[Qemu-devel] [Bug 757702] Re: Undefined instruction exception starts at offset 0x8 instead of 0x4

[Peter Maydell]

> Also, in the test case hits 0x8 after encountering UNDEF instruction
at 0x100058.

So if you run qemu like this:
qemu-system-arm -M realview-pb-a8 -serial stdio -kernel arm_test.elf -s -S

and run arm-none-gnueabi-gdb with no arguments and in gdb type these
commands:

(gdb) target remote :1234
Remote debugging using :1234
0x00100000 in ?? ()
(gdb) break *0x4
Breakpoint 1 at 0x4
(gdb) break *0x8
Breakpoint 2 at 0x8
(gdb) c
Continuing.

...what does gdb do? 
(For me it says "Breakpoint 1, 0x00000004 in ?? ()" which is what I expect.)

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https://bugs.launchpad.net/bugs/757702

Title:
  Undefined instruction exception starts at offset 0x8 instead of 0x4

Status in QEMU:
  New

Bug description:
  ARMv7a has lot of undefined instruction from its instruction opcode
  space. This undefined instructions are very useful for replacing
  sensitive non-priviledged instructions of guest operating systems
  (virtualization). The undefined instruction exception executes at
  <exception_base> + 0x4, where <exception_base> can be 0x0 or
  0xfff00000. Currently, in qemu 0.14.0 undefined instruction fault at
  0x8 offset instead of 0x4. This was not a problem with qemu 0.13.0,
  seems like this is a new bug. As as example, if we try to execute
  value "0xec019800" in qemu 0.14.0 then it should cause undefined
  exception at <exception_base>+0x4 since "0xec019800" is an undefined
  instruction.