Merge tag 'v3.3.7' into 3.3/master

[zen-stable.git] / arch / tile / kernel / process.c

/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   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, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/sched.h>
#include <linux/preempt.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/kprobes.h>
#include <linux/elfcore.h>
#include <linux/tick.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/compat.h>
#include <linux/hardirq.h>
#include <linux/syscalls.h>
#include <linux/kernel.h>
#include <linux/tracehook.h>
#include <linux/signal.h>
#include <asm/system.h>
#include <asm/stack.h>
#include <asm/homecache.h>
#include <asm/syscalls.h>
#include <asm/traps.h>
#ifdef CONFIG_HARDWALL
#include <asm/hardwall.h>
#endif
#include <arch/chip.h>
#include <arch/abi.h>


/*
 * Use the (x86) "idle=poll" option to prefer low latency when leaving the
 * idle loop over low power while in the idle loop, e.g. if we have
 * one thread per core and we want to get threads out of futex waits fast.
 */
static int no_idle_nap;
static int __init idle_setup(char *str)
{
        if (!str)
                return -EINVAL;

        if (!strcmp(str, "poll")) {
                pr_info("using polling idle threads.\n");
                no_idle_nap = 1;
        } else if (!strcmp(str, "halt"))
                no_idle_nap = 0;
        else
                return -1;

        return 0;
}
early_param("idle", idle_setup);

/*
 * The idle thread. There's no useful work to be
 * done, so just try to conserve power and have a
 * low exit latency (ie sit in a loop waiting for
 * somebody to say that they'd like to reschedule)
 */
void cpu_idle(void)
{
        int cpu = smp_processor_id();


        current_thread_info()->status |= TS_POLLING;

        if (no_idle_nap) {
                while (1) {
                        while (!need_resched())
                                cpu_relax();
                        schedule();
                }
        }

        /* endless idle loop with no priority at all */
        while (1) {
                tick_nohz_idle_enter();
                rcu_idle_enter();
                while (!need_resched()) {
                        if (cpu_is_offline(cpu))
                                BUG();  /* no HOTPLUG_CPU */

                        local_irq_disable();
                        __get_cpu_var(irq_stat).idle_timestamp = jiffies;
                        current_thread_info()->status &= ~TS_POLLING;
                        /*
                         * TS_POLLING-cleared state must be visible before we
                         * test NEED_RESCHED:
                         */
                        smp_mb();

                        if (!need_resched())
                                _cpu_idle();
                        else
                                local_irq_enable();
                        current_thread_info()->status |= TS_POLLING;
                }
                rcu_idle_exit();
                tick_nohz_idle_exit();
                preempt_enable_no_resched();
                schedule();
                preempt_disable();
        }
}

struct thread_info *alloc_thread_info_node(struct task_struct *task, int node)
{
        struct page *page;
        gfp_t flags = GFP_KERNEL;

#ifdef CONFIG_DEBUG_STACK_USAGE
        flags |= __GFP_ZERO;
#endif

        page = alloc_pages_node(node, flags, THREAD_SIZE_ORDER);
        if (!page)
                return NULL;

        return (struct thread_info *)page_address(page);
}

/*
 * Free a thread_info node, and all of its derivative
 * data structures.
 */
void free_thread_info(struct thread_info *info)
{
        struct single_step_state *step_state = info->step_state;

#ifdef CONFIG_HARDWALL
        /*
         * We free a thread_info from the context of the task that has
         * been scheduled next, so the original task is already dead.
         * Calling deactivate here just frees up the data structures.
         * If the task we're freeing held the last reference to a
         * hardwall fd, it would have been released prior to this point
         * anyway via exit_files(), and "hardwall" would be NULL by now.
         */
        if (info->task->thread.hardwall)
                hardwall_deactivate(info->task);
#endif

