Date:	Thu, 28 May 2009 15:28:59 +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 01/12] Prepare the code for Hardware Breakpoint
	interfaces

On Mon, May 11, 2009 at 05:22:13PM +0530, K.Prasad wrote:
> This patch introduces header files containing constants, structure
> definitions and declaration of functions used by the hardware
> breakpoint interface code.

Hrm.  I don't really like splitting prototypes from their
implementations as you have done.  Patches in a series should be split
into logical, self-contained units, not just sliced up by file.

[snip]
> Index: include/asm-generic/hw_breakpoint.h
> ===================================================================
> --- /dev/null
> +++ include/asm-generic/hw_breakpoint.h
> @@ -0,0 +1,139 @@
> +#ifndef	_ASM_GENERIC_HW_BREAKPOINT_H
> +#define	_ASM_GENERIC_HW_BREAKPOINT_H

Is there a reason this is asm-generic, rather than
linux/hw_breakpoint.h?

> +#ifndef __ARCH_HW_BREAKPOINT_H
> +#error "Please don't include this file directly"
> +#endif
> +
> +#ifdef	__KERNEL__
> +#include <linux/list.h>
> +#include <linux/types.h>
> +#include <linux/kallsyms.h>
> +
> +/**
> + * struct hw_breakpoint - unified kernel/user-space hardware breakpoint
> + * @triggered: callback invoked after target address access

"triggered" seems an odd name to me - it suggests a boolean flag, not
a function pointer.

> + * @info: arch-specific breakpoint info (address, length, and type)
> + *
> + * %hw_breakpoint structures are the kernel's way of representing
> + * hardware breakpoints.  These are data breakpoints
> + * (also known as "watchpoints", triggered on data access), and the breakpoint's
> + * target address can be located in either kernel space or user space.

I thought this infrastructure could also handle instruction
breakpoints.  The comment above seems to contradict that.

> + * The breakpoint's address, length, and type are highly
> + * architecture-specific.  The values are encoded in the @info field; you
> + * specify them when registering the breakpoint.  To examine the encoded
> + * values use hw_breakpoint_get_{kaddress,uaddress,len,type}(), declared
> + * below.
> + *
> + * The address is specified as a regular kernel pointer (for kernel-space
> + * breakponts) or as an %__user pointer (for user-space breakpoints).
> + * With register_user_hw_breakpoint(), the address must refer to a
> + * location in user space.  The breakpoint will be active only while the
> + * requested task is running.  Conversely with
> + * register_kernel_hw_breakpoint(), the address must refer to a location
> + * in kernel space, and the breakpoint will be active on all CPUs
> + * regardless of the current task.
> + *
> + * The length is the breakpoint's extent in bytes, which is subject to
> + * certain limitations.  include/asm/hw_breakpoint.h contains macros
> + * defining the available lengths for a specific architecture.  Note that
> + * the address's alignment must match the length.  The breakpoint will
> + * catch accesses to any byte in the range from address to address +
> + * (length - 1).
> + *
> + * The breakpoint's type indicates the sort of access that will cause it
> + * to trigger.  Possible values may include:
> + *
> + * 	%HW_BREAKPOINT_RW (triggered on read or write access),
> + * 	%HW_BREAKPOINT_WRITE (triggered on write access), and
> + * 	%HW_BREAKPOINT_READ (triggered on read access).
> + *
> + * Appropriate macros are defined in include/asm/hw_breakpoint.h; not all
> + * possibilities are available on all architectures.  Execute breakpoints
> + * must have length equal to the special value %HW_BREAKPOINT_LEN_EXECUTE.

Why do instruction breakpoints have a special length, rather than a
special type?  Or do they have both?

