riscv: vector: Support calling schedule() for preemptible Vector

Each function entry implies a call to ftrace infrastructure. And it may
call into schedule in some cases. So, it is possible for preemptible
kernel-mode Vector to implicitly call into schedule. Since all V-regs
are caller-saved, it is possible to drop all V context when a thread
voluntarily call schedule(). Besides, we currently don't pass argument
through vector register, so we don't have to save/restore V-regs in
ftrace trampoline.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20250407180838.42877-7-andybnac@gmail.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 5f56eb9..9c1cc71 100644 (file)
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -79,6 +79,10 @@ struct pt_regs;
  *       Thus, the task does not own preempt_v. Any use of Vector will have to
  *       save preempt_v, if dirty, and fallback to non-preemptible kernel-mode
  *       Vector.
+ *  - bit 29: The thread voluntarily calls schedule() while holding an active
+ *    preempt_v. All preempt_v context should be dropped in such case because
+ *    V-regs are caller-saved. Only sstatus.VS=ON is persisted across a
+ *    schedule() call.
  *  - bit 30: The in-kernel preempt_v context is saved, and requries to be
  *    restored when returning to the context that owns the preempt_v.
  *  - bit 31: The in-kernel preempt_v context is dirty, as signaled by the
@@ -93,6 +97,7 @@ struct pt_regs;
 #define RISCV_PREEMPT_V                        0x00000100
 #define RISCV_PREEMPT_V_DIRTY          0x80000000
 #define RISCV_PREEMPT_V_NEED_RESTORE   0x40000000
+#define RISCV_PREEMPT_V_IN_SCHEDULE    0x20000000
 
 /* CPU-specific state of a task */
 struct thread_struct {

diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index e8a83f5..45c9b42 100644 (file)
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -120,6 +120,11 @@ static __always_inline void riscv_v_disable(void)
                csr_clear(CSR_SSTATUS, SR_VS);
 }
 
+static __always_inline bool riscv_v_is_on(void)
+{
+       return !!(csr_read(CSR_SSTATUS) & SR_VS);
+}
+
 static __always_inline void __vstate_csr_save(struct __riscv_v_ext_state *dest)
 {
        asm volatile (
@@ -366,6 +371,11 @@ static inline void __switch_to_vector(struct task_struct *prev,
        struct pt_regs *regs;
 
        if (riscv_preempt_v_started(prev)) {
+               if (riscv_v_is_on()) {
+                       WARN_ON(prev->thread.riscv_v_flags & RISCV_V_CTX_DEPTH_MASK);
+                       riscv_v_disable();
+                       prev->thread.riscv_v_flags |= RISCV_PREEMPT_V_IN_SCHEDULE;
+               }
                if (riscv_preempt_v_dirty(prev)) {
                        __riscv_v_vstate_save(&prev->thread.kernel_vstate,
                                              prev->thread.kernel_vstate.datap);
@@ -376,10 +386,16 @@ static inline void __switch_to_vector(struct task_struct *prev,
                riscv_v_vstate_save(&prev->thread.vstate, regs);
        }
 
-       if (riscv_preempt_v_started(next))
-               riscv_preempt_v_set_restore(next);
-       else
+       if (riscv_preempt_v_started(next)) {
+               if (next->thread.riscv_v_flags & RISCV_PREEMPT_V_IN_SCHEDULE) {
+                       next->thread.riscv_v_flags &= ~RISCV_PREEMPT_V_IN_SCHEDULE;
+                       riscv_v_enable();
+               } else {
+                       riscv_preempt_v_set_restore(next);
+               }
+       } else {
                riscv_v_vstate_set_restore(next, task_pt_regs(next));
+       }
 }
 
 void riscv_v_vstate_ctrl_init(struct task_struct *tsk);