RISC-V: Task implementation

This patch contains the implementation of tasks on RISC-V, most of which
is involved in task switching.
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

arch/riscv/include/asm/asm-offsets.h

+#include <generated/asm-offsets.h>

arch/riscv/include/asm/current.h

+/*
+ * Based on arm/arm64/include/asm/current.h
+ *
+ * Copyright (C) 2016 ARM
+ * Copyright (C) 2017 SiFive
+ *
+ *   This program is free software; you can redistribute it and/or
+ *   modify it under the terms of the GNU General Public License
+ *   as published by the Free Software Foundation, version 2.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+
+
+#ifndef __ASM_CURRENT_H
+#define __ASM_CURRENT_H
+
+#include <linux/bug.h>
+#include <linux/compiler.h>
+
+#ifndef __ASSEMBLY__
+
+struct task_struct;
+
+/*
+ * This only works because "struct thread_info" is at offset 0 from "struct
+ * task_struct".  This constraint seems to be necessary on other architectures
+ * as well, but __switch_to enforces it.  We can't check TASK_TI here because
+ * <asm/asm-offsets.h> includes this, and I can't get the definition of "struct
+ * task_struct" here due to some header ordering problems.
+ */
+static __always_inline struct task_struct *get_current(void)
+{
+	register struct task_struct *tp __asm__("tp");
+	return tp;
+}
+
+#define current get_current()
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* __ASM_CURRENT_H */

arch/riscv/include/asm/kprobes.h

+/*
+ * Copied from arch/arm64/include/asm/kprobes.h
+ *
+ * Copyright (C) 2013 Linaro Limited
+ * Copyright (C) 2017 SiFive
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ */
+
+#ifndef _RISCV_KPROBES_H
+#define _RISCV_KPROBES_H
+
+#include <asm-generic/kprobes.h>
+
+#endif /* _RISCV_KPROBES_H */

arch/riscv/include/asm/processor.h

+/*
+ * Copyright (C) 2012 Regents of the University of California
+ *
+ *   This program is free software; you can redistribute it and/or
+ *   modify it under the terms of the GNU General Public License
+ *   as published by the Free Software Foundation, version 2.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+
+#ifndef _ASM_RISCV_PROCESSOR_H
+#define _ASM_RISCV_PROCESSOR_H
+
+#include <linux/const.h>
+
+#include <asm/ptrace.h>
+
+/*
+ * This decides where the kernel will search for a free chunk of vm
+ * space during mmap's.
+ */
+#define TASK_UNMAPPED_BASE	PAGE_ALIGN(TASK_SIZE >> 1)
+
+#define STACK_TOP		TASK_SIZE
+#define STACK_TOP_MAX		STACK_TOP
+#define STACK_ALIGN		16
+
+#ifndef __ASSEMBLY__
+
+struct task_struct;
+struct pt_regs;
+
+/*
+ * Default implementation of macro that returns current
+ * instruction pointer ("program counter").
+ */
+#define current_text_addr()	({ __label__ _l; _l: &&_l; })
+
+/* CPU-specific state of a task */
+struct thread_struct {
+	/* Callee-saved registers */
+	unsigned long ra;
+	unsigned long sp;	/* Kernel mode stack */
+	unsigned long s[12];	/* s[0]: frame pointer */
+	struct __riscv_d_ext_state fstate;
+};
+
+#define INIT_THREAD {					\
+	.sp = sizeof(init_stack) + (long)&init_stack,	\
+}
+
+#define task_pt_regs(tsk)						\
+	((struct pt_regs *)(task_stack_page(tsk) + THREAD_SIZE		\
+			    - ALIGN(sizeof(struct pt_regs), STACK_ALIGN)))
+
+#define KSTK_EIP(tsk)		(task_pt_regs(tsk)->sepc)
+#define KSTK_ESP(tsk)		(task_pt_regs(tsk)->sp)
+
+
+/* Do necessary setup to start up a newly executed thread. */
+extern void start_thread(struct pt_regs *regs,
+			unsigned long pc, unsigned long sp);
+
+/* Free all resources held by a thread. */
+static inline void release_thread(struct task_struct *dead_task)
+{
+}
+
+extern unsigned long get_wchan(struct task_struct *p);
+
+
+static inline void cpu_relax(void)
+{
+#ifdef __riscv_muldiv
+	int dummy;
+	/* In lieu of a halt instruction, induce a long-latency stall. */
+	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
+#endif
+	barrier();
+}
+
+static inline void wait_for_interrupt(void)
+{
+	__asm__ __volatile__ ("wfi");
+}
+
+struct device_node;
+extern int riscv_of_processor_hart(struct device_node *node);
+
+extern void riscv_fill_hwcap(void);
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_RISCV_PROCESSOR_H */

