[RFC:PATCH 00/03] powerpc: Expose BookE debug registers through extended ptrace interface

[Kumar Gala]

On Dec 10, 2009, at 8:29 PM, Dave Kleikamp wrote:

> On Thu, 2009-12-10 at 20:24 -0600, Kumar Gala wrote:
>> On Dec 10, 2009, at 9:57 AM, Dave Kleikamp wrote:
>> 
>>> These patches implement an extention to the ptrace interface proposed by
>>> Thiago Bauermann and the the PowerPC gdb team.
>>> 
>>> GDB intends to support the following hardware debug features of BookE
>>> processors:
>>> 
>>> 4 hardware breakpoints (IAC)
>>> 2 hardware watchpoints (read, write and read-write) (DAC)
>>> 2 value conditions for the hardware watchpoints (DVC)
>>> 
>>> For that, we need to extend ptrace so that GDB can query and set these
>>> resources. Since we're extending, we're trying to create an interface
>>> that's extendable and that covers both BookE and server processors, so
>>> that GDB doesn't need to special-case each of them. We propose the
>>> following 3 new ptrace requests described below.
>>> 
>>> There have been discussions of a generic hardware debug interface for the
>>> kernel which would hopefully contemplate all the functionality below and
>>> supersede it.  But we need something that works now, and which enables GDB
>>> to be simpler and work with both Server and Embedded processors without
>>> special cases.
>>> 
>>> 1. PTRACE_PPC_GETHWDEBUGINFO
>>> 
>>> Query for GDB to discover the hardware debug features. The main info to
>>> be returned here is the minimum alignment for the hardware watchpoints.
>>> BookE processors don't have restrictions here, but server processors have
>>> an 8-byte alignment restriction for hardware watchpoints. We'd like to avoid
>>> adding special cases to GDB based on what it sees in AUXV.
>>> 
>>> Since we're at it, we added other useful info that the kernel can return to
>>> GDB: this query will return the number of hardware breakpoints, hardware
>>> watchpoints and whether it supports a range of addresses and a condition.
>>> The query will fill the following structure provided by the requesting process:
>>> 
>>> struct ppc_debug_info {
>>>      unit32_t version;
>>>      unit32_t num_instruction_bps;
>>>      unit32_t num_data_bps;
>>>      unit32_t num_condition_regs;
>>>      unit32_t data_bp_alignment;
>>>      unit32_t sizeof_condition; /* size of the DVC register */
>>>      uint64_t features; /* bitmask of the individual flags */
>>> };
>>> 
>>> features will have bits indicating whether there is support for:
>>> 
>>> #define PPC_DEBUG_FEATURE_INSN_BP_RANGE		0x1
>>> #define PPC_DEBUG_FEATURE_INSN_BP_MASK		0x2
>>> #define PPC_DEBUG_FEATURE_DATA_BP_RANGE		0x4
>>> #define PPC_DEBUG_FEATURE_DATA_BP_MASK		0x8
>>> 
>>> 2. PTRACE_SETHWDEBUG
>>> 
>>> Sets a hardware breakpoint or watchpoint, according to the provided structure:
>>> 
>>> struct ppc_hw_breakpoint {
>>>       uint32_t version;
>>> #define PPC_BREAKPOINT_TRIGGER_EXECUTE  0x1
>>> #define PPC_BREAKPOINT_TRIGGER_READ     0x2
>>> #define PPC_BREAKPOINT_TRIGGER_WRITE    0x4
>>>       uint32_t trigger_type;       /* only some combinations allowed */
>>> #define PPC_BREAKPOINT_MODE_EXACT               0x0
>>> #define PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE     0x1
>>> #define PPC_BREAKPOINT_MODE_RANGE_EXCLUSIVE     0x2
>>> #define PPC_BREAKPOINT_MODE_MASK                0x3
>>>       uint32_t addr_mode;          /* address match mode */
>>> 
>>> #define PPC_BREAKPOINT_CONDITION_NONE   0x0
