Re: [Qemu-devel] [PATCH 0/3] scripts/qemu-gdb: Add event tracing support

[Markus Armbruster]

Jan Kiszka <address@hidden> writes:

> On 2015-05-15 14:18, Markus Armbruster wrote:
>> Peter Maydell <address@hidden> writes:
>> 
>>> On 15 May 2015 at 08:58, Markus Armbruster <address@hidden> wrote:
>>>> Since you're touching qemu-gdb.py anyway, could you stick in a brief
>>>> comment explaining how to put it to use?
>>>
>>> Good idea. It turns out the answer is just "source it from gdb",
>>> but it took me a little while to find that out, so worth commenting.
>>> I also have a patch which makes it do the 'ignore SIGUSR1' bit
>>> by doing 'handle SIGUSR1 pass noprint nostop' for you.
>> 
>> Here's how to load scripts/qemu-gdb.py automatically:
>> 
>> * Apply the appended patch to turn it into a gdb init file
>> 
>>   That's what it is, after all.  It's not a standalone Python program.
>> 
>> * Tell gdb to trust it
>> 
>>   Add a line like
>> 
>>       add-auto-load-safe-path ~/work/qemu/scripts/qemu-gdb.py
>> 
>>   to your ~/.gdbinit
>> 
>> * Link it into the directory where you run gdb --args qemu...
>> 
>> * Verify it works:
>> 
>>       $ gdb
>>       [...]
>>       (gdb) help qemu
>>       Prefix for QEMU debug support commands
>> 
>>       List of qemu subcommands:
>> 
>>       qemu coroutine -- Display coroutine backtrace
>>       qemu mtree -- Display the memory tree hierarchy
>> 
>>       Type "help qemu" followed by qemu subcommand name for full 
>> documentation.
>>       Type "apropos word" to search for commands related to "word".
>>       Command name abbreviations are allowed if unambiguous.
>>       (gdb) 
>> 
>> If you know a better way to do this, please post it.
>> 
>
> Yep, that's the basic idea behind gdb python scripts: myapp-gdb.py gets
> auto-pulled on "gdb myapp". Since some gdb 7.x, we have that security
> feature above which prevents pulling from arbitrary sources.
>
>> 
>> diff --git a/scripts/qemu-gdb.py b/scripts/qemu-gdb.py
>> index 6c7f4fb..ac3087c 100644
>> --- a/scripts/qemu-gdb.py
>> +++ b/scripts/qemu-gdb.py
>> @@ -1,5 +1,3 @@
>> -#!/usr/bin/python
>> -
>>  # GDB debugging support
>>  #
>>  # Copyright 2012 Red Hat, Inc. and/or its affiliates
>> @@ -13,7 +11,7 @@
>>  # Contributions after 2012-01-13 are licensed under the terms of the
>>  # GNU GPL, version 2 or (at your option) any later version.
>>  
>> -
>> +python
>
> What is this line doing?

Found here:
https://stackoverflow.com/questions/16553283/python-code-in-gdb-init-file-gdbinit

It doesn't work for me without it.

>>  import gdb
>>  
>>  def isnull(ptr):
>> 
>> 
>
> See also linux-4.x/scripts/gdb and
> linux-4.x/Documentation/gdb-kernel-debugging.txt for more references.
>
> For the kernel, we set the link during the build. Given the many aliases
> for qemu and the fact that quite a few developers run from the build
> directory, I guess that would make sense here as well.
>
> Helper functions are another interesting facility to consider. Trivial
> example: the kernel has $container_of(PTR, "TYPE", "ELEMENT").

Neat!