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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-05-15 14:18, Markus Armbruster wrote:
> Peter Maydell <peter.maydell@linaro.org> writes:
> 
>> On 15 May 2015 at 08:58, Markus Armbruster <armbru@redhat.com> 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?

>  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").

Jan

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