Document revision date: 19 July 1999

• Loading the watchpoint driver
• Creating and deleting watchpoints
• Looking at watchpoint driver data
• Acquiring collected watchpoint data
• Looking at the protection attributes and access fault mechanism
• Looking at some watchpoint restrictions

This chapter describes the Watchpoint utility, which enables monitoring of write access to user-specified locations. The chapter contains the following sections:

Section 9.1 presents an introduction of the Watchpoint facility.

Section 9.2 describes how to load the watchpoint driver.

Section 9.3 describes the creation and deletion of watchpoints and the constraints upon watchpoint locations.

Section 9.4 contains detailed descriptions of the watchpoint driver data structures, knowledge of which may be required to analyze collected watchpoint data.

Section 9.5 discusses acquiring collected watchpoint data.

Section 9.6 describes the watchpoint protection facility.

Section 9.7 describes its restrictions.

9.1 Introduction

A watchpoint is a data field to which write access is monitored. The field is from 1 to 8 bytes long and must be contained within a single page. Typically, watchpoints are in a nonpaged pool. However, subject to certain constraints (see Section 9.3.1), they can be defined in other areas of system space. The Watchpoint facility can simultaneously monitor a large number (50 or more) watchpoints.

The facility is implemented in the WPDRIVER device driver and the utility program WP. This document concentrates on the device driver, which can be invoked directly or through the WP utility.

For information on the WP utility, see its help files, which can be displayed with the following DCL command:

$ HELP/LIBRARY=SYS$HELP:WP

Once the driver has been loaded, a suitably privileged user can designate a watchpoint in system space. Any write to a location designated as a watchpoint is trapped. Information is recorded about the write, including its time, the register contents, and the program counter (PC) and processor status longword (PSL) of the writing instruction. Optionally, one or both of the following user-specified actions can be taken:

• An XDELTA breakpoint¹ or system code debugger breakpoint which occurs just after the write to the watchpoint
• A fatal watchpoint bugcheck which occurs just after the write to the watchpoint

A user defines a watchpoint by issuing QIO requests to the watchpoint driver; entering commands to the WP utility, which issues requests to the driver; or, from kernel mode code, invoking a routine within the watchpoint driver.

The information recorded about writes to a watchpoint is stored in WPDRIVER data structures. It can be obtained through QIO requests to the WPDRIVER, commands to the WP utility, XDELTA commands issued during a requested breakpoint, or SDA commands issued during the analysis of a requested crashdump.

9.2 Initializing the Watchpoint Facility

From a process with CMKRNL privilege, run the SYSMAN Utility to load the watchpoint driver, SYS$WPDRIVER.EXE. Enter the following commands:

$ RUN SYS$SYSTEM:SYSMAN SYSMAN> IO CONNECT WPA0:/NOADAPTER/DRIVER=SYS$WPDRIVER SYSMAN> EXIT

SYSMAN creates system I/O data structures for the pseudodevice WPA0, loads WPDRIVER, and invokes its initialization routines. WPDRIVER initialization includes the following actions:

• Allocating nonpaged pool and physical memory for WPDRIVER data structures
• Appropriating the SCB vector specific to access violations
• Recording in system space the addresses of the WPDRIVER routines invoked by kernel mode code to create and delete watchpoints

Memory requirements for WPDRIVER and its data structures are:

• Device driver and UCB---approximately 3K bytes of nonpaged pool
• Trace table and a related array---36 bytes for each of SYSGEN parameter WPTTE_SIZE trace table entries
• Watchpoint restore entries---SYSGEN parameter WPRE_SIZE pages of physically contiguous memory
• Each watchpoint---176 bytes of nonpaged pool

It is advisable to load the watchpoint driver relatively soon after system initialization to ensure its allocation of physically contiguous memory. If the driver cannot allocate enough physically contiguous memory, it does not set WPA0: online. If the unit is offline, you will not be able to use the watchpoint facility.

9.3 Creating and Deleting Watchpoints

There are three different ways to create and delete watchpoints:

• An image can assign a channel to device WPA0: and then request the Queue I/O Request ($QIO) system service to create or delete a watchpoint.
• Code running in kernel mode can dispatch directly to routines within the WPDRIVER to create and delete watchpoints.
• A user can enter commands to the WP utility.