arch/riscv/include/asm/switch_to.h

+/*
+ * Copyright (C) 2012 Regents of the University of California
+ *
+ *   This program is free software; you can redistribute it and/or
+ *   modify it under the terms of the GNU General Public License
+ *   as published by the Free Software Foundation, version 2.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+
+#ifndef _ASM_RISCV_SWITCH_TO_H
+#define _ASM_RISCV_SWITCH_TO_H
+
+#include <asm/processor.h>
+#include <asm/ptrace.h>
+#include <asm/csr.h>
+
+extern void __fstate_save(struct task_struct *save_to);
+extern void __fstate_restore(struct task_struct *restore_from);
+
+static inline void __fstate_clean(struct pt_regs *regs)
+{
+	regs->sstatus |= (regs->sstatus & ~(SR_FS)) | SR_FS_CLEAN;
+}
+
+static inline void fstate_save(struct task_struct *task,
+			       struct pt_regs *regs)
+{
+	if ((regs->sstatus & SR_FS) == SR_FS_DIRTY) {
+		__fstate_save(task);
+		__fstate_clean(regs);
+	}
+}
+
+static inline void fstate_restore(struct task_struct *task,
+				  struct pt_regs *regs)
+{
+	if ((regs->sstatus & SR_FS) != SR_FS_OFF) {
+		__fstate_restore(task);
+		__fstate_clean(regs);
+	}
+}
+
+static inline void __switch_to_aux(struct task_struct *prev,
+				   struct task_struct *next)
+{
+	struct pt_regs *regs;
+
+	regs = task_pt_regs(prev);
+	if (unlikely(regs->sstatus & SR_SD))
+		fstate_save(prev, regs);
+	fstate_restore(next, task_pt_regs(next));
+}
+
+extern struct task_struct *__switch_to(struct task_struct *,
+				       struct task_struct *);
+
+#define switch_to(prev, next, last)			\
+do {							\
+	struct task_struct *__prev = (prev);		\
+	struct task_struct *__next = (next);		\
+	__switch_to_aux(__prev, __next);		\
+	((last) = __switch_to(__prev, __next));		\
+} while (0)
+
+#endif /* _ASM_RISCV_SWITCH_TO_H */

