linux-kernel - [PATCH v3 5/7] riscv: vector: Support calling schedule() for preemptible Vector

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Message-Id: <20241127172908.17149-6-andybnac@gmail.com>
Date: Thu, 28 Nov 2024 01:29:06 +0800
From: Andy Chiu <andybnac@...il.com>
To: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>
Cc: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	bjorn@...osinc.com,
	puranjay12@...il.com,
	alexghiti@...osinc.com,
	yongxuan.wang@...ive.com,
	greentime.hu@...ive.com,
	nick.hu@...ive.com,
	nylon.chen@...ive.com,
	tommy.wu@...ive.com,
	eric.lin@...ive.com,
	viccent.chen@...ive.com,
	zong.li@...ive.com,
	samuel.holland@...ive.com
Subject: [PATCH v3 5/7] riscv: vector: Support calling schedule() for preemptible Vector

From: Andy Chiu <andy.chiu@...ive.com>

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@...ive.com>
---
 arch/riscv/include/asm/processor.h |  5 +++++
 arch/riscv/include/asm/vector.h    | 22 +++++++++++++++++++---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 5f56eb9d114a..9c1cc716b891 100644
--- 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 c7c023afbacd..c5b6070db99f 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -76,6 +76,11 @@ 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 (
@@ -244,6 +249,11 @@ 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);
@@ -254,10 +264,16 @@ 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);
-- 
2.39.3 (Apple Git-145)