gdb command: qemu handlers

A new gdb commands are added:

  qemu handlers

     That dumps an AioContext list (by default qemu_aio_context)
     possibly including a backtrace for cases it knows about
     (with the verbose option).  Intended to help find why something
     is hanging waiting for IO.

  Use 'qemu handlers --verbose iohandler_ctx'  to find out why
your incoming migration is stuck.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1445951385-11924-1-git-send-email-dgilbert@redhat.com

V2:
  Merge into one command with optional handlers arg, and only do
    backtrace in verbose mode

 (gdb) qemu handlers
 ----
 {pfd = {fd = 6, events = 25, revents = 0}, io_read = 0x55869656ffd0
 <event_notifier_dummy_cb>, io_write = 0x0, deleted = 0, opaque =
 0x558698c4ce08, node = {le_next = 0x0, le_prev = 0x558698c4cdc0}}

 (gdb) qemu handlers iohandler_ctx
 ----
 {pfd = {fd = 9, events = 25, revents = 0}, io_read = 0x558696581380
 <fd_coroutine_enter>, io_write = 0x0, deleted = 0, opaque =
 0x558698dc99d0, node = {le_next = 0x558698c4cca0, le_prev =
 0x558698c4c1d0}}
 ----
 {pfd = {fd = 4, events = 25, revents = 0}, io_read = 0x55869657b330
 <sigfd_handler>, io_write = 0x0, deleted = 0, opaque = 0x4, node =
 {le_next = 0x558698c4c260, le_prev = 0x558699f72508}}
 ----
 {pfd = {fd = 5, events = 25, revents = 0}, io_read = 0x55869656ffd0
 <event_notifier_dummy_cb>, io_write = 0x0, deleted = 0, opaque =
 0x558698c4c218, node = {le_next = 0x0, le_prev = 0x558698c4ccc8}}
 ----
 (gdb) qemu handlers --verbose iohandler_ctx
 ----
 {pfd = {fd = 9, events = 25, revents = 0}, io_read = 0x558696581380
 <fd_coroutine_enter>, io_write = 0x0, deleted = 0, opaque =
 0x558698dc99d0, node = {le_next = 0x558698c4cca0, le_prev =
 0x558698c4c1d0}}
 #0  0x0000558696581820 in qemu_coroutine_switch
 (from_=from_@entry=0x558698cb3cf0, to_=to_@entry=0x7f421c37eac8,
 action=action@entry=COROUTINE_YIELD) at
 /home/dgilbert/git/qemu/coroutine-ucontext.c:177
 #1  0x0000558696580c00 in qemu_coroutine_yield () at
 /home/dgilbert/git/qemu/qemu-coroutine.c:145
 #2  0x00005586965814f5 in yield_until_fd_readable (fd=9) at
 /home/dgilbert/git/qemu/qemu-coroutine-io.c:90
 #3  0x0000558696523937 in socket_get_buffer (opaque=0x55869a3dc620,
 buf=0x558698c505a0 "", pos=<optimized out>, size=32768) at
 /home/dgilbert/git/qemu/migration/qemu-file-unix.c:101
 #4  0x0000558696521fac in qemu_fill_buffer (f=0x558698c50570) at
 /home/dgilbert/git/qemu/migration/qemu-file.c:227
 #5  0x0000558696522989 in qemu_peek_byte (f=0x558698c50570, offset=0)
     at /home/dgilbert/git/qemu/migration/qemu-file.c:507
 #6  0x0000558696522bf4 in qemu_get_be32 (f=0x558698c50570) at
 /home/dgilbert/git/qemu/migration/qemu-file.c:520
 #7  0x0000558696522bf4 in qemu_get_be32 (f=f@entry=0x558698c50570)
     at /home/dgilbert/git/qemu/migration/qemu-file.c:604
 #8  0x0000558696347e5c in qemu_loadvm_state (f=f@entry=0x558698c50570)
     at /home/dgilbert/git/qemu/migration/savevm.c:1821
 #9  0x000055869651de8c in process_incoming_migration_co
 (opaque=0x558698c50570)
     at /home/dgilbert/git/qemu/migration/migration.c:336
 #10 0x000055869658188a in coroutine_trampoline (i0=<optimized out>,
 i1=<optimized out>)
     at /home/dgilbert/git/qemu/coroutine-ucontext.c:80
 #11 0x00007f420f05df10 in __start_context () at /lib64/libc.so.6
 #12 0x00007ffc40815f50 in  ()
 #13 0x0000000000000000 in  ()

  ----
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/qemu-gdb.py

@@ -26,7 +26,7 @@
 
 sys.path.append(os.path.dirname(__file__))
 
-from qemugdb import mtree, coroutine
+from qemugdb import aio, mtree, coroutine
 
 class QemuCommand(gdb.Command):
     '''Prefix for QEMU debug support commands'''
@@ -37,6 +37,7 @@ def __init__(self):
 QemuCommand()
 coroutine.CoroutineCommand()
 mtree.MtreeCommand()
+aio.HandlersCommand()
 
 coroutine.CoroutineSPFunction()
 coroutine.CoroutinePCFunction()

scripts/qemugdb/aio.py

@@ -0,0 +1,58 @@
+#!/usr/bin/python
+
+# GDB debugging support: aio/iohandler debug
+#
+# Copyright (c) 2015 Red Hat, Inc.
+#
+# Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
+#
+# This work is licensed under the terms of the GNU GPL, version 2 or
+# later.  See the COPYING file in the top-level directory.
+#
+
+import gdb
+from qemugdb import coroutine
+
+def isnull(ptr):
+    return ptr == gdb.Value(0).cast(ptr.type)
+
+def dump_aiocontext(context, verbose):
+    '''Display a dump and backtrace for an aiocontext'''
+    cur = context['aio_handlers']['lh_first']
+    # Get pointers to functions we're going to process specially
+    sym_fd_coroutine_enter = gdb.parse_and_eval('fd_coroutine_enter')
+
+    while not isnull(cur):
+        entry = cur.dereference()
+        gdb.write('----\n%s\n' % entry)
+        if verbose and cur['io_read'] == sym_fd_coroutine_enter:
+            coptr = (cur['opaque'].cast(gdb.lookup_type('FDYieldUntilData').pointer()))['co']
+            coptr = coptr.cast(gdb.lookup_type('CoroutineUContext').pointer())
+            coroutine.bt_jmpbuf(coptr['env']['__jmpbuf'])
+        cur = cur['node']['le_next'];
+
+    gdb.write('----\n')
+
+class HandlersCommand(gdb.Command):
+    '''Display aio handlers'''
+    def __init__(self):
+        gdb.Command.__init__(self, 'qemu handlers', gdb.COMMAND_DATA,
+                             gdb.COMPLETE_NONE)
+
+    def invoke(self, arg, from_tty):
+        verbose = False
+        argv = gdb.string_to_argv(arg)
+
+        if len(argv) > 0 and argv[0] == '--verbose':
+            verbose = True
+            argv.pop(0)
+
+        if len(argv) > 1:
+            gdb.write('usage: qemu handlers [--verbose] [handler]\n')
+            return
+
+        if len(argv) == 1:
+            handlers_name = argv[0]
+        else:
+            handlers_name = 'qemu_aio_context'
+        dump_aiocontext(gdb.parse_and_eval(handlers_name), verbose)