[v3, 5/5] riscv: vector: allow kernel-mode Vector with preemption

[Andy Chiu <andy.chiu@sifive.com>]

Add kernel_vstate to keep track of kernel-mode Vector registers when
trap introduced context switch happens. Also, provide trap_pt_regs to
let context save/restore routine reference status.VS at which the trap
takes place. The thread flag TIF_RISCV_V_KERNEL_MODE indicates whether
a task is running in kernel-mode Vector with preemption 'ON'. So context
switch routines know and would save V-regs to kernel_vstate and restore
V-regs immediately from kernel_vstate if the bit is set.

Apart from a task's preemption status, the capability of
running preemptive kernel-mode Vector is as well controlled by the
RISCV_V_VSTATE_CTRL_PREEMPTIBLE mask in the task's
thread.vstate_ctrl. This bit is masked whenever a trap takes place in
kernel mode while executing preemptive Vector code.

Also, provide a config CONFIG_RISCV_ISA_V_PREEMPTIVE to give users an
option to disable preemptible kernel-mode Vector at build time. Users
with constraint memory may want to disable this config as preemptible
kernel-mode Vector needs extra space for tracking per thread's
kernel-mode V context. Or, users might as well want to disable it if all
kernel-mode Vector code is time sensitive and cannot tolerate context
switch overhead.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
---
Changelog v3:
 - Guard vstate_save with {get,set}_cpu_vector_context
 - Add comments on preventions of nesting V contexts
 - remove warnings in context switch when trap's reg is not pressent (Conor)
 - refactor code (Björn)
Changelog v2:
 - fix build fail when compiling without RISCV_ISA_V (Conor)
 - 's/TIF_RISCV_V_KMV/TIF_RISCV_V_KERNEL_MODE' and add comment (Conor)
 - merge Kconfig patch into this oine (Conor).
 - 's/CONFIG_RISCV_ISA_V_PREEMPTIVE_KMV/CONFIG_RISCV_ISA_V_PREEMPTIVE/'
   (Conor)
 - fix some typos (Conor)
 - enclose assembly with RISCV_ISA_V_PREEMPTIVE.
 - change riscv_v_vstate_ctrl_config_kmv() to
   kernel_vector_allow_preemption() for better understanding. (Conor)
 - 's/riscv_v_kmv_preempitble/kernel_vector_preemptible/'
---
 arch/riscv/Kconfig                     | 10 +++++
 arch/riscv/include/asm/processor.h     |  2 +
 arch/riscv/include/asm/simd.h          |  9 +++-
 arch/riscv/include/asm/thread_info.h   |  4 ++
 arch/riscv/include/asm/vector.h        | 25 +++++++++--
 arch/riscv/kernel/asm-offsets.c        |  2 +
 arch/riscv/kernel/entry.S              | 49 ++++++++++++++++++++++
 arch/riscv/kernel/kernel_mode_vector.c | 57 ++++++++++++++++++++++++--
 arch/riscv/kernel/process.c            |  8 +++-
 arch/riscv/kernel/vector.c             |  3 +-
 10 files changed, 159 insertions(+), 10 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d607ab0f7c6d..dc51164b8fd4 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -520,6 +520,16 @@ config RISCV_ISA_V_DEFAULT_ENABLE
 
 	  If you don't know what to do here, say Y.
 
+config RISCV_ISA_V_PREEMPTIVE
+	bool "Run kernel-mode Vector with kernel preemption"
+	depends on PREEMPTION
+	depends on RISCV_ISA_V
+	default y
+	help
+	  Ordinarily the kernel disables preemption before running in-kernel
+	  Vector code. This config frees the kernel from disabling preemption
+	  by adding memory on demand for tracking kernel's V-context.
+
 config TOOLCHAIN_HAS_ZBB
 	bool
 	default y
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 3e23e1786d05..f9b85e37e624 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -82,6 +82,8 @@ struct thread_struct {
 	unsigned long bad_cause;
 	unsigned long vstate_ctrl;
 	struct __riscv_v_ext_state vstate;
+	struct pt_regs *trap_pt_regs;
+	struct __riscv_v_ext_state kernel_vstate;
 };
 
