RISC-V: decr_pc_after_break causing problems

[Jim Wilson <jimw@sifive.com>]

The RISC-V port in the riscv-tdep.c file has
  set_gdbarch_decr_pc_after_break (gdbarch, (has_compressed_isa ? 2 : 4));

The privileged architecture spec v1.10 states in section 3.2.1 that
the ebreak instruction causes the receiving privilege mode's epc
register to be set to the address of the ebreak instruction, not the
address of the following instruction.  So gdb should not be
decrementing from the pc after a breakpoint is hit.

It isn't clear why this code is even here, as it isn't present in the
original gdb port in the github riscv/riscv-binutils-gdb tree.

Curiously, there is a corresponding bug in the riscv linux kernel
sources, where it is adding 4 to the sepc in the breakpoint trap
handling code for no apparent reason.  This might be OK if this was a
4-byte breakpoint instruction, but is not OK if this is a 2-byte
breakpoint instruction.

In order to get compressed breakpoints working on a SiFive HiFive
Unleashed board running linux, I need both the gdb and the linux
kernel bugs fixed.  The 4-byte breakpoint instruction works OK now,
but is not safe to use in code compiled with compressed instructions.
A good example is in the shared library support where _dl_debug_state
is a 2-byte function located 2-bytes before _dl_debug_initialize, so
placing a 4-byte breakpoint at _dl_debug_state overwrites the first
two bytes of the first instruction of _dl_debug_initialize causing it
to segfault.

I can submit patches for gdb and the linux kernel, but it would be
useful to know why gdb is trying to subtract from the pc after a
break.  Maybe someone has a part that doesn't conform to the v1.10
privilege architecture spec?  I noticed that this epc == breakpoint
address is not stated in earlier versions of the spec, which makes
earlier spec versions potentially ambiguous.  If we need to support
parts that don't conform to v1.10 priv spec then that makes the fix
more complicated.  It isn't clear how gdb is supposed to detect
whether a part conforms or not.  Maybe we can add an option to turn
this decrementing on
or off?  Maybe a configure option to select whether it is on/off by default?

There is another problem here incidentally that there is an option to
turn on/off compressed breakpoints, but it doesn't affect the amount
we subtract from the pc, which means this option can't work as
currently written.  This problem goes away if we stop decrementing the
pc in gdb.  If we have to keep the code that decrements the pc for
some targets, then maybe we should just eliminate the option.  It
isn't safe to use 4-byte breakpoints in code with compressed
instructions anyways.  And there is no point in using 2-byte
breakpoints in code with no compressed instructions.

Jim