arch/riscv/include/asm/thread_info.h

+/*
+ * Copyright (C) 2009 Chen Liqin <liqin.chen@sunplusct.com>
+ * Copyright (C) 2012 Regents of the University of California
+ * Copyright (C) 2017 SiFive
+ *
+ *   This program is free software; you can redistribute it and/or
+ *   modify it under the terms of the GNU General Public License
+ *   as published by the Free Software Foundation, version 2.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ */
+
+#ifndef _ASM_RISCV_THREAD_INFO_H
+#define _ASM_RISCV_THREAD_INFO_H
+
+#include <asm/page.h>
+#include <linux/const.h>
+
+/* thread information allocation */
+#define THREAD_SIZE_ORDER	(1)
+#define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
+
+#ifndef __ASSEMBLY__
+
+#include <asm/processor.h>
+#include <asm/csr.h>
+
+typedef unsigned long mm_segment_t;
+
+/*
+ * low level task data that entry.S needs immediate access to
+ * - this struct should fit entirely inside of one cache line
+ * - if the members of this struct changes, the assembly constants
+ *   in asm-offsets.c must be updated accordingly
+ * - thread_info is included in task_struct at an offset of 0.  This means that
+ *   tp points to both thread_info and task_struct.
+ */
+struct thread_info {
+	unsigned long		flags;		/* low level flags */
+	int                     preempt_count;  /* 0=>preemptible, <0=>BUG */
+	mm_segment_t		addr_limit;
+	/*
+	 * These stack pointers are overwritten on every system call or
+	 * exception.  SP is also saved to the stack it can be recovered when
+	 * overwritten.
+	 */
+	long			kernel_sp;	/* Kernel stack pointer */
+	long			user_sp;	/* User stack pointer */
+	int			cpu;
+};
+
+/*
+ * macros/functions for gaining access to the thread information structure
+ *
+ * preempt_count needs to be 1 initially, until the scheduler is functional.
+ */
+#define INIT_THREAD_INFO(tsk)			\
+{						\
+	.flags		= 0,			\
+	.preempt_count	= INIT_PREEMPT_COUNT,	\
+	.addr_limit	= KERNEL_DS,		\
+}
+
+#define init_stack		(init_thread_union.stack)
+
+#endif /* !__ASSEMBLY__ */
+
+/*
+ * thread information flags
+ * - these are process state flags that various assembly files may need to
+ *   access
+ * - pending work-to-be-done flags are in lowest half-word
+ * - other flags in upper half-word(s)
+ */
+#define TIF_SYSCALL_TRACE	0	/* syscall trace active */
+#define TIF_NOTIFY_RESUME	1	/* callback before returning to user */
+#define TIF_SIGPENDING		2	/* signal pending */
+#define TIF_NEED_RESCHED	3	/* rescheduling necessary */
+#define TIF_RESTORE_SIGMASK	4	/* restore signal mask in do_signal() */
+#define TIF_MEMDIE		5	/* is terminating due to OOM killer */
+#define TIF_SYSCALL_TRACEPOINT  6       /* syscall tracepoint instrumentation */
+
+#define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
+#define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
+#define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
+#define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
+
+#define _TIF_WORK_MASK \
+	(_TIF_NOTIFY_RESUME | _TIF_SIGPENDING | _TIF_NEED_RESCHED)
+
+#endif /* _ASM_RISCV_THREAD_INFO_H */