 /* Whitelist the fstate from the task_struct for hardened usercopy */
diff --git a/arch/riscv/include/asm/simd.h b/arch/riscv/include/asm/simd.h
index 0c5ba555b460..93d9015b4751 100644
--- a/arch/riscv/include/asm/simd.h
+++ b/arch/riscv/include/asm/simd.h
@@ -12,6 +12,7 @@
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/types.h>
+#include <linux/thread_info.h>
 
 #ifdef CONFIG_RISCV_ISA_V
 
@@ -33,8 +34,14 @@ static __must_check inline bool may_use_simd(void)
 	 * cannot change under our feet -- if it's set we cannot be
 	 * migrated, and if it's clear we cannot be migrated to a CPU
 	 * where it is set.
+	 *
+	 * The TIF_RISCV_V_KERNEL_MODE check here prevent us from nesting a
+	 * non-preemptible V context on top of a preemptible one. For example,
+	 * executing V in a softirq context is prevented if the core is
+	 * interrupted during the execution of preemptible V.
 	 */
-	return !in_hardirq() && !in_nmi() && !this_cpu_read(vector_context_busy);
+	return !in_hardirq() && !in_nmi() && !this_cpu_read(vector_context_busy) &&
+	       !test_thread_flag(TIF_RISCV_V_KERNEL_MODE);
 }
 
 #else /* ! CONFIG_RISCV_ISA_V */
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index b182f2d03e25..8797d520e8ef 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -94,6 +94,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 #define TIF_UPROBE		10	/* uprobe breakpoint or singlestep */
 #define TIF_32BIT		11	/* compat-mode 32bit process */
 #define TIF_RISCV_V_DEFER_RESTORE	12 /* restore Vector before returing to user */
+#define TIF_RISCV_V_KERNEL_MODE			13 /* kernel-mode Vector run with preemption-on */
 
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
@@ -101,9 +102,12 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 #define _TIF_NOTIFY_SIGNAL	(1 << TIF_NOTIFY_SIGNAL)
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
 #define _TIF_RISCV_V_DEFER_RESTORE	(1 << TIF_RISCV_V_DEFER_RESTORE)
+#define _TIF_RISCV_V_KERNEL_MODE	(1 << TIF_RISCV_V_KERNEL_MODE)
 
 #define _TIF_WORK_MASK \
 	(_TIF_NOTIFY_RESUME | _TIF_SIGPENDING | _TIF_NEED_RESCHED | \
 	 _TIF_NOTIFY_SIGNAL | _TIF_UPROBE)
 
+#define RISCV_V_VSTATE_CTRL_PREEMPTIBLE	0x20
+
 #endif /* _ASM_RISCV_THREAD_INFO_H */
diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index d356eac8c0b4..27bb49e97af8 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -198,9 +198,22 @@ static inline void __switch_to_vector(struct task_struct *prev,
 {
 	struct pt_regs *regs;
 
-	regs = task_pt_regs(prev);
-	riscv_v_vstate_save(&prev->thread.vstate, regs);
-	riscv_v_vstate_set_restore(next, task_pt_regs(next));
+	if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE) &&
+	    test_tsk_thread_flag(prev, TIF_RISCV_V_KERNEL_MODE)) {
+		regs = prev->thread.trap_pt_regs;
+		riscv_v_vstate_save(&prev->thread.kernel_vstate, regs);
+	} else {
+		regs = task_pt_regs(prev);
+		riscv_v_vstate_save(&prev->thread.vstate, regs);
+	}
+
+	if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE) &&
+	    test_tsk_thread_flag(next, TIF_RISCV_V_KERNEL_MODE)) {
+		regs = next->thread.trap_pt_regs;
+		riscv_v_vstate_restore(&next->thread.kernel_vstate, regs);
+	} else {
+		riscv_v_vstate_set_restore(next, task_pt_regs(next));
+	}
 }
 
 void riscv_v_vstate_ctrl_init(struct task_struct *tsk);
@@ -225,4 +238,10 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
 
