riscv: vector: Check SR_SD before saving vstate

The SD bit summarizes the dirty states of FS, VS, or XS fields,
providing a "fast check" before saving fstate or vstate.

Let __switch_to_vector() check SD bit as __switch_to_fpu() does.

Fixes: 3a2df63 ("riscv: Add task switch support for vector")
Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>

arch/riscv/include/asm/vector.h

@@ -190,7 +190,8 @@ static inline void __switch_to_vector(struct task_struct *prev,
 	struct pt_regs *regs;
 
 	regs = task_pt_regs(prev);
-	riscv_v_vstate_save(prev, regs);
+	if (unlikely(regs->status & SR_SD))
+		riscv_v_vstate_save(prev, regs);
 	riscv_v_vstate_restore(next, task_pt_regs(next));
 }