scripts/gdb/symbols: make lx-symbols skip the s390 decompressor

When one starts QEMU with the -S flag and attaches GDB, the kernel is
not yet loaded, and the current instruction is an entry point to the
decompressor. In case the intention is to debug the early kernel boot,
and not the decompressor, e.g., put a breakpoint on some kernel
function and see all the invocations, one has to skip the decompressor.

There are many ways to do this, and so far people wrote private scripts
or memorized certain command sequences.

Make it work out of the box like this:

    $ gdb -ex 'target remote :6812' -ex 'source vmlinux-gdb.py' vmlinux
    Remote debugging using :6812
    0x0000000000010000 in ?? ()
    (gdb) lx-symbols
    loading vmlinux
    (gdb) x/i $pc
    => 0x3ffe0100000 <startup_continue>:    lghi    %r2,0

Implement this by reading the address of the jump_to_kernel() function
from the lowcore, and step until DAT is turned on.

Signed-off-by: Ilya Leoshkevich <[email protected]>
Acked-by: Jan Kiszka <[email protected]>
Tested-by: Alexander Gordeev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexander Gordeev <[email protected]>

Author: iii-i

scripts/gdb/linux/symbols.py

@@ -84,6 +84,30 @@ def get_kerneloffset():
     return None
 
 
+def is_in_s390_decompressor():
+    # DAT is always off in decompressor. Use this as an indicator.
+    # Note that in the kernel, DAT can be off during kexec() or restart.
+    # Accept this imprecision in order to avoid complicating things.
+    # It is unlikely that someone will run lx-symbols at these points.
+    pswm = int(gdb.parse_and_eval("$pswm"))
+    return (pswm & 0x0400000000000000) == 0
+
+
+def skip_decompressor():
+    if utils.is_target_arch("s390"):
+        if is_in_s390_decompressor():
+            # The address of the jump_to_kernel function is statically placed
+            # into svc_old_psw.addr (see ipl_data.c); read it from there. DAT
+            # is off, so we do not need to care about lowcore relocation.
+            svc_old_pswa = 0x148
+            jump_to_kernel = int(gdb.parse_and_eval("*(unsigned long long *)" +
+                                                    hex(svc_old_pswa)))
+            gdb.execute("tbreak *" + hex(jump_to_kernel))
+            gdb.execute("continue")
+            while is_in_s390_decompressor():
+                gdb.execute("stepi")
+
+
 class LxSymbols(gdb.Command):
     """(Re-)load symbols of Linux kernel and currently loaded modules.
 
@@ -204,6 +228,8 @@ def load_all_symbols(self):
             saved_state['breakpoint'].enabled = saved_state['enabled']
 
     def invoke(self, arg, from_tty):
+        skip_decompressor()
+
         self.module_paths = [os.path.abspath(os.path.expanduser(p))
                              for p in arg.split()]
         self.module_paths.append(os.getcwd())