 #endif /* CONFIG_RISCV_ISA_V */
 
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+void kernel_vector_allow_preemption(void);
+#else
+#define kernel_vector_allow_preemption()	do {} while (0)
+#endif
+
 #endif /* ! __ASM_RISCV_VECTOR_H */
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index d6a75aac1d27..4b062f7741b2 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -38,6 +38,8 @@ void asm_offsets(void)
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
 	OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+	OFFSET(TASK_THREAD_TRAP_REGP, task_struct, thread.trap_pt_regs);
+	OFFSET(TASK_THREAD_VSTATE_CTRL, task_struct, thread.vstate_ctrl);
 
 	OFFSET(TASK_THREAD_F0,  task_struct, thread.fstate.f[0]);
 	OFFSET(TASK_THREAD_F1,  task_struct, thread.fstate.f[1]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 143a2bb3e697..ec8baada608f 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -66,6 +66,33 @@ _save_context:
 	REG_S s4, PT_CAUSE(sp)
 	REG_S s5, PT_TP(sp)
 
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+	/*
+	 * Record the register set at the frame where in-kernel V registers are
+	 * last alive.
+	 */
+	REG_L s0, TASK_TI_FLAGS(tp)
+	li s1, 1 << TIF_RISCV_V_KERNEL_MODE
+	and s0, s0, s1
+	beqz s0, 1f
+	li s0, TASK_THREAD_TRAP_REGP
+	add s0, s0, tp
+	REG_L s1, (s0)
+	bnez s1, 1f
+	REG_S sp, (s0)
+	li s0, TASK_THREAD_VSTATE_CTRL
+	add s0, s0, tp
+	REG_L s1, (s0)
+	/*
+	 * Nesting preemptible Vector context is prevented by unsetting
+	 * RISCV_V_VSTATE_CTRL_PREEMPTIBLE here.
+	 */
+	li s2, ~RISCV_V_VSTATE_CTRL_PREEMPTIBLE
+	and s1, s1, s2
+	REG_S s1, (s0)
+1:
+#endif
+
 	/*
 	 * Set the scratch register to 0, so that if a recursive exception
 	 * occurs, the exception vector knows it came from the kernel
@@ -129,6 +156,28 @@ SYM_CODE_START_NOALIGN(ret_from_exception)
 	 */
 	csrw CSR_SCRATCH, tp
 1:
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+	/*
+	 * Clear tracking of the trap registers when we return to the frame
+	 * that uses kernel mode Vector.
+	 */
+	REG_L s0, TASK_TI_FLAGS(tp)
+	li s1, 1 << TIF_RISCV_V_KERNEL_MODE
+	and s0, s0, s1
+	beqz s0, 1f
+	li s0, TASK_THREAD_TRAP_REGP
+	add s0, s0, tp
+	REG_L s1, (s0)
+	bne s1, sp, 1f
+	REG_S x0, (s0)
+	li s0, TASK_THREAD_VSTATE_CTRL
+	add s0, s0, tp
+	REG_L s1, (s0)
+	ori s1, s1, RISCV_V_VSTATE_CTRL_PREEMPTIBLE
+	REG_S s1, (s0)
+1:
+#endif
+
 	REG_L a0, PT_STATUS(sp)
 	/*
 	 * The current load reservation is effectively part of the processor's
diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
index 2344817f8640..6203990476b3 100644
--- a/arch/riscv/kernel/kernel_mode_vector.c
+++ b/arch/riscv/kernel/kernel_mode_vector.c
@@ -10,6 +10,7 @@
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/types.h>
+#include <linux/slab.h>
 
 #include <asm/vector.h>
 #include <asm/switch_to.h>
@@ -48,6 +49,50 @@ void put_cpu_vector_context(void)
 	preempt_enable();
 }
 
