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

[Tristan Gingold <gingold@adacore.com>]

> 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.