The first two methods are described in detail in the sections that follow.

9.3.1 Using the $QIO Interface

An image first assigns a channel to pseudodevice WPA0: and then issues a $QIO request on that channel. The process must have the privilege PHY_IO. Otherwise, the $QIO request is rejected with the error SS$_NOPRIV.

Table 9-1 shows the functions that the driver supports.

Table 9-1 Driver Supported Functions

Function	Activity
IO$_ACCESS	Creates a watchpoint
IO$_DEACCESS	Deletes a watchpoint
IO$_RDSTATS	Receives trace information on a watchpoint

The IO$_ACCESS function requires the following device/function dependent arguments:

• P2---Length of the watchpoint. A number larger than 8 is reduced to 8.
• P3---Starting address of the watchpoint area.

There are a number of constraints on the watchpoint area. It must be:

• Nonpageable system space.
• Write-accessible from kernel mode.
• Within one page. If it is not, the requested length is reduced to what will fit within the page containing the starting address.
• Within a page accessed only from kernel mode and by instructions which incur no pagefaults.
• Within a page whose protection is not altered while the watchpoint is in place.
• Outside of certain address ranges. These are the WPDRIVER code, its data structures, and the system page table.

Because of the current behavior of the driver, there is an additional requirement that there be no "unexpected" access violations referencing a page containing a watchpoint. See Section 9.7 for further details.

To specify that an XDELTA breakpoint or a fatal bugcheck occur if the watchpoint is written, use the following I/O function code modifiers:

• IO$M_CTRL to request an XDELTA breakpoint
• IO$M_ABORT to request a fatal bugcheck

For an XDELTA breakpoint to be taken, OpenVMS must have been booted specifying that XDELTA and/or the system code debugger be resident (bit 1 or bit 15 in the boot flags must be set). If both watchpoint options are requested, the XDELTA breakpoint is taken first. At exit from the breakpoint, the driver crashes the system.

A request to create a watchpoint can succeed completely, succeed partially, or fail. Table 9-2 shows the status codes that can be returned in the I/O status block.

Table 9-2 Returned Status Codes

Status Code	Meaning
SS$_NORMAL	Success.
SS$_BUFFEROVF	A watchpoint was established, but its length is less than was requested because the requested watchpoint would have straddled a page boundary.
SS$_EXQUOTA	The watchpoint could not be created because too many watchpoints already exist.
SS$_INSFMEM	The watchpoint could not be created because there was insufficient nonpaged pool to create data structures specific to this watchpoint.
SS$_IVADDR	The requested watchpoint resides in one of the areas in which the WPDRIVER is unable to create watchpoints.
SS$_WASSET	An existing watchpoint either coincides or overlaps with the requested watchpoint.

The following example MACRO program assigns a channel to the WPA0 device and creates a watchpoint of 4 bytes, at starting address 80001068. The program requests neither an XDELTA breakpoint nor a system crash for that watchpoint.

$IODEF .PSECT RWDATA,NOEXE,RD,WRT,LONG ; WP_IOSB: .BLKL 2 ; I/O status block. WP_ADDR: .LONG ^X80001068 ; Address of watchpoint to create. WP_NAM: .ASCID /WPA0:/ ; Device to which to assign channel. WP_CHAN: .BLKW 1 ; Channel number. .PSECT PROG,EXE,NOWRT ; START: .CALL_ENTRY $ASSIGN_S DEVNAM=WP_NAM,CHAN=WP_CHAN BLBC R0,RETURN $QIOW_S CHAN=WP_CHAN,- FUNC=#IO$_ACCESS,- IOSB=WP_IOSB,- P2=#4,- P3=WP_ADDR MOVL WP_IOSB,R0 ; Move status to R0. RETURN: RET ; Return to caller. .END START

A watchpoint remains in effect until it is explicitly deleted. (Note, however, that watchpoint definitions do not persist across system reboots.) To delete an existing watchpoint, issue an IO$_DEACCESS QIO request.