> + * When a breakpoint gets hit, the @triggered callback is
> + * invoked in_interrupt with a pointer to the %hw_breakpoint structure and the
> + * processor registers.
> + * Data breakpoints occur after the memory access has taken place.
> + * Breakpoints are disabled during execution @triggered, to avoid
> + * recursive traps and allow unhindered access to breakpointed memory.
> + *
> + * This sample code sets a breakpoint on pid_max and registers a callback
> + * function for writes to that variable.  Note that it is not portable
> + * as written, because not all architectures support HW_BREAKPOINT_LEN_4.
> + *
> + * ----------------------------------------------------------------------
> + *
> + * #include <asm/hw_breakpoint.h>
> + *
> + * struct hw_breakpoint my_bp;
> + *
> + * static void my_triggered(struct hw_breakpoint *bp, struct pt_regs *regs)
> + * {
> + * 	printk(KERN_DEBUG "Inside triggered routine of breakpoint exception\n");
> + * 	dump_stack();
> + *  	.......<more debugging output>........
> + * }
> + *
> + * static struct hw_breakpoint my_bp;
> + *
> + * static int init_module(void)
> + * {
> + *	..........<do anything>............
> + *	my_bp.info.type = HW_BREAKPOINT_WRITE;
> + *	my_bp.info.len = HW_BREAKPOINT_LEN_4;
> + *
> + *	my_bp.installed = (void *)my_bp_installed;
> + *
> + *	rc = register_kernel_hw_breakpoint(&my_bp);
> + *	..........<do anything>............
> + * }
> + *
> + * static void cleanup_module(void)
> + * {
> + *	..........<do anything>............
> + *	unregister_kernel_hw_breakpoint(&my_bp);
> + *	..........<do anything>............
> + * }
> + *
> + * ----------------------------------------------------------------------
> + */
> +struct hw_breakpoint {
> +	void (*triggered)(struct hw_breakpoint *, struct pt_regs *);
> +	struct arch_hw_breakpoint info;
> +};
> +
> +/*
> + * len and type values are defined in include/asm/hw_breakpoint.h.
> + * Available values vary according to the architecture.  On i386 the
> + * possibilities are:
> + *
> + *	HW_BREAKPOINT_LEN_1
> + *	HW_BREAKPOINT_LEN_2
> + *	HW_BREAKPOINT_LEN_4
> + *	HW_BREAKPOINT_RW
> + *	HW_BREAKPOINT_READ
> + *
> + * On other architectures HW_BREAKPOINT_LEN_8 may be available, and the
> + * 1-, 2-, and 4-byte lengths may be unavailable.  There also may be
> + * HW_BREAKPOINT_WRITE.  You can use #ifdef to check at compile time.
> + */
> +
> +int register_user_hw_breakpoint(struct task_struct *tsk,
> +					struct hw_breakpoint *bp);
> +void unregister_user_hw_breakpoint(struct task_struct *tsk,
> +						struct hw_breakpoint *bp);
> +/*
> + * Kernel breakpoints are not associated with any particular thread.
> + */
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp);
> +void unregister_kernel_hw_breakpoint(struct hw_breakpoint *bp);
> +void switch_to_none_hw_breakpoint(void);
> +
> +extern unsigned int hbp_kernel_pos;
> +extern spinlock_t hw_breakpoint_lock;
> +
> +#endif	/* __KERNEL__ */
> +#endif	/* _ASM_GENERIC_HW_BREAKPOINT_H */
> Index: arch/x86/include/asm/hw_breakpoint.h
> ===================================================================
> --- /dev/null
> +++ arch/x86/include/asm/hw_breakpoint.h
> @@ -0,0 +1,66 @@
> +#ifndef	_I386_HW_BREAKPOINT_H
> +#define	_I386_HW_BREAKPOINT_H
> +
> +#ifdef	__KERNEL__
> +#define	__ARCH_HW_BREAKPOINT_H
> +
> +struct arch_hw_breakpoint {
> +	char		*name; /* Contains name of the symbol to set bkpt */
> +	unsigned long	address;
> +	u8		len;
> +	u8		type;
> +};
> +
> +#include <linux/kdebug.h>
> +#include <asm-generic/hw_breakpoint.h>
> +
> +/* Available HW breakpoint length encodings */
> +#define HW_BREAKPOINT_LEN_1		0x40
> +#define HW_BREAKPOINT_LEN_2		0x44
> +#define HW_BREAKPOINT_LEN_4		0x4c
> +#define HW_BREAKPOINT_LEN_EXECUTE	0x40
> +
> +#ifdef CONFIG_X86_64
> +#define HW_BREAKPOINT_LEN_8		0x48
> +#endif
> +
> +/* Available HW breakpoint type encodings */
> +
> +/* trigger on instruction execute */
> +#define HW_BREAKPOINT_EXECUTE	0x80
> +/* trigger on memory write */
> +#define HW_BREAKPOINT_WRITE	0x81
> +/* trigger on memory read or write */
> +#define HW_BREAKPOINT_RW	0x83

What is the reason for making the encoding of all these values
arch-specific?  How would you set up the encoding for an arch where
different CPU variants have different debug facilities (as we do on
powerpc).

> +/* Total number of available HW breakpoint registers */
> +#define HB_NUM 4
> +
> +extern struct hw_breakpoint *hbp_kernel[HB_NUM];
> +DECLARE_PER_CPU(struct hw_breakpoint*, this_hbp_kernel[HB_NUM]);
> +extern unsigned int hbp_user_refcount[HB_NUM];
> +
> +/*
> + * Ptrace support: breakpoint trigger routine.
> + */
> +int __register_user_hw_breakpoint(int pos, struct task_struct *tsk,
> +			struct hw_breakpoint *bp);
> +int __modify_user_hw_breakpoint(int pos, struct task_struct *tsk,
> +			struct hw_breakpoint *bp);
> +void __unregister_user_hw_breakpoint(int pos, struct task_struct *tsk);
> +
> +void arch_install_thread_hw_breakpoint(struct task_struct *tsk);
> +void arch_uninstall_thread_hw_breakpoint(void);
> +int arch_check_va_in_userspace(unsigned long va, u8 hbp_len);
> +int arch_check_va_in_kernelspace(unsigned long va, u8 hbp_len);
> +int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
> +						struct task_struct *tsk);
> +void arch_update_user_hw_breakpoint(int pos, struct task_struct *tsk);
> +void arch_flush_thread_hw_breakpoint(struct task_struct *tsk);
> +void arch_update_kernel_hw_breakpoint(void *);
> +int hw_breakpoint_handler(struct die_args *args);
> +int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
> +				     unsigned long val, void *data);