arch/riscv/kernel/process.c

+/*
+ * Copyright (C) 2009 Sunplus Core Technology Co., Ltd.
+ *  Chen Liqin <liqin.chen@sunplusct.com>
+ *  Lennox Wu <lennox.wu@sunplusct.com>
+ * Copyright (C) 2012 Regents of the University of California
+ * Copyright (C) 2017 SiFive
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see the file COPYING, or write
+ * to the Free Software Foundation, Inc.,
+ */
+
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/sched/task_stack.h>
+#include <linux/tick.h>
+#include <linux/ptrace.h>
+
+#include <asm/unistd.h>
+#include <asm/uaccess.h>
+#include <asm/processor.h>
+#include <asm/csr.h>
+#include <asm/string.h>
+#include <asm/switch_to.h>
+
+extern asmlinkage void ret_from_fork(void);
+extern asmlinkage void ret_from_kernel_thread(void);
+
+void arch_cpu_idle(void)
+{
+	wait_for_interrupt();
+	local_irq_enable();
+}
+
+void show_regs(struct pt_regs *regs)
+{
+	show_regs_print_info(KERN_DEFAULT);
+
+	pr_cont("sepc: " REG_FMT " ra : " REG_FMT " sp : " REG_FMT "\n",
+		regs->sepc, regs->ra, regs->sp);
+	pr_cont(" gp : " REG_FMT " tp : " REG_FMT " t0 : " REG_FMT "\n",
+		regs->gp, regs->tp, regs->t0);
+	pr_cont(" t1 : " REG_FMT " t2 : " REG_FMT " s0 : " REG_FMT "\n",
+		regs->t1, regs->t2, regs->s0);
+	pr_cont(" s1 : " REG_FMT " a0 : " REG_FMT " a1 : " REG_FMT "\n",
+		regs->s1, regs->a0, regs->a1);
+	pr_cont(" a2 : " REG_FMT " a3 : " REG_FMT " a4 : " REG_FMT "\n",
+		regs->a2, regs->a3, regs->a4);
+	pr_cont(" a5 : " REG_FMT " a6 : " REG_FMT " a7 : " REG_FMT "\n",
+		regs->a5, regs->a6, regs->a7);
+	pr_cont(" s2 : " REG_FMT " s3 : " REG_FMT " s4 : " REG_FMT "\n",
+		regs->s2, regs->s3, regs->s4);
+	pr_cont(" s5 : " REG_FMT " s6 : " REG_FMT " s7 : " REG_FMT "\n",
+		regs->s5, regs->s6, regs->s7);
+	pr_cont(" s8 : " REG_FMT " s9 : " REG_FMT " s10: " REG_FMT "\n",
+		regs->s8, regs->s9, regs->s10);
+	pr_cont(" s11: " REG_FMT " t3 : " REG_FMT " t4 : " REG_FMT "\n",
+		regs->s11, regs->t3, regs->t4);
+	pr_cont(" t5 : " REG_FMT " t6 : " REG_FMT "\n",
+		regs->t5, regs->t6);
+
+	pr_cont("sstatus: " REG_FMT " sbadaddr: " REG_FMT " scause: " REG_FMT "\n",
+		regs->sstatus, regs->sbadaddr, regs->scause);
+}
+
+void start_thread(struct pt_regs *regs, unsigned long pc,
+	unsigned long sp)
+{
+	regs->sstatus = SR_PIE /* User mode, irqs on */ | SR_FS_INITIAL;
+	regs->sepc = pc;
+	regs->sp = sp;
+	set_fs(USER_DS);
+}
+
+void flush_thread(void)
+{
+	/*
+	 * Reset FPU context
+	 *	frm: round to nearest, ties to even (IEEE default)
+	 *	fflags: accrued exceptions cleared
+	 */
+	memset(&current->thread.fstate, 0, sizeof(current->thread.fstate));
+}
+
+int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
+{
+	fstate_save(src, task_pt_regs(src));
+	*dst = *src;
+	return 0;
+}
+
+int copy_thread(unsigned long clone_flags, unsigned long usp,
+	unsigned long arg, struct task_struct *p)
+{
+	struct pt_regs *childregs = task_pt_regs(p);
+
+	/* p->thread holds context to be restored by __switch_to() */
+	if (unlikely(p->flags & PF_KTHREAD)) {
+		/* Kernel thread */
+		const register unsigned long gp __asm__ ("gp");
+		memset(childregs, 0, sizeof(struct pt_regs));
+		childregs->gp = gp;
+		childregs->sstatus = SR_PS | SR_PIE; /* Supervisor, irqs on */
+
+		p->thread.ra = (unsigned long)ret_from_kernel_thread;
+		p->thread.s[0] = usp; /* fn */
+		p->thread.s[1] = arg;
+	} else {
+		*childregs = *(current_pt_regs());
+		if (usp) /* User fork */
+			childregs->sp = usp;
+		if (clone_flags & CLONE_SETTLS)
+			childregs->tp = childregs->a5;
+		childregs->a0 = 0; /* Return value of fork() */
+		p->thread.ra = (unsigned long)ret_from_fork;
+	}
+	p->thread.sp = (unsigned long)childregs; /* kernel sp */
+	return 0;
+}