scripts/gdb: fix 'qemu coroutine' when users selects a non topmost stack frame

The code that dumps the stack frame works like that:
* save current registers
* overwrite current registers (including rip/rsp) with coroutine snapshot
  in the jmpbuf
* print backtrace
* restore the saved registers.

If the user has currently selected a non topmost stack frame in gdb,
the above code will still restore the selected frame registers,
but the gdb will then lose the selected frame index, which makes it impossible
to switch back to frame 0, to continue debugging the executable.

Therefore switch temporarily to the topmost frame of the stack
for the above code.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201217155436.927320-2-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/qemugdb/coroutine.py

@@ -70,6 +70,11 @@ def bt_jmpbuf(jmpbuf):
     regs = get_jmpbuf_regs(jmpbuf)
     old = dict()
 
+    # remember current stack frame and select the topmost
+    # so that register modifications don't wreck it
+    selected_frame = gdb.selected_frame()
+    gdb.newest_frame().select()
+
     for i in regs:
         old[i] = gdb.parse_and_eval('(uint64_t)$%s' % i)
 
@@ -81,6 +86,8 @@ def bt_jmpbuf(jmpbuf):
     for i in regs:
         gdb.execute('set $%s = %s' % (i, old[i]))
 
+    selected_frame.select()
+
 def coroutine_to_jmpbuf(co):
     coroutine_pointer = co.cast(gdb.lookup_type('CoroutineUContext').pointer())
     return coroutine_pointer['env']['__jmpbuf']