[Qemu-devel] [RFC] qemu-system-arm: cortex-m gdb registers

[Christopher Friedt <chrisfriedt@gmail.com>]

At least for Cortex-M3 devices (but also M0, M0+, M4, ...), while
JTAG debugging using OpenOCD's built-in GDB server, the general purpose
register layout (i.e. `info reg' in GDB) should contain slightly more than
the usual ARM core registers.
    
The non-addressable core registers that appear in OpenOCD's listing are:
    
    r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, sp (r13), lr (r14),
    pc (r15), xpsr, primask, basepri, faultmask, and control.
    
These registers are well documented in the ARMv7M Architecture Reference
Manual.
    
This change addes preliminary support for those registers via a custom 
qXfer:features:read+ and XML response that follows the GNU convention
documented here [1] for org.gnu.gdb.arm.m-profile and
org.gnu.gdb.arm.m-system.
    
[1] https://goo.gl/NMxlC5

The default behaviour for non-cortex-m will be to mimic the original ARM
behaviour of referring the GDB client to an <xi:include href="arm-core.xml">.

A simple test environment for this patch is to compile and link the following
assembly fragment:

		.syntax unified
		.cpu cortex-m3

		.section .interp
		.word 0x20020000
		.word 0x9

		.text

		.global _start
		.thumb
		.thumb_func
		.type _start, %function
	_start:
		b _start
		.size _start, .-_start

Using the command below:

    arm-none-eabi-gcc -g -O0 -mthumb -march=armv7-m -Wl,-Ttext-segment,0 \
        -static -nostartfiles -o foo foo.S

Launch the resulting binary with qemu:

    qemu-system-arm -S -s -M netduino2 -nographic -kernel foo

Run gdb, attaching to the qemu's GDB server:

    arm-none-eabi-gdb foo
    (gdb) target remote localhost:1234
    Remote debugging using localhost:1234
    0x00000000 in _start ()
    (gdb) info all-registers
    r0             0x0	0
    r1             0x0	0
    r2             0x0	0
    r3             0x0	0
    r4             0x0	0
    r5             0x0	0
    r6             0x0	0
    r7             0x0	0
    r8             0x0	0
    r9             0x0	0
    r10            0x0	0
    r11            0x0	0
    r12            0x0	0
    sp             0xbffef7fc	0xbffef7fc
    lr             0x0	0
    pc             0x0	0x0 <_start>
    xpsr           0x40000000	1073741824
    msp            0xbffef7fc	0xbffef7fc
    psp            0x0	0x0 <_start>
    primask        0x0	0
    basepri        0x0	0
    faultmask      0x1	1
    control        0x0	0

The changes have been tested with other firmware images and also via Eclipse
Mars.

Christopher Friedt (1):
  qemu-system-arm: cortex-m gdb registers

 gdbstub.c            |  29 ++++---
 include/qom/cpu.h    |   1 +
 target-arm/cpu-qom.h |   4 +
 target-arm/cpu.c     |   5 +-
 target-arm/gdbstub.c | 215 +++++++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 241 insertions(+), 13 deletions(-)

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