        if (step_state) {

                /*
                 * FIXME: we don't munmap step_state->buffer
                 * because the mm_struct for this process (info->task->mm)
                 * has already been zeroed in exit_mm().  Keeping a
                 * reference to it here seems like a bad move, so this
                 * means we can't munmap() the buffer, and therefore if we
                 * ptrace multiple threads in a process, we will slowly
                 * leak user memory.  (Note that as soon as the last
                 * thread in a process dies, we will reclaim all user
                 * memory including single-step buffers in the usual way.)
                 * We should either assign a kernel VA to this buffer
                 * somehow, or we should associate the buffer(s) with the
                 * mm itself so we can clean them up that way.
                 */
                kfree(step_state);
        }

        free_pages((unsigned long)info, THREAD_SIZE_ORDER);
}

static void save_arch_state(struct thread_struct *t);

int copy_thread(unsigned long clone_flags, unsigned long sp,
                unsigned long stack_size,
                struct task_struct *p, struct pt_regs *regs)
{
        struct pt_regs *childregs;
        unsigned long ksp;

        /*
         * When creating a new kernel thread we pass sp as zero.
         * Assign it to a reasonable value now that we have the stack.
         */
        if (sp == 0 && regs->ex1 == PL_ICS_EX1(KERNEL_PL, 0))
                sp = KSTK_TOP(p);

        /*
         * Do not clone step state from the parent; each thread
         * must make its own lazily.
         */
        task_thread_info(p)->step_state = NULL;

        /*
         * Start new thread in ret_from_fork so it schedules properly
         * and then return from interrupt like the parent.
         */
        p->thread.pc = (unsigned long) ret_from_fork;

        /* Save user stack top pointer so we can ID the stack vm area later. */
        p->thread.usp0 = sp;

        /* Record the pid of the process that created this one. */
        p->thread.creator_pid = current->pid;

        /*
         * Copy the registers onto the kernel stack so the
         * return-from-interrupt code will reload it into registers.
         */
        childregs = task_pt_regs(p);
        *childregs = *regs;
        childregs->regs[0] = 0;         /* return value is zero */
        childregs->sp = sp;  /* override with new user stack pointer */

        /*
         * If CLONE_SETTLS is set, set "tp" in the new task to "r4",
         * which is passed in as arg #5 to sys_clone().
         */
        if (clone_flags & CLONE_SETTLS)
                childregs->tp = regs->regs[4];

        /*
         * Copy the callee-saved registers from the passed pt_regs struct
         * into the context-switch callee-saved registers area.
         * This way when we start the interrupt-return sequence, the
         * callee-save registers will be correctly in registers, which
         * is how we assume the compiler leaves them as we start doing
         * the normal return-from-interrupt path after calling C code.
         * Zero out the C ABI save area to mark the top of the stack.
         */
        ksp = (unsigned long) childregs;
        ksp -= C_ABI_SAVE_AREA_SIZE;   /* interrupt-entry save area */
        ((long *)ksp)[0] = ((long *)ksp)[1] = 0;
        ksp -= CALLEE_SAVED_REGS_COUNT * sizeof(unsigned long);
        memcpy((void *)ksp, &regs->regs[CALLEE_SAVED_FIRST_REG],
               CALLEE_SAVED_REGS_COUNT * sizeof(unsigned long));
        ksp -= C_ABI_SAVE_AREA_SIZE;   /* __switch_to() save area */
        ((long *)ksp)[0] = ((long *)ksp)[1] = 0;
        p->thread.ksp = ksp;

#if CHIP_HAS_TILE_DMA()
        /*
         * No DMA in the new thread.  We model this on the fact that
         * fork() clears the pending signals, alarms, and aio for the child.
         */
        memset(&p->thread.tile_dma_state, 0, sizeof(struct tile_dma_state));
        memset(&p->thread.dma_async_tlb, 0, sizeof(struct async_tlb));
#endif

#if CHIP_HAS_SN_PROC()
        /* Likewise, the new thread is not running static processor code. */
        p->thread.sn_proc_running = 0;
        memset(&p->thread.sn_async_tlb, 0, sizeof(struct async_tlb));
#endif

