cortex-m xml register descriptions for m-system

[Christopher Friedt <chrisfriedt@gmail.com>]

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>

The first question I would ask for clarification from the binutils-gdb
developers, is, which regnum is appropriate to assign to each of those
m-system registers? Should these registers enumerate starting with 26
(resuming from the xpsr)?

Just for comparison, the current binutils-gdb arm-m-profile.xml is
here (https://goo.gl/hpTye8), and the openocd variant is here
(http://goo.gl/FFn56X).

The second question I would like to ask is, what is the best way to
add this XML? Should it

1) Should it be inserted directly into arm-m-profile.xml?
2) Should it be included from arm-m-profile.xml as arm-m-system.xml?

IMHO, the 1st or 2nd option would make sense, as all Cortex-M's
contain these registers.

I'm asking because I have a patch ready to submit for this on a whim's
notice, but would just like to get some buy-in ahead of time.


C