>>> #define PPC_BREAKPOINT_CONDITION_AND    0x1
>>> #define PPC_BREAKPOINT_CONDITION_EXACT  0x1	/* different name for the same thing as above */
>>> #define PPC_BREAKPOINT_CONDITION_OR     0x2
>>> #define PPC_BREAKPOINT_CONDITION_AND_OR 0x3
>>> #define PPC_BREAKPOINT_CONDITION_BE_ALL 0x00ff0000	/* byte enable bits */
>>> #define PPC_BREAKPOINT_CONDITION_BE(n)  (1<<((n)+16))
>>>       uint32_t condition_mode;     /* break/watchpoint condition flags */
>>> 
>>>       uint64_t addr;
>>>       uint64_t addr2;
>>>       uint64_t condition_value;
>>> };
>>> 
>>> A request specifies one event, not necessarily just one register to be set.
>>> For instance, if the request is for a watchpoint with a condition, both the
>>> DAC and DVC registers will be set in the same request.
>>> 
>>> With this GDB can ask for all kinds of hardware breakpoints and watchpoints
>>> that the BookE supports. COMEFROM breakpoints available in server processors
>>> are not contemplated, but that is out of the scope of this work.
>>> 
>>> ptrace will return an integer (handle) uniquely identifying the breakpoint or
>>> watchpoint just created. This integer will be used in the PTRACE_DELHWDEBUG
>>> request to ask for its removal. Return -ENOSPC if the requested breakpoint
>>> can't be allocated on the registers.
>>> 
>>> Some examples of using the structure to:
>>> 
>>> - set a breakpoint in the first breakpoint register
>>> 
>>> p.version         = PPC_DEBUG_CURRENT_VERSION;
>>> p.trigger_type    = PPC_BREAKPOINT_TRIGGER_EXECUTE;
>>> p.addr_mode       = PPC_BREAKPOINT_MODE_EXACT;
>>> p.condition_mode  = PPC_BREAKPOINT_CONDITION_NONE;
>>> p.addr            = (uint64_t) address;
>>> p.addr2           = 0;
>>> p.condition_value = 0;
>>> 
>>> - set a watchpoint which triggers on reads in the second watchpoint register
>>> 
>>> p.version         = PPC_DEBUG_CURRENT_VERSION;
>>> p.trigger_type    = PPC_BREAKPOINT_TRIGGER_READ;
>>> p.addr_mode       = PPC_BREAKPOINT_MODE_EXACT;
>>> p.condition_mode  = PPC_BREAKPOINT_CONDITION_NONE;
>>> p.addr            = (uint64_t) address;
>>> p.addr2           = 0;
>>> p.condition_value = 0;
>>> 
>>> - set a watchpoint which triggers only with a specific value
>>> 
>>> p.version         = PPC_DEBUG_CURRENT_VERSION;
>>> p.trigger_type    = PPC_BREAKPOINT_TRIGGER_READ;
>>> p.addr_mode       = PPC_BREAKPOINT_MODE_EXACT;
>>> p.condition_mode  = PPC_BREAKPOINT_CONDITION_AND | PPC_BREAKPOINT_CONDITION_BE_ALL;
>>> p.addr            = (uint64_t) address;
>>> p.addr2           = 0;
>>> p.condition_value = (uint64_t) condition;
>>> 
>>> - set a ranged hardware breakpoint
>>> 
>>> p.version         = PPC_DEBUG_CURRENT_VERSION;
>>> p.trigger_type    = PPC_BREAKPOINT_TRIGGER_EXECUTE;
>>> p.addr_mode       = PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE;
>>> p.condition_mode  = PPC_BREAKPOINT_CONDITION_NONE;
>>> p.addr            = (uint64_t) begin_range;
>>> p.addr2           = (uint64_t) end_range;
>>> p.condition_value = 0;
>>> 
>>> 3. PTRACE_DELHWDEBUG
>>> 
>>> Takes an integer which identifies an existing breakpoint or watchpoint
>>> (i.e., the value returned from PTRACE_SETHWDEBUG), and deletes the
>>> corresponding breakpoint or watchpoint..
>> 
>> This is a good write up.  We should have it as a commit message for the first patch.
> 
> I have to give credit to Thiago for this.
> 
> Would it be worth adding to Documentation/powerpc/ ?

That would also work.

- k