#if CHIP_HAS_PROC_STATUS_SPR()
        /* New thread has its miscellaneous processor state bits clear. */
        p->thread.proc_status = 0;
#endif

#ifdef CONFIG_HARDWALL
        /* New thread does not own any networks. */
        p->thread.hardwall = NULL;
#endif


        /*
         * Start the new thread with the current architecture state
         * (user interrupt masks, etc.).
         */
        save_arch_state(&p->thread);

        return 0;
}

/*
 * Return "current" if it looks plausible, or else a pointer to a dummy.
 * This can be helpful if we are just trying to emit a clean panic.
 */
struct task_struct *validate_current(void)
{
        static struct task_struct corrupt = { .comm = "<corrupt>" };
        struct task_struct *tsk = current;
        if (unlikely((unsigned long)tsk < PAGE_OFFSET ||
                     (void *)tsk > high_memory ||
                     ((unsigned long)tsk & (__alignof__(*tsk) - 1)) != 0)) {
                pr_err("Corrupt 'current' %p (sp %#lx)\n", tsk, stack_pointer);
                tsk = &corrupt;
        }
        return tsk;
}

/* Take and return the pointer to the previous task, for schedule_tail(). */
struct task_struct *sim_notify_fork(struct task_struct *prev)
{
        struct task_struct *tsk = current;
        __insn_mtspr(SPR_SIM_CONTROL, SIM_CONTROL_OS_FORK_PARENT |
                     (tsk->thread.creator_pid << _SIM_CONTROL_OPERATOR_BITS));
        __insn_mtspr(SPR_SIM_CONTROL, SIM_CONTROL_OS_FORK |
                     (tsk->pid << _SIM_CONTROL_OPERATOR_BITS));
        return prev;
}

int dump_task_regs(struct task_struct *tsk, elf_gregset_t *regs)
{
        struct pt_regs *ptregs = task_pt_regs(tsk);
        elf_core_copy_regs(regs, ptregs);
        return 1;
}

#if CHIP_HAS_TILE_DMA()

/* Allow user processes to access the DMA SPRs */
void grant_dma_mpls(void)
{
#if CONFIG_KERNEL_PL == 2
        __insn_mtspr(SPR_MPL_DMA_CPL_SET_1, 1);
        __insn_mtspr(SPR_MPL_DMA_NOTIFY_SET_1, 1);
#else
        __insn_mtspr(SPR_MPL_DMA_CPL_SET_0, 1);
        __insn_mtspr(SPR_MPL_DMA_NOTIFY_SET_0, 1);
#endif
}

/* Forbid user processes from accessing the DMA SPRs */
void restrict_dma_mpls(void)
{
#if CONFIG_KERNEL_PL == 2
        __insn_mtspr(SPR_MPL_DMA_CPL_SET_2, 1);
        __insn_mtspr(SPR_MPL_DMA_NOTIFY_SET_2, 1);
#else
        __insn_mtspr(SPR_MPL_DMA_CPL_SET_1, 1);
        __insn_mtspr(SPR_MPL_DMA_NOTIFY_SET_1, 1);
#endif
}

/* Pause the DMA engine, then save off its state registers. */
static void save_tile_dma_state(struct tile_dma_state *dma)
{
        unsigned long state = __insn_mfspr(SPR_DMA_USER_STATUS);
        unsigned long post_suspend_state;

        /* If we're running, suspend the engine. */
        if ((state & DMA_STATUS_MASK) == SPR_DMA_STATUS__RUNNING_MASK)
                __insn_mtspr(SPR_DMA_CTR, SPR_DMA_CTR__SUSPEND_MASK);

