Date:	Thu, 28 May 2009 16:15:18 +1000
From:	David Gibson <dwg@....ibm.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [Patch 02/12] Introducing generic hardware breakpoint handler
	interfaces

On Mon, May 11, 2009 at 05:22:40PM +0530, K.Prasad wrote:
> This patch introduces the generic Hardware Breakpoint interfaces for
> both user and kernel space requests.

[snip]
> Index: kernel/hw_breakpoint.c
> ===================================================================
> --- /dev/null
> +++ kernel/hw_breakpoint.c
> @@ -0,0 +1,367 @@
> +/*
> + * 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, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright (C) 2007 Alan Stern
> + * Copyright (C) IBM Corporation, 2009
> + */
> +
> +/*
> + * HW_breakpoint: a unified kernel/user-space hardware breakpoint facility,
> + * using the CPU's debug registers.
> + *
> + * This file contains the arch-independent routines.  It is not meant
> + * to be compiled as a standalone source file; rather it should be
> + * #include'd by the arch-specific implementation.
> + */
> +
> +#include <linux/irqflags.h>
> +#include <linux/kallsyms.h>
> +#include <linux/notifier.h>
> +#include <linux/kprobes.h>
> +#include <linux/kdebug.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/percpu.h>
> +#include <linux/mutex.h>
> +#include <linux/sched.h>
> +#include <linux/init.h>
> +#include <linux/smp.h>
> +
> +#include <asm/hw_breakpoint.h>
> +#include <asm/processor.h>
> +
> +#ifdef CONFIG_X86

That's a nasty #ifdef.  Surely this include should instead go in the
x86 version of hw_breakpoint.h.

> +#include <asm/debugreg.h>
> +#endif
> +/*
> + * Spinlock that protects all (un)register operations over kernel/user-space
> + * breakpoint requests
> + */
> +DEFINE_SPINLOCK(hw_breakpoint_lock);
> +
> +/* Array of kernel-space breakpoint structures */
> +struct hw_breakpoint *hbp_kernel[HB_NUM];
> +
> +/*
> + * Per-processor copy of hbp_kernel[]. Used only when hbp_kernel is being
> + * modified but we need the older copy to handle any hbp exceptions. It will
> + * sync with hbp_kernel[] value after updation is done through IPIs.
> + */
> +DEFINE_PER_CPU(struct hw_breakpoint*, this_hbp_kernel[HB_NUM]);

Ok.. this seems to assume that all breakpoints are equal as far as
allocation goes - i.e. that the total number of breakpoints is the
only allocation limit.  That's apparently true for x86, but on
embedded powerpc (4xx), for example, we can have up to 2 data
breakpoints and up to 4 instruction breakpoints independent of each
other.

> +/*
> + * Kernel breakpoints grow downwards, starting from HB_NUM
> + * 'hbp_kernel_pos' denotes lowest numbered breakpoint register occupied for
> + * kernel-space request. We will initialise it here and not in an __init
> + * routine because load_debug_registers(), which uses this variable can be
> + * called very early during CPU initialisation.
> + */
> +unsigned int hbp_kernel_pos = HB_NUM;

Likewise here.  For 44x we would need separate tables and separate
pointers for data and instruction breakpoints.

> +/*
> + * An array containing refcount of threads using a given bkpt register
> + * Accesses are synchronised by acquiring hw_breakpoint_lock
> + */
> +unsigned int hbp_user_refcount[HB_NUM];
> +
> +/*
> + * Install the debug register values for a new thread.
> + */
> +void switch_to_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	/* Set the debug register */
> +	arch_install_thread_hw_breakpoint(tsk);

Um.. and the point of this wrapper which does nothing extra around the
arch-specific function would be...?

> +}
> +
> +/*
> + * Install the debug register values for just the kernel, no thread.
> + */
> +void switch_to_none_hw_breakpoint(void)
> +{
> +	arch_uninstall_thread_hw_breakpoint();

And again.

> +}
> +
> +/*
> + * Load the debug registers during startup of a CPU.
> + */
> +void load_debug_registers(void)

This name does not suggest a boot and/or cpu hotplug only function.

