[Bug 1877136] Re: Qemu GDB Arm core registers XML description not valid for M-profile

[Peter Maydell]

Patch submitted:
https://patchew.org/QEMU/address@hidden/


** Changed in: qemu
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1877136

Title:
  Qemu GDB Arm core registers XML description not valid for M-profile

Status in QEMU:
  In Progress

Bug description:
  When trying to debug an armv7-m binary running on Qemu, GDB makes some
  mistakes due to mistakenly believing the target is not M-profile.

  One observable is that backtraces over signal handlers are not handled
  correctly -- since the special M-profile EXC_RETURN value is not
  recognised.  That happens because GDB doesn't think the target is
  M-profile.

  This happens because GDB sees a reported feature set from the Qemu
  remote connection that includes the feature `org.gnu.gdb.arm.core`.

  As described in the GDB online docs, for "M-profile targets (e.g. Cortex-M3), 
the ‘org.gnu.gdb.arm.core’ feature is replaced by ‘org.gnu.gdb.arm.m-profile’"
  https://sourceware.org/gdb/current/onlinedocs/gdb/ARM-Features.html

  From a scan of the Qemu source code on commit
  ea1329bb3a8d5cd25b70e3dbf73e7ded4d5ad756 it seems that when emulating
  an arm core it uses `arm-core.xml` unconditionally for
  `CPUClass->gdb_core_xml_file`, and that means the only feature
  provided is `org.gnu.gdb.arm.core`.

  Note that even though there is a command to set the architecture in GDB, 
setting the target architecture to an M-profile core is still not a valid 
workaround.
  This is because the target description overrides everything in setting the 
`is_m` attribute within GDB.

  Reproduction of the observable:
  Using the examples here 
https://git.linaro.org/people/peter.maydell/m-profile-tests.git/tree/ .
  Build the examples, and run 
  ```
  qemu-system-arm -s -S -no-reboot -M lm3s6965evb -m 16 -serial stdio -display 
none -net nic -net user,restrict=on -d guest_errors,unimp -kernel test3-kern.bin
  ```

  Then in a GDB session
  ```
  vshcmd: > arm-none-eabi-gdb -q                                                
                                                                                
                                    
  (gdb)
  vshcmd: > file test3-kern.elf
  Reading symbols from test3-kern.elf...
  (gdb)
  vshcmd: > target remote localhost:1234
  Remote debugging using localhost:1234
  _start () at init-m.S:53
  53        mov r0, #0
  (gdb)
  vshcmd: > show architecture
  The target architecture is set automatically (currently armv7)
  (gdb)
  vshcmd: > break svc
  Breakpoint 1 at 0x6fc: svc. (2 locations)
  (gdb)
  vshcmd: > cont
  Continuing.

  Breakpoint 1, svc () at test3.c:16
  16          int test = SEQ();
  (gdb)
  vshcmd: > bt
  #0  svc () at test3.c:16
  #1  0xfffffff8 in ?? ()
  Backtrace stopped: previous frame identical to this frame (corrupt stack?)
  (gdb)
  vshcmd: > print/x $lr
  $1 = 0xfffffff9
  (gdb)
  ```

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