[PATCH RFC 01/12] riscv: Add support for kernel mode vector

[Heiko Stuebner]

From: Greentime Hu <greentime.hu at sifive.com>

Add kernel_rvv_begin() and kernel_rvv_end() function declarations
and corresponding definitions in kernel_mode_vector.c

These are needed to wrap uses of vector in kernel mode.

Co-developed-by: Vincent Chen <vincent.chen at sifive.com>
Signed-off-by: Vincent Chen <vincent.chen at sifive.com>
Signed-off-by: Greentime Hu <greentime.hu at sifive.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner at vrull.eu>
---
 arch/riscv/include/asm/vector.h        |  14 +++
 arch/riscv/kernel/Makefile             |   1 +
 arch/riscv/kernel/kernel_mode_vector.c | 132 +++++++++++++++++++++++++
 3 files changed, 147 insertions(+)
 create mode 100644 arch/riscv/kernel/kernel_mode_vector.c

diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index 7c77696d704a..f38266ec483a 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -134,6 +134,20 @@ static inline void vstate_restore(struct task_struct *task,
 	}
 }
 
+static inline void vector_flush_cpu_state(void)
+{
+	asm volatile (
+		"vsetvli	t0, x0, e8, m8, ta, ma\n\t"
+		"vmv.v.i	v0, 0\n\t"
+		"vmv.v.i	v8, 0\n\t"
+		"vmv.v.i	v16, 0\n\t"
+		"vmv.v.i	v24, 0\n\t"
+		: : : "t0");
+}
+
+void kernel_rvv_begin(void);
+void kernel_rvv_end(void);
+
 #else /* ! CONFIG_RISCV_ISA_V  */
 
 struct pt_regs;
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 48d345a5f326..304c500cc1f7 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -56,6 +56,7 @@ obj-$(CONFIG_MMU) += vdso.o vdso/
 obj-$(CONFIG_RISCV_M_MODE)	+= traps_misaligned.o
 obj-$(CONFIG_FPU)		+= fpu.o
 obj-$(CONFIG_RISCV_ISA_V)	+= vector.o
+obj-$(CONFIG_RISCV_ISA_V)	+= kernel_mode_vector.o
 obj-$(CONFIG_SMP)		+= smpboot.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_SMP)		+= cpu_ops.o
diff --git a/arch/riscv/kernel/kernel_mode_vector.c b/arch/riscv/kernel/kernel_mode_vector.c
new file mode 100644
index 000000000000..0277168af0c5
--- /dev/null
+++ b/arch/riscv/kernel/kernel_mode_vector.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2012 ARM Ltd.
+ * Author: Catalin Marinas <catalin.marinas at arm.com>
+ * Copyright (C) 2017 Linaro Ltd. <ard.biesheuvel at linaro.org>
+ * Copyright (C) 2021 SiFive
+ */
+#include <linux/compiler.h>
+#include <linux/irqflags.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
+#include <linux/types.h>
+
+#include <asm/vector.h>
+#include <asm/switch_to.h>
+
+DECLARE_PER_CPU(bool, vector_context_busy);
+DEFINE_PER_CPU(bool, vector_context_busy);
+
+/*
+ * may_use_vector - whether it is allowable at this time to issue vector
+ *                instructions or access the vector register file
+ *
+ * Callers must not assume that the result remains true beyond the next
+ * preempt_enable() or return from softirq context.
+ */
+static __must_check inline bool may_use_vector(void)
+{
+	/*
+	 * vector_context_busy is only set while preemption is disabled,
+	 * and is clear whenever preemption is enabled. Since
+	 * this_cpu_read() is atomic w.r.t. preemption, vector_context_busy
+	 * 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.
+	 */
+	return !in_irq() && !irqs_disabled() && !in_nmi() &&
+	       !this_cpu_read(vector_context_busy);
+}
+
+/*
+ * Claim ownership of the CPU vector context for use by the calling context.
+ *
+ * The caller may freely manipulate the vector context metadata until
+ * put_cpu_vector_context() is called.
+ */
+static void get_cpu_vector_context(void)
+{
+	bool busy;
+
+	preempt_disable();
+	busy = __this_cpu_xchg(vector_context_busy, true);
+
+	WARN_ON(busy);
+}
+
+/*
+ * Release the CPU vector context.
+ *
+ * Must be called from a context in which get_cpu_vector_context() was
+ * previously called, with no call to put_cpu_vector_context() in the
+ * meantime.
+ */
+static void put_cpu_vector_context(void)
+{
+	bool busy = __this_cpu_xchg(vector_context_busy, false);
+
+	WARN_ON(!busy);
+	preempt_enable();
+}
+
+/*
+ * kernel_rvv_begin(): obtain the CPU vector registers for use by the calling
+ * context
+ *
+ * Must not be called unless may_use_vector() returns true.
+ * Task context in the vector registers is saved back to memory as necessary.
+ *
+ * A matching call to kernel_rvv_end() must be made before returning from the
+ * calling context.
+ *
+ * The caller may freely use the vector registers until kernel_rvv_end() is
+ * called.
+ */
+void kernel_rvv_begin(void)
+{
+	if (WARN_ON(!has_vector()))
+		return;
+
+	WARN_ON(!may_use_vector());
+
+	/* Acquire kernel mode vector */
+	get_cpu_vector_context();
+
+	/* Save vector state, if any */
+	vstate_save(current, task_pt_regs(current));
+
+	/* Enable vector */
+	rvv_enable();
+
+	/* Invalidate vector regs */
+	vector_flush_cpu_state();
+}
+EXPORT_SYMBOL_GPL(kernel_rvv_begin);
+
+/*
+ * kernel_rvv_end(): give the CPU vector registers back to the current task
+ *
+ * Must be called from a context in which kernel_rvv_begin() was previously
+ * called, with no call to kernel_rvv_end() in the meantime.
+ *
+ * The caller must not use the vector registers after this function is called,
+ * unless kernel_rvv_begin() is called again in the meantime.
+ */
+void kernel_rvv_end(void)
+{
+	if (WARN_ON(!has_vector()))
+		return;
+
+	/* Invalidate vector regs */
+	vector_flush_cpu_state();
+
+	/* Restore vector state, if any */
+	vstate_restore(current, task_pt_regs(current));
+
+	/* disable vector */
+	rvv_disable();
+
+	/* release kernel mode vector */
+	put_cpu_vector_context();
+}
+EXPORT_SYMBOL_GPL(kernel_rvv_end);
-- 
2.39.0