+#ifdef CONFIG_RISCV_ISA_V_PREEMPTIVE
+void kernel_vector_allow_preemption(void)
+{
+	current->thread.vstate_ctrl |= RISCV_V_VSTATE_CTRL_PREEMPTIBLE;
+}
+
+static bool kernel_vector_preemptible(void)
+{
+	return !!(current->thread.vstate_ctrl & RISCV_V_VSTATE_CTRL_PREEMPTIBLE);
+}
+
+static int riscv_v_start_kernel_context(void)
+{
+	struct __riscv_v_ext_state *vstate;
+
+	if (!kernel_vector_preemptible())
+		return -EBUSY;
+
+	vstate = &current->thread.kernel_vstate;
+	if (!vstate->datap) {
+		vstate->datap = kmalloc(riscv_v_vsize, GFP_KERNEL);
+		if (!vstate->datap)
+			return -ENOMEM;
+	}
+
+	get_cpu_vector_context();
+	riscv_v_vstate_save(&current->thread.vstate, task_pt_regs(current));
+	put_cpu_vector_context();
+
+	current->thread.trap_pt_regs = NULL;
+	WARN_ON(test_and_set_thread_flag(TIF_RISCV_V_KERNEL_MODE));
+	return 0;
+}
+
+static void riscv_v_stop_kernel_context(void)
+{
+	WARN_ON(!test_and_clear_thread_flag(TIF_RISCV_V_KERNEL_MODE));
+	current->thread.trap_pt_regs = NULL;
+}
+#else
+#define riscv_v_start_kernel_context()	(0)
+#define riscv_v_stop_kernel_context()	do {} while (0)
+#endif /* CONFIG_RISCV_ISA_V_PREEMPTIVE */
+
 /*
  * kernel_vector_begin(): obtain the CPU vector registers for use by the calling
  * context
@@ -68,9 +113,10 @@ void kernel_vector_begin(void)
 
 	BUG_ON(!may_use_simd());
 
-	get_cpu_vector_context();
-
-	riscv_v_vstate_save(&current->thread.vstate, task_pt_regs(current));
+	if (!preemptible() || riscv_v_start_kernel_context()) {
+		get_cpu_vector_context();
+		riscv_v_vstate_save(&current->thread.vstate, task_pt_regs(current));
+	}
 
 	riscv_v_enable();
 }
@@ -94,6 +140,9 @@ void kernel_vector_end(void)
 
 	riscv_v_disable();
 
-	put_cpu_vector_context();
+	if (!test_thread_flag(TIF_RISCV_V_KERNEL_MODE))
+		put_cpu_vector_context();
+	else
+		riscv_v_stop_kernel_context();
 }
 EXPORT_SYMBOL_GPL(kernel_vector_end);
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index ec89e7edb6fd..18cb37c305ab 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -160,8 +160,11 @@ void flush_thread(void)
 void arch_release_task_struct(struct task_struct *tsk)
 {
 	/* Free the vector context of datap. */
-	if (has_vector())
+	if (has_vector()) {
 		kfree(tsk->thread.vstate.datap);
+		if (IS_ENABLED(CONFIG_RISCV_ISA_V_PREEMPTIVE))
+			kfree(tsk->thread.kernel_vstate.datap);
+	}
 }
 
 int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
@@ -170,7 +173,9 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	*dst = *src;
 	/* clear entire V context, including datap for a new task */
 	memset(&dst->thread.vstate, 0, sizeof(struct __riscv_v_ext_state));
+	memset(&dst->thread.kernel_vstate, 0, sizeof(struct __riscv_v_ext_state));
 	clear_tsk_thread_flag(dst, TIF_RISCV_V_DEFER_RESTORE);
+	clear_tsk_thread_flag(dst, TIF_RISCV_V_KERNEL_MODE);
 
 	return 0;
 }
@@ -205,6 +210,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		childregs->a0 = 0; /* Return value of fork() */
 		p->thread.s[0] = 0;
 	}
+	kernel_vector_allow_preemption();
 	p->thread.ra = (unsigned long)ret_from_fork;
 	p->thread.sp = (unsigned long)childregs; /* kernel sp */
 	return 0;
diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
index 9d583b760db4..42f227077ee5 100644
--- a/arch/riscv/kernel/vector.c
+++ b/arch/riscv/kernel/vector.c
@@ -122,7 +122,8 @@ static inline void riscv_v_ctrl_set(struct task_struct *tsk, int cur, int nxt,
 	ctrl |= VSTATE_CTRL_MAKE_NEXT(nxt);
 	if (inherit)
 		ctrl |= PR_RISCV_V_VSTATE_CTRL_INHERIT;
-	tsk->thread.vstate_ctrl = ctrl;
+	tsk->thread.vstate_ctrl &= ~PR_RISCV_V_VSTATE_CTRL_MASK;
+	tsk->thread.vstate_ctrl |= ctrl;
 }
 
 bool riscv_v_vstate_ctrl_user_allowed(void)
-- 
2.17.1


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