Re: cortex-m xml register descriptions for m-system

[Pedro Alves <palves@redhat.com>]

On 12/15/2015 11:44 AM, Tristan Gingold wrote:
> 
>> On 15 Dec 2015, at 00:11, Christopher Friedt <chrisfriedt@gmail.com> wrote:
>>
>> On Dec 14, 2015 1:55 PM, "Pedro Alves" <palves@redhat.com> wrote:
>>>
>>> On 12/14/2015 05:04 PM, Christopher Friedt wrote:
>>>> Hi list,
>>>>
>>>> I've been using GDB and OpenOCD to debug ARM Cortex-M devices for
>>>> quite a while. One thing that I always noticed when using OpenOCD is
>>>> that the m-system registers are listed, which is *incredibly* useful
>>>> for writing code on just about any Cortex-M microcontroller.
>>>>
>>>> Somewhat recently, Qemu has also begun to support Cortex-M based
>>>> virtual devices, and it seems to be fairly usable.
>>>>
>>>> The down side, is that they do not expose the m-system registers,
>>>> simply because binutils-gdb does not (at this time) have an XML file
>>>> for them.
>>>>
>>>> Just to catch anyone up to speed who might be reading this, the
>>>> m-system registers are
>>>>
>>>> MSP (main stack pointer)
>>>> PSP (process stack pointer)
>>>> PRIMASK (1-bit register that says if interrupts are enabled)
>>>> BASEPRI (8-bit register that sets the NVIC base priority)
>>>> FAULTMASK (1-bit register that says if fault interrupts are enabled)
>>>> CONTROL (3-bit register that indicates presence of FP, whether PSP is
>>>> selected, and whether running in unprivileged mode)
>>>>
>>>> Now, these are "system" registers, and on a full blown microprocessor,
>>>> it might be unusual to expose them, but on a microcontroller, it's
>>>> quite important. The other debuggers that I have seen (IAR,
>>>> specifically) also list the m-system registers along with the general
>>>> purpose ones for Cortex-M.
>>>>
>>>> The following XML is sufficient to describe the m-system registers so
>>>> that they appear to the GDB client.
>>>>
>>>> <feature name="org.gnu.gdb.arm.m-system">
>>>>  <reg name="msp" bitsize="32" type="data_ptr"/>
>>>>  <reg name="psp" bitsize="32" type="data_ptr"/>
>>>>  <reg name="primask" bitsize="1" type="int8"/>
>>>>  <reg name="basepri" bitsize="8" type="int8"/>
>>>>  <reg name="faultmask" bitsize="1" type="int8"/>
>>>>  <reg name="control" bitsize="3" type="int8"/>
>>>> </feature>
>>>
>>> Does GDB need to be aware of these registers at all?  That is, does gdb
>>> need to be aware of org.gnu.gdb.arm.m-system?  Usually GDB needs to
>>> be aware of specific registers if for instance Dwarf can refer to them.
>>> Otherwise, the design of xml descriptions is such that you're free
>>> to send any additional registers you want without a specific feature.
>>> GDB will show them.
>>
>> Hmm... It's hard for me to say. The MSP and PSP are banked stack
>> pointers, control instructs the core which stack pointer to use, and
>> they are also tightly coupled to exception entry, so I would lean
>> towards yes?
> 
> I do think so too.
> 
> I have just written a patch so that gdb unwinds correctly on cortex-m
> exceptions, and this of course requires that gdb knows about at least
> psp.
> 
> I plan to submit it early January.
> 
> Tristan.
> 


Thanks,
Pedro Alves