        /*
         * Wait for the engine to idle, then save regs.  Note that we
         * want to record the "running" bit from before suspension,
         * and the "done" bit from after, so that we can properly
         * distinguish a case where the user suspended the engine from
         * the case where the kernel suspended as part of the context
         * swap.
         */
        do {
                post_suspend_state = __insn_mfspr(SPR_DMA_USER_STATUS);
        } while (post_suspend_state & SPR_DMA_STATUS__BUSY_MASK);

        dma->src = __insn_mfspr(SPR_DMA_SRC_ADDR);
        dma->src_chunk = __insn_mfspr(SPR_DMA_SRC_CHUNK_ADDR);
        dma->dest = __insn_mfspr(SPR_DMA_DST_ADDR);
        dma->dest_chunk = __insn_mfspr(SPR_DMA_DST_CHUNK_ADDR);
        dma->strides = __insn_mfspr(SPR_DMA_STRIDE);
        dma->chunk_size = __insn_mfspr(SPR_DMA_CHUNK_SIZE);
        dma->byte = __insn_mfspr(SPR_DMA_BYTE);
        dma->status = (state & SPR_DMA_STATUS__RUNNING_MASK) |
                (post_suspend_state & SPR_DMA_STATUS__DONE_MASK);
}

/* Restart a DMA that was running before we were context-switched out. */
static void restore_tile_dma_state(struct thread_struct *t)
{
        const struct tile_dma_state *dma = &t->tile_dma_state;

        /*
         * The only way to restore the done bit is to run a zero
         * length transaction.
         */
        if ((dma->status & SPR_DMA_STATUS__DONE_MASK) &&
            !(__insn_mfspr(SPR_DMA_USER_STATUS) & SPR_DMA_STATUS__DONE_MASK)) {
                __insn_mtspr(SPR_DMA_BYTE, 0);
                __insn_mtspr(SPR_DMA_CTR, SPR_DMA_CTR__REQUEST_MASK);
                while (__insn_mfspr(SPR_DMA_USER_STATUS) &
                       SPR_DMA_STATUS__BUSY_MASK)
                        ;
        }

        __insn_mtspr(SPR_DMA_SRC_ADDR, dma->src);
        __insn_mtspr(SPR_DMA_SRC_CHUNK_ADDR, dma->src_chunk);
        __insn_mtspr(SPR_DMA_DST_ADDR, dma->dest);
        __insn_mtspr(SPR_DMA_DST_CHUNK_ADDR, dma->dest_chunk);
        __insn_mtspr(SPR_DMA_STRIDE, dma->strides);
        __insn_mtspr(SPR_DMA_CHUNK_SIZE, dma->chunk_size);
        __insn_mtspr(SPR_DMA_BYTE, dma->byte);

        /*
         * Restart the engine if we were running and not done.
         * Clear a pending async DMA fault that we were waiting on return
         * to user space to execute, since we expect the DMA engine
         * to regenerate those faults for us now.  Note that we don't
         * try to clear the TIF_ASYNC_TLB flag, since it's relatively
         * harmless if set, and it covers both DMA and the SN processor.
         */
        if ((dma->status & DMA_STATUS_MASK) == SPR_DMA_STATUS__RUNNING_MASK) {
                t->dma_async_tlb.fault_num = 0;
                __insn_mtspr(SPR_DMA_CTR, SPR_DMA_CTR__REQUEST_MASK);
        }
}