> +{
> +	unsigned long flags;
> +	struct task_struct *tsk = current;
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +
> +	/* Prevent IPIs for new kernel breakpoint updates */
> +	local_irq_save(flags);
> +	arch_update_kernel_hw_breakpoint(NULL);
> +	local_irq_restore(flags);
> +
> +	if (test_tsk_thread_flag(tsk, TIF_DEBUG))
> +		switch_to_thread_hw_breakpoint(tsk);
> +
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +
> +/*
> + * Erase all the hardware breakpoint info associated with a thread.
> + *
> + * If tsk != current then tsk must not be usable (for example, a
> + * child being cleaned up from a failed fork).
> + */
> +void flush_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	int i;
> +	struct thread_struct *thread = &(tsk->thread);
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +
> +	/* The thread no longer has any breakpoints associated with it */
> +	clear_tsk_thread_flag(tsk, TIF_DEBUG);
> +	for (i = 0; i < HB_NUM; i++) {
> +		if (thread->hbp[i]) {
> +			hbp_user_refcount[i]--;
> +			kfree(thread->hbp[i]);
> +			thread->hbp[i] = NULL;
> +		}
> +	}
> +
> +	arch_flush_thread_hw_breakpoint(tsk);
> +
> +	/* Actually uninstall the breakpoints if necessary */
> +	if (tsk == current)
> +		switch_to_none_hw_breakpoint();
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +
> +/*
> + * Copy the hardware breakpoint info from a thread to its cloned
> child.

This comment contradicts what the code actually does.

> + */
> +int copy_thread_hw_breakpoint(struct task_struct *tsk,
> +		struct task_struct *child, unsigned long clone_flags)
> +{
> +	/*
> +	 * We will assume that breakpoint settings are not inherited
> +	 * and the child starts out with no debug registers set.
> +	 * But what about CLONE_PTRACE?
> +	 */
> +	clear_tsk_thread_flag(child, TIF_DEBUG);
> +
> +	/* We will call flush routine since the debugregs are not inherited */
> +	arch_flush_thread_hw_breakpoint(child);

> +
> +	return 0;
> +}
> +
> +int __register_user_hw_breakpoint(int pos, struct task_struct *tsk,
> +					struct hw_breakpoint *bp)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +	int rc;
> +
> +	/* Do not overcommit. Fail if kernel has used the hbp registers */
> +	if (pos >= hbp_kernel_pos)
> +		return -ENOSPC;

Again, this is generic code making a non-portable assumption about how
breakpoints must be allocated.