The IO$_DEACCESS function requires the following device/function dependent argument: P3 - Starting address of the watchpoint to be deleted.

Table 9-3 shows the status values that are returned in the I/O status block.

Table 9-3 Returned Status Values

Status Value	Meaning
SS$_NORMAL	Success.
SS$_IVADDR	The specified watchpoint does not exist.

Section 9.5 describes the use of the IO$_RDSTATS QIO request.

9.3.2 Invoking WPDRIVER Entry Points from System Routines

When the WPDRIVER is loaded, it initializes two locations in system space with the addresses of routines within the driver. These locations, WP$CREATE_WATCHPOINT and WP$DELETE_WATCHPOINT, enable dispatch to create and delete watchpoint routines within the loaded driver. Input arguments for both routines are passed in registers.

Code running in kernel mode can execute the following instructions:

JSB @G^WP$CREATE_WATCHPOINT ; create a watchpoint

and

JSB @G^WP$DELETE_WATCHPOINT ; delete a watchpoint

Both these routines save IPL at entry and set it to the fork IPL of the WPDRIVER, IPL 11. Thus, they should not be invoked by code threads running above IPL 11. At exit, the routines restore the entry IPL.

These two locations contain an RSB instruction prior to the loading of the driver. As a result, if a system routine tries to create or delete a watchpoint before the WPDRIVER is loaded, control immediately returns.

WP$CREATE_WATCHPOINT has the following register arguments:

• R0---User-specified watchpoint options
  • Bit 1 equal to 1 specifies that a fatal OPERCRASH bugcheck should occur after a write to the watchpoint area.
  • Bit 2 equal to 1 specifies that an XDELTA breakpoint should occur after a write to the watchpoint area.
• R1---Length of the watchpoint area
• R2---Starting address of the watchpoint area

Status is returned in R0. The status values and their interpretations are identical to those for the QIO interface to create a watchpoint. The only difference is that the SS$_NOPRIV status cannot be returned with this interface.

WPS$DELETE_WATCHPOINT has the following register argument: R2 - Starting address of the watchpoint area

Status is returned in R0. The status values and their interpretations are identical to those for the QIO interface.

9.4 Data Structures

The WPDRIVER uses three different kinds of data structures:

• One watchpoint restore entry (WPRE) for each page of system space in which one or more active watchpoints are located
• One watchpoint control block (WPCB) for each active watchpoint
• Trace table entries (WPTTEs) in a circular trace buffer which maintains a history of watchpoint writes

These data structures are described in detail and illustrated in the sections that follow.

9.4.1 Watchpoint Restore Entry (WPRE)

There is one WPRE for each system page that contains a watchpoint. That is, if nine watchpoints are defined which are in four different system pages, four WPREs are required to describe those pages. When WPDRIVER is loaded, its initialization routine allocates physically contiguous memory for the maximum number of WPREs. The number of pages to be allocated is specified by SYSGEN parameter WPRE_SIZE.

The WPDRIVER allocates WPREs starting at the beginning of the table and maintains a tightly packed list. That is, when a WPRE in the middle of those in use is "deallocated," its current contents are replaced with the contents of the last WPRE in use. The number in use at any given time is in the driver variable WP$L_WP_COUNT. The system global EXE$GA_WP_WPRE points to the beginning of the WPRE table.

The WPRE for a page contains information useful for:

• Determining whether a given access violation refers to an address in the page associated with this WPRE
• Restoring the original SPTE value for the associated page
• Reestablishing the modified SPTE value when watchpoints are reenabled
• Invalidating the translation buffer when the SPTE is modified
• Locating the data structures associated with individual watchpoints defined in this system page

The WPCBs associated with a given system page are singly-linked to a list header in the associated WPRE. A WPCB is allocated from a nonpaged pool when a watchpoint is created. A WPCB contains static information about the watchpoint such as:

• Its starting address and length
• Original contents of the watchpoint at the time it was established
• User-specified options for this watchpoint

In addition, the WPCB contains dynamic data associated with the most recent write reference to the watchpoint. This data includes:

• Number of times that the watchpoint has been written.
• Address of the first byte within the watchpoint that was modified at the last write reference.
• PC-PSL pair that made the last write reference.
• System time at the last write reference.
• Contents of the general registers at the time of the last write reference.
• A copy of up to 15 bytes of instruction stream data beginning at the program counter (PC) of the instruction that made the last write reference. The amount of instruction stream data that is copied here is the lesser of 15 bytes and the remaining bytes on the page containing the PC.
• Contents of the watchpoint before the last write reference.
• Contents of the watchpoint after the last write reference. This value is presumably the current contents of the watchpoint.
• A pointer to an entry in the global circular trace buffer where all recent references to watchpoints are traced.