#endif

static void save_arch_state(struct thread_struct *t)
{
#if CHIP_HAS_SPLIT_INTR_MASK()
        t->interrupt_mask = __insn_mfspr(SPR_INTERRUPT_MASK_0_0) |
                ((u64)__insn_mfspr(SPR_INTERRUPT_MASK_0_1) << 32);
#else
        t->interrupt_mask = __insn_mfspr(SPR_INTERRUPT_MASK_0);
#endif
        t->ex_context[0] = __insn_mfspr(SPR_EX_CONTEXT_0_0);
        t->ex_context[1] = __insn_mfspr(SPR_EX_CONTEXT_0_1);
        t->system_save[0] = __insn_mfspr(SPR_SYSTEM_SAVE_0_0);
        t->system_save[1] = __insn_mfspr(SPR_SYSTEM_SAVE_0_1);
        t->system_save[2] = __insn_mfspr(SPR_SYSTEM_SAVE_0_2);
        t->system_save[3] = __insn_mfspr(SPR_SYSTEM_SAVE_0_3);
        t->intctrl_0 = __insn_mfspr(SPR_INTCTRL_0_STATUS);
#if CHIP_HAS_PROC_STATUS_SPR()
        t->proc_status = __insn_mfspr(SPR_PROC_STATUS);
#endif
#if !CHIP_HAS_FIXED_INTVEC_BASE()
        t->interrupt_vector_base = __insn_mfspr(SPR_INTERRUPT_VECTOR_BASE_0);
#endif
#if CHIP_HAS_TILE_RTF_HWM()
        t->tile_rtf_hwm = __insn_mfspr(SPR_TILE_RTF_HWM);
#endif
#if CHIP_HAS_DSTREAM_PF()
        t->dstream_pf = __insn_mfspr(SPR_DSTREAM_PF);
#endif
}

static void restore_arch_state(const struct thread_struct *t)
{
#if CHIP_HAS_SPLIT_INTR_MASK()
        __insn_mtspr(SPR_INTERRUPT_MASK_0_0, (u32) t->interrupt_mask);
        __insn_mtspr(SPR_INTERRUPT_MASK_0_1, t->interrupt_mask >> 32);
#else
        __insn_mtspr(SPR_INTERRUPT_MASK_0, t->interrupt_mask);
#endif
        __insn_mtspr(SPR_EX_CONTEXT_0_0, t->ex_context[0]);
        __insn_mtspr(SPR_EX_CONTEXT_0_1, t->ex_context[1]);
        __insn_mtspr(SPR_SYSTEM_SAVE_0_0, t->system_save[0]);
        __insn_mtspr(SPR_SYSTEM_SAVE_0_1, t->system_save[1]);
        __insn_mtspr(SPR_SYSTEM_SAVE_0_2, t->system_save[2]);
        __insn_mtspr(SPR_SYSTEM_SAVE_0_3, t->system_save[3]);
        __insn_mtspr(SPR_INTCTRL_0_STATUS, t->intctrl_0);
#if CHIP_HAS_PROC_STATUS_SPR()
        __insn_mtspr(SPR_PROC_STATUS, t->proc_status);
#endif
#if !CHIP_HAS_FIXED_INTVEC_BASE()
        __insn_mtspr(SPR_INTERRUPT_VECTOR_BASE_0, t->interrupt_vector_base);
#endif
#if CHIP_HAS_TILE_RTF_HWM()
        __insn_mtspr(SPR_TILE_RTF_HWM, t->tile_rtf_hwm);
#endif
#if CHIP_HAS_DSTREAM_PF()
        __insn_mtspr(SPR_DSTREAM_PF, t->dstream_pf);
#endif
}


void _prepare_arch_switch(struct task_struct *next)
{
#if CHIP_HAS_SN_PROC()
        int snctl;
#endif
#if CHIP_HAS_TILE_DMA()
        struct tile_dma_state *dma = &current->thread.tile_dma_state;
        if (dma->enabled)
                save_tile_dma_state(dma);
#endif
#if CHIP_HAS_SN_PROC()
        /*
         * Suspend the static network processor if it was running.
         * We do not suspend the fabric itself, just like we don't
         * try to suspend the UDN.
         */
        snctl = __insn_mfspr(SPR_SNCTL);
        current->thread.sn_proc_running =
                (snctl & SPR_SNCTL__FRZPROC_MASK) == 0;
        if (current->thread.sn_proc_running)
                __insn_mtspr(SPR_SNCTL, snctl | SPR_SNCTL__FRZPROC_MASK);
#endif
}


struct task_struct *__sched _switch_to(struct task_struct *prev,
                                       struct task_struct *next)
{
        /* DMA state is already saved; save off other arch state. */
        save_arch_state(&prev->thread);

