Re: [Xen-devel] [RFC PATCH 2/2] build: convert crash_debug to Kconfig

[Doug Goldstein <cardoe@xxxxxxxxxx>]

On 3/25/16 2:49 PM, Konrad Rzeszutek Wilk wrote:
> On Thu, Mar 24, 2016 at 11:48:19AM -0500, Doug Goldstein wrote:
>> Convert the crash_debug option to Kconfig as CONFIG_CRASH_DEBUG. This
>> was previously togglable on the command line so this adds a message for
>> users enabling it from the command line to tell them to enable it from
>> make menuconfig.
>>
>> Signed-off-by: Doug Goldstein <cardoe@xxxxxxxxxx>
>> ---
>> This is an example of using the debug menu.
>>
>>
>> CC: Keir Fraser <keir@xxxxxxx>
>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>>  docs/misc/crashdb.txt          |  4 ++--
>>  xen/Kconfig.debug              | 11 +++++++++++
>>  xen/Rules.mk                   |  5 +++--
>>  xen/arch/x86/Makefile          |  3 +--
>>  xen/arch/x86/x86_64/Makefile   |  2 +-
>>  xen/common/Makefile            |  2 +-
>>  xen/include/asm-x86/debugger.h |  2 +-
>>  xen/include/xen/gdbstub.h      |  2 +-
>>  8 files changed, 21 insertions(+), 10 deletions(-)
>>
>> diff --git a/docs/misc/crashdb.txt b/docs/misc/crashdb.txt
>> index b41a538..9733666 100644
>> --- a/docs/misc/crashdb.txt
>> +++ b/docs/misc/crashdb.txt
>> @@ -5,7 +5,7 @@ Xen has a simple gdb stub for doing post-mortem debugging 
>> i.e. once
>>  you've crashed it, you get to poke around and find out why.  There's
>>  also a special key handler for making it crash, which is handy.
>>  
>> -You need to have crash_debug=y set when compiling , and you also need
>> +You need to have CRASH_DEBUG=y set when compiling, and you also need
>>  to enable it on the Xen command line, eg by gdb=com1.
>>  
>>  If you need to have a serial port shared between gdb and the console,
>> @@ -19,7 +19,7 @@ if you have a simple null modem connection between the 
>> test box and
>>  the workstation, and aren't using a H/L split console:
>>  
>>    * Set debug=y in Config.mk
>> -  * Set crash_debug=y in xen/Rules.mk
>> +  * Set CRASH_DEBUG=y with `make -C xen menuconfig`
>>    * Make the changes in the attached patch, and build.
>>    * Arrange to pass gdb=com1 as a hypervisor command line argument
>>      (I already have com1=38400,8n1 console=com1,vga sync_console)
>> diff --git a/xen/Kconfig.debug b/xen/Kconfig.debug
>> index 36890bd..3336a64 100644
>> --- a/xen/Kconfig.debug
>> +++ b/xen/Kconfig.debug
>> @@ -4,3 +4,14 @@ menuconfig DEBUG
>>      ---help---
>>        If you want to debug Xen say Y and select any additional debugging
>>        support options.
>> +
>> +if DEBUG
> 
> Perhaps if !defined then atuomatically enable it? Looking at Config.mk
> it seems you could do crash_debug without debug=y?

debug=y unfortunately is more than just a "meta" option that selects
verbose=y and frame_pointer=y. It also turns off NDEBUG so that debug
messages appear. I'm not sure how that should be mapped in the context
of this patch. Should the messages be enabled when DEBUG is enabled or
should there be another option?

If debug messages are enabled with DEBUG that would certainly be a
behavior change since now someone could have crash_debug=y debug=n but
I'm not sure if that's desired?

> 
> Why is this called crash_debug and not 'crash_gdb' ..? Ah [after reading