Whenever a watchpoint is written, all the relevant data is recorded in the WPCB associated with the watchpoint. In addition, to maintain a history, the WPDRIVER copies a subset of the data to the oldest WPTTE in the circular trace buffer. Thus, the circular trace buffer contains a history of the last N references to watchpoints. The driver allocates nonpaged pool to accommodate the number of trace table entries specified by the SYSGEN parameter WPTTE_SIZE. The WPTTEs for all watchpoints are together in the table, but the ones for a particular watchpoint are chained together.

The subset of data in a WPTTE includes:

• Starting address of the watchpoint
• Relative offset of the first byte modified on this reference
• Opcode of the instruction that modified the watchpoint
• A relative backptr to the previous WPTTE of this watchpoint
• PC-PSL of the write reference
• System time of the write reference
• Contents of the watchpoint before this reference

Analyzing watchpoint results is a function of the mode in which the WPDRIVER is used. For example, if you have only one watchpoint and have specified that an XDELTA breakpoint and/or a bugcheck occur on a write to the watchpoint, then when the reference occurs, simply find the program counter (PC) that caused the reference.

This PC (actually the PC of the next instruction) and its processor status longword (PSL) are on the stack at the time of the breakpoint and/or bugcheck. The layout that follows is the stack as it appears within an XDELTA breakpoint. Examined from a crash dump, the stack is similar but does not contain the return address from the JSB to INI$BRK.

+--------------------------------------+ |address in WPDRIVER from JSB G^INI$BRK| :SP |PC of next instruction | |PSL at watchpoint access | +--------------------------------------+

Furthermore, R0 contains the address of the WPCB associated with that watchpoint. You can examine the WPCB to determine the original contents of the watchpoint area and the registers at the time of the write.

Definitions for the watchpoint data structures are in SYS$LIBRARY:LIB.MLB. Build an object module with their symbol definitions by entering the following DCL commands:

$MACRO/OBJ=SYS$LOGIN:WPDEFS SYS$INPUT: + SYS$LIBRARY:LIB/LIB $WPCBDEF GLOBAL !n.b. GLOBAL must be capitalized $WPREDEF GLOBAL $WPTTEDEF GLOBAL .end CTRL/Z

Then, within SDA, you can format watchpoint data structures. For example, enter the following SDA commands:

SDA>READ SYS$LOGIN:WPDEFS.OBJ SDA>FORMAT @R0 /TYPE=WPCB !type definition is required SDA>DEF WPTTE = @R0 + WPCB$L_TTE SDA>FORMAT WPTTE /TYPE=WPTTE

An alternative to crashing the system or using XDELTA to get watchpoint information is the QIO function IO$_RDSTAT. This function returns watchpoint control block contents and trace table entries for a particular watchpoint.

It requires the following device/function dependent arguments:

• P1---Address of buffer to receive watchpoint data.
• P2---Length of the buffer. The minimum size buffer of 188 bytes is only large enough for WPCB contents.
• P3---Watchpoint address.

The data returned in the buffer has the format shown in Figure 9-1.

Figure 9-1 Format of Data Returned in Buffer

+---------------------------------------------+ | Number of bytes copied to buffer | +---------------------------------------------+ | Total number of WPTTEs for watchpoint | +---------------------------------------------+ | Number of WPTTEs copied to buffer | +---------------------------------------------+ | WPCB | +---------------------------------------------+ | Most recent WPTTE | +---------------------------------------------+ | Next recent WPTTE | +---------------------------------------------+ | Next WPTTE | +---------------------------------------------+ ~ Next WPTTE ~ +---------------------------------------------+

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