#if CHIP_HAS_TILE_DMA()
        /*
         * Restore DMA in new task if desired.
         * Note that it is only safe to restart here since interrupts
         * are disabled, so we can't take any DMATLB miss or access
         * interrupts before we have finished switching stacks.
         */
        if (next->thread.tile_dma_state.enabled) {
                restore_tile_dma_state(&next->thread);
                grant_dma_mpls();
        } else {
                restrict_dma_mpls();
        }
#endif

        /* Restore other arch state. */
        restore_arch_state(&next->thread);

#if CHIP_HAS_SN_PROC()
        /*
         * Restart static network processor in the new process
         * if it was running before.
         */
        if (next->thread.sn_proc_running) {
                int snctl = __insn_mfspr(SPR_SNCTL);
                __insn_mtspr(SPR_SNCTL, snctl & ~SPR_SNCTL__FRZPROC_MASK);
        }
#endif

#ifdef CONFIG_HARDWALL
        /* Enable or disable access to the network registers appropriately. */
        if (prev->thread.hardwall != NULL) {
                if (next->thread.hardwall == NULL)
                        restrict_network_mpls();
        } else if (next->thread.hardwall != NULL) {
                grant_network_mpls();
        }
#endif

        /*
         * Switch kernel SP, PC, and callee-saved registers.
         * In the context of the new task, return the old task pointer
         * (i.e. the task that actually called __switch_to).
         * Pass the value to use for SYSTEM_SAVE_K_0 when we reset our sp.
         */
        return __switch_to(prev, next, next_current_ksp0(next));
}

/*
 * This routine is called on return from interrupt if any of the
 * TIF_WORK_MASK flags are set in thread_info->flags.  It is
 * entered with interrupts disabled so we don't miss an event
 * that modified the thread_info flags.  If any flag is set, we
 * handle it and return, and the calling assembly code will
 * re-disable interrupts, reload the thread flags, and call back
 * if more flags need to be handled.
 *
 * We return whether we need to check the thread_info flags again
 * or not.  Note that we don't clear TIF_SINGLESTEP here, so it's
 * important that it be tested last, and then claim that we don't
 * need to recheck the flags.
 */
int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
{
        if (thread_info_flags & _TIF_NEED_RESCHED) {
                schedule();
                return 1;
        }
#if CHIP_HAS_TILE_DMA() || CHIP_HAS_SN_PROC()
        if (thread_info_flags & _TIF_ASYNC_TLB) {
                do_async_page_fault(regs);
                return 1;
        }
#endif
        if (thread_info_flags & _TIF_SIGPENDING) {
                do_signal(regs);
                return 1;
        }
        if (thread_info_flags & _TIF_NOTIFY_RESUME) {
                clear_thread_flag(TIF_NOTIFY_RESUME);
                tracehook_notify_resume(regs);
                if (current->replacement_session_keyring)
                        key_replace_session_keyring();
                return 1;
        }
        if (thread_info_flags & _TIF_SINGLESTEP) {
                if ((regs->ex1 & SPR_EX_CONTEXT_1_1__PL_MASK) == 0)
                        single_step_once(regs);
                return 0;
        }
        panic("work_pending: bad flags %#x\n", thread_info_flags);
}

/* Note there is an implicit fifth argument if (clone_flags & CLONE_SETTLS). */
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
                void __user *, parent_tidptr, void __user *, child_tidptr,
                struct pt_regs *, regs)
{
        if (!newsp)
                newsp = regs->sp;
        return do_fork(clone_flags, newsp, regs, 0,
                       parent_tidptr, child_tidptr);
}

/*
 * sys_execve() executes a new program.
 */
SYSCALL_DEFINE4(execve, const char __user *, path,
                const char __user *const __user *, argv,
                const char __user *const __user *, envp,
                struct pt_regs *, regs)
{
        long error;
        char *filename;

        filename = getname(path);
        error = PTR_ERR(filename);
        if (IS_ERR(filename))
                goto out;
        error = do_execve(filename, argv, envp, regs);
        putname(filename);
        if (error == 0)
                single_step_execve();
out:
        return error;
}

