct *thread)

arch/arm64/kernel/fpsimd.c

@@ -1819,6 +1819,17 @@ void fpsimd_flush_task_state(struct task_struct *t)
 	barrier();
 }
 
+void fpsimd_save_and_flush_current_state(void)
+{
+	if (!system_supports_fpsimd())
+		return;
+
+	get_cpu_fpsimd_context();
+	fpsimd_save_user_state();
+	fpsimd_flush_task_state(current);
+	put_cpu_fpsimd_context();
+}
+
 /*
  * Save the FPSIMD state to memory and invalidate cpu view.
  * This function must be called with preemption disabled.