It looks like most if not all of these functions should have the same
prototype (though a different implementation) on all archs.  In which
case the prototypes should go in a generic header.

> +#endif	/* __KERNEL__ */
> +#endif	/* _I386_HW_BREAKPOINT_H */
> +
> Index: arch/x86/include/asm/debugreg.h
> ===================================================================
> --- arch/x86/include/asm/debugreg.h.orig
> +++ arch/x86/include/asm/debugreg.h
> @@ -18,6 +18,7 @@
>  #define DR_TRAP1	(0x2)		/* db1 */
>  #define DR_TRAP2	(0x4)		/* db2 */
>  #define DR_TRAP3	(0x8)		/* db3 */
> +#define DR_TRAP_BITS	(DR_TRAP0|DR_TRAP1|DR_TRAP2|DR_TRAP3)
>  
>  #define DR_STEP		(0x4000)	/* single-step */
>  #define DR_SWITCH	(0x8000)	/* task switch */
> @@ -49,6 +50,8 @@
>  
>  #define DR_LOCAL_ENABLE_SHIFT 0    /* Extra shift to the local enable bit */
>  #define DR_GLOBAL_ENABLE_SHIFT 1   /* Extra shift to the global enable bit */
> +#define DR_LOCAL_ENABLE (0x1)      /* Local enable for reg 0 */
> +#define DR_GLOBAL_ENABLE (0x2)     /* Global enable for reg 0 */
>  #define DR_ENABLE_SIZE 2           /* 2 enable bits per register */
>  
>  #define DR_LOCAL_ENABLE_MASK (0x55)  /* Set  local bits for all 4 regs */
> @@ -67,4 +70,32 @@
>  #define DR_LOCAL_SLOWDOWN (0x100)   /* Local slow the pipeline */
>  #define DR_GLOBAL_SLOWDOWN (0x200)  /* Global slow the pipeline */
>  
> +/*
> + * HW breakpoint additions
> + */
> +#ifdef __KERNEL__
> +
> +/* For process management */
> +void flush_thread_hw_breakpoint(struct task_struct *tsk);
> +int copy_thread_hw_breakpoint(struct task_struct *tsk,
> +		struct task_struct *child, unsigned long clone_flags);
> +void dump_thread_hw_breakpoint(struct task_struct *tsk, int u_debugreg[8]);
> +void switch_to_thread_hw_breakpoint(struct task_struct *tsk);
> +
> +/* For CPU management */
> +void load_debug_registers(void);
> +static inline void hw_breakpoint_disable(void)
> +{
> +	/* Zero the control register for HW Breakpoint */
> +	set_debugreg(0UL, 7);
> +
> +	/* Zero-out the individual HW breakpoint address registers */
> +	set_debugreg(0UL, 0);
> +	set_debugreg(0UL, 1);
> +	set_debugreg(0UL, 2);
> +	set_debugreg(0UL, 3);
> +}
> +
> +#endif	/* __KERNEL__ */
> +
>  #endif /* _ASM_X86_DEBUGREG_H */
> Index: arch/x86/include/asm/processor.h
> ===================================================================
> --- arch/x86/include/asm/processor.h.orig
> +++ arch/x86/include/asm/processor.h
> @@ -29,6 +29,7 @@ struct mm_struct;
>  #include <linux/threads.h>
>  #include <linux/init.h>
>  
> +#define HB_NUM 4

Uh.. this #define appears to be duplicated in asm-x86/hw_breakpoint.h
and here.

>  /*
>   * Default implementation of macro that returns current
>   * instruction pointer ("program counter").
> @@ -435,8 +436,11 @@ struct thread_struct {
>  	unsigned long		debugreg1;
>  	unsigned long		debugreg2;
>  	unsigned long		debugreg3;
> +	unsigned long		debugreg[HB_NUM];
>  	unsigned long		debugreg6;
>  	unsigned long		debugreg7;
> +	/* Hardware breakpoint info */
> +	struct hw_breakpoint	*hbp[HB_NUM];
>  	/* Fault info: */
>  	unsigned long		cr2;
>  	unsigned long		trap_no;
> 

-- 
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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