#ifdef CONFIG_COMPAT
long compat_sys_execve(const char __user *path,
                       compat_uptr_t __user *argv,
                       compat_uptr_t __user *envp,
                       struct pt_regs *regs)
{
        long error;
        char *filename;

        filename = getname(path);
        error = PTR_ERR(filename);
        if (IS_ERR(filename))
                goto out;
        error = compat_do_execve(filename, argv, envp, regs);
        putname(filename);
        if (error == 0)
                single_step_execve();
out:
        return error;
}
#endif

unsigned long get_wchan(struct task_struct *p)
{
        struct KBacktraceIterator kbt;

        if (!p || p == current || p->state == TASK_RUNNING)
                return 0;

        for (KBacktraceIterator_init(&kbt, p, NULL);
             !KBacktraceIterator_end(&kbt);
             KBacktraceIterator_next(&kbt)) {
                if (!in_sched_functions(kbt.it.pc))
                        return kbt.it.pc;
        }

        return 0;
}

/*
 * We pass in lr as zero (cleared in kernel_thread) and the caller
 * part of the backtrace ABI on the stack also zeroed (in copy_thread)
 * so that backtraces will stop with this function.
 * Note that we don't use r0, since copy_thread() clears it.
 */
static void start_kernel_thread(int dummy, int (*fn)(int), int arg)
{
        do_exit(fn(arg));
}

/*
 * Create a kernel thread
 */
int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
{
        struct pt_regs regs;

        memset(&regs, 0, sizeof(regs));
        regs.ex1 = PL_ICS_EX1(KERNEL_PL, 0);  /* run at kernel PL, no ICS */
        regs.pc = (long) start_kernel_thread;
        regs.flags = PT_FLAGS_CALLER_SAVES;   /* need to restore r1 and r2 */
        regs.regs[1] = (long) fn;             /* function pointer */
        regs.regs[2] = (long) arg;            /* parameter register */

        /* Ok, create the new process.. */
        return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs,
                       0, NULL, NULL);
}
EXPORT_SYMBOL(kernel_thread);

/* Flush thread state. */
void flush_thread(void)
{
        /* Nothing */
}

/*
 * Free current thread data structures etc..
 */
void exit_thread(void)
{
        /* Nothing */
}

void show_regs(struct pt_regs *regs)
{
        struct task_struct *tsk = validate_current();
        int i;

        pr_err("\n");
        pr_err(" Pid: %d, comm: %20s, CPU: %d\n",
               tsk->pid, tsk->comm, smp_processor_id());
#ifdef __tilegx__
        for (i = 0; i < 51; i += 3)
                pr_err(" r%-2d: "REGFMT" r%-2d: "REGFMT" r%-2d: "REGFMT"\n",
                       i, regs->regs[i], i+1, regs->regs[i+1],
                       i+2, regs->regs[i+2]);
        pr_err(" r51: "REGFMT" r52: "REGFMT" tp : "REGFMT"\n",
               regs->regs[51], regs->regs[52], regs->tp);
        pr_err(" sp : "REGFMT" lr : "REGFMT"\n", regs->sp, regs->lr);
#else
        for (i = 0; i < 52; i += 4)
                pr_err(" r%-2d: "REGFMT" r%-2d: "REGFMT
                       " r%-2d: "REGFMT" r%-2d: "REGFMT"\n",
                       i, regs->regs[i], i+1, regs->regs[i+1],
                       i+2, regs->regs[i+2], i+3, regs->regs[i+3]);
        pr_err(" r52: "REGFMT" tp : "REGFMT" sp : "REGFMT" lr : "REGFMT"\n",
               regs->regs[52], regs->tp, regs->sp, regs->lr);
#endif
        pr_err(" pc : "REGFMT" ex1: %ld     faultnum: %ld\n",
               regs->pc, regs->ex1, regs->faultnum);

        dump_stack_regs(regs);
}