[PATCH v3 2/4] riscv: clarify vector state semantics on syscall and context switch

[Andy Chiu]

From: daichengrong <daichengrong at iscas.ac.cn>

The RISC-V vector specification states that executing a system call
causes all caller-saved vector registers (v0-v31, vl, vtype) and vstart
to become unspecified.

Currently, after calling riscv_v_vstate_discard(), the vector state
may still be marked as DIRTY, which can mislead the context switch
logic into treating the registers as containing valid user data.

This patch clarifies and tightens the kernel-side semantics:

1. On syscall entry, the kernel checks the vector state via sstatus
   and explicitly set it to INIT, indicating that the vector registers
   no longer contain meaningful user data.

2. During context switch, the vector state is saved only if the state is
   DIRTY. (no change)

3. On restore, if the state is INIT, the vector registers are treated
   as invalid and are not restored from memory. Instead, they are
   overwritten with a known initial value to avoid data leaakge.

Signed-off-by: daichengrong <daichengrong at iscas.ac.cn>
Co-developed-by: Andy Chiu <tchiu at tenstorrent.com>
Signed-off-by: Andy Chiu <tchiu at tenstorrent.com>
---
Changelog v3:
 - rename vstate_on to vstate_init to prevent confusion
 - set context as clean at first-use trap to return zero'ed context
 - reduce context nulling operations by defering __vstate_discard to
   exit_to_user_mode_prepare.
---
 arch/riscv/include/asm/vector.h | 32 ++++++++++++++++----------------
 arch/riscv/kernel/vector.c      |  2 +-
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index 45820cd900d6..769f30779867 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -95,7 +95,7 @@ static inline void riscv_v_vstate_off(struct pt_regs *regs)
 	regs->status = __riscv_v_vstate_or(regs->status, OFF);
 }
 
-static inline void riscv_v_vstate_on(struct pt_regs *regs)
+static inline void riscv_v_vstate_init(struct pt_regs *regs)
 {
 	regs->status = __riscv_v_vstate_or(regs->status, INITIAL);
 }
@@ -288,16 +288,6 @@ static inline void __riscv_v_vstate_discard(void)
 		: "=&r" (vl) : "r" (vtype_inval));
 }
 
-static inline void riscv_v_vstate_discard(struct pt_regs *regs)
-{
-	if (riscv_v_vstate_query(regs)) {
-		riscv_v_enable();
-		__riscv_v_vstate_discard();
-		riscv_v_disable();
-		__riscv_v_vstate_dirty(regs);
-	}
-}
-
 static inline void riscv_v_vstate_save(struct __riscv_v_ext_state *vstate,
 				       struct pt_regs *regs)
 {
@@ -312,20 +302,29 @@ static inline void riscv_v_vstate_save(struct __riscv_v_ext_state *vstate,
 static inline void riscv_v_vstate_restore(struct __riscv_v_ext_state *vstate,
 					  struct pt_regs *regs)
 {
-	if (riscv_v_vstate_query(regs)) {
+	if (__riscv_v_vstate_check(regs->status, INITIAL)) {
+		riscv_v_enable();
+		__riscv_v_vstate_discard();
+		riscv_v_disable();
+	} else if (__riscv_v_vstate_check(regs->status, CLEAN)) {
 		riscv_v_enable();
 		__riscv_v_vstate_restore(vstate, vstate->datap);
 		riscv_v_disable();
-		__riscv_v_vstate_clean(regs);
 	}
 }
 
 static inline void riscv_v_vstate_set_restore(struct task_struct *task,
 					      struct pt_regs *regs)
 {
-	if (riscv_v_vstate_query(regs)) {
+	if (riscv_v_vstate_query(regs))
 		set_tsk_thread_flag(task, TIF_RISCV_V_DEFER_RESTORE);
-		riscv_v_vstate_on(regs);
+}
+
+static inline void riscv_v_vstate_discard(struct pt_regs *regs)
+{
+	if (riscv_v_vstate_query(regs)) {
+		riscv_v_vstate_set_restore(current, regs);
+		riscv_v_vstate_init(regs);
 	}
 }
 
@@ -396,6 +395,7 @@ static inline void __switch_to_vector(struct task_struct *prev,
 			riscv_preempt_v_set_restore(next);
 		}
 	} else {
+		/* VS is never DIRTY at this point, there's no need to alter vstate here */
 		riscv_v_vstate_set_restore(next, task_pt_regs(next));
 	}
 }
@@ -421,7 +421,7 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
 #define riscv_v_vstate_restore(vstate, regs)	do {} while (0)
 #define __switch_to_vector(__prev, __next)	do {} while (0)
 #define riscv_v_vstate_off(regs)		do {} while (0)
-#define riscv_v_vstate_on(regs)			do {} while (0)
+#define riscv_v_vstate_init(regs)		do {} while (0)
 #define riscv_v_thread_free(tsk)		do {} while (0)
 #define  riscv_v_setup_ctx_cache()		do {} while (0)
 #define riscv_v_thread_alloc(tsk)		do {} while (0)
diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index b112166d51e9..4eef51f6d432 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -221,7 +221,7 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
 		return true;
 	}
 
-	riscv_v_vstate_on(regs);
+	__riscv_v_vstate_clean(regs);
 	riscv_v_vstate_set_restore(current, regs);
 
 	return true;
-- 
2.43.0