> +	rc = arch_validate_hwbkpt_settings(bp, tsk);
> +	if (rc)
> +		return rc;
> +
> +	thread->hbp[pos] = bp;
> +	hbp_user_refcount[pos]++;
> +
> +	arch_update_user_hw_breakpoint(pos, tsk);
> +	/*
> +	 * Does it need to be installed right now?
> +	 * Otherwise it will get installed the next time tsk runs
> +	 */
> +	if (tsk == current)
> +		switch_to_thread_hw_breakpoint(tsk);
> +
> +	return rc;
> +}
> +
> +/*
> + * Modify the address of a hbp register already in use by the task
> + * Do not invoke this in-lieu of a __unregister_user_hw_breakpoint()
> + */
> +int __modify_user_hw_breakpoint(int pos, struct task_struct *tsk,
> +					struct hw_breakpoint *bp)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +
> +	if ((pos >= hbp_kernel_pos) || (arch_validate_hwbkpt_settings(bp, tsk)))
> +		return -EINVAL;
> +
> +	if (thread->hbp[pos] == NULL)
> +		return -EINVAL;
> +
> +	thread->hbp[pos] = bp;
> +	/*
> +	 * 'pos' must be that of a hbp register already used by 'tsk'
> +	 * Otherwise arch_modify_user_hw_breakpoint() will fail
> +	 */
> +	arch_update_user_hw_breakpoint(pos, tsk);
> +
> +	if (tsk == current)
> +		switch_to_thread_hw_breakpoint(tsk);
> +
> +	return 0;
> +}
> +
> +void __unregister_user_hw_breakpoint(int pos, struct task_struct *tsk)
> +{
> +	hbp_user_refcount[pos]--;
> +	tsk->thread.hbp[pos] = NULL;
> +
> +	arch_update_user_hw_breakpoint(pos, tsk);
> +
> +	if (tsk == current)
> +		switch_to_thread_hw_breakpoint(tsk);
> +}
> +
> +/**
> + * register_user_hw_breakpoint - register a hardware breakpoint for user space
> + * @tsk: pointer to 'task_struct' of the process to which the address belongs
> + * @bp: the breakpoint structure to register
> + *
> + * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> + * @bp->triggered must be set properly before invocation
> + *
> + */
> +int register_user_hw_breakpoint(struct task_struct *tsk,
> +					struct hw_breakpoint *bp)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +	int i, rc = -ENOSPC;
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +
> +	for (i = 0; i < hbp_kernel_pos; i++) {
> +		if (!thread->hbp[i]) {
> +			rc = __register_user_hw_breakpoint(i, tsk, bp);
> +			break;
> +		}
> +	}
> +
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);
> +
> +/**
> + * unregister_user_hw_breakpoint - unregister a user-space hardware breakpoint
> + * @tsk: pointer to 'task_struct' of the process to which the address belongs
> + * @bp: the breakpoint structure to unregister
> + *
> + */
> +void unregister_user_hw_breakpoint(struct task_struct *tsk,
> +						struct hw_breakpoint *bp)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +	int i;
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +	for (i = 0; i < hbp_kernel_pos; i++) {
> +		if (bp == thread->hbp[i]) {
> +			__unregister_user_hw_breakpoint(i, tsk);
> +			break;
> +		}
> +	}
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +EXPORT_SYMBOL_GPL(unregister_user_hw_breakpoint);
> +
> +/**
> + * register_kernel_hw_breakpoint - register a hardware breakpoint for kernel space
> + * @bp: the breakpoint structure to register
> + *
> + * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> + * @bp->triggered must be set properly before invocation
> + *
> + */
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> +	int rc;
> +
> +	rc = arch_validate_hwbkpt_settings(bp, NULL);
> +	if (rc)
> +		return rc;
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +
> +	rc = -EINVAL;
> +	/* Check if we are over-committing */
> +	if ((hbp_kernel_pos > 0) && (!hbp_user_refcount[hbp_kernel_pos-1])) {
> +		hbp_kernel_pos--;
> +		hbp_kernel[hbp_kernel_pos] = bp;
> +		on_each_cpu(arch_update_kernel_hw_breakpoint, NULL, 1);
> +		rc = 0;
> +	}
> +
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(register_kernel_hw_breakpoint);
> +
> +/**
> + * unregister_kernel_hw_breakpoint - unregister a HW breakpoint for kernel space
> + * @bp: the breakpoint structure to unregister
> + *
> + * Uninstalls and unregisters @bp.
> + */
> +void unregister_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> +	int i, j;
> +
> +	spin_lock_bh(&hw_breakpoint_lock);
> +
> +	/* Find the 'bp' in our list of breakpoints for kernel */
> +	for (i = hbp_kernel_pos; i < HB_NUM; i++)
> +		if (bp == hbp_kernel[i])
> +			break;
> +
> +	/* Check if we did not find a match for 'bp'. If so return early */
> +	if (i == HB_NUM) {
> +		spin_unlock_bh(&hw_breakpoint_lock);
> +		return;
> +	}
> +
> +	/*
> +	 * We'll shift the breakpoints one-level above to compact if
> +	 * unregistration creates a hole
> +	 */
> +	for (j = i; j > hbp_kernel_pos; j--)
> +		hbp_kernel[j] = hbp_kernel[j-1];
> +
> +	hbp_kernel[hbp_kernel_pos] = NULL;
> +	on_each_cpu(arch_update_kernel_hw_breakpoint, NULL, 1);
> +	hbp_kernel_pos++;
> +
> +	spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +EXPORT_SYMBOL_GPL(unregister_kernel_hw_breakpoint);
> +
> +static struct notifier_block hw_breakpoint_exceptions_nb = {
> +	.notifier_call = hw_breakpoint_exceptions_notify,
> +	/* we need to be notified first */
> +	.priority = 0x7fffffff
> +};
> +
> +static int __init init_hw_breakpoint(void)
> +{
> +	return register_die_notifier(&hw_breakpoint_exceptions_nb);
> +}
> +
> +core_initcall(init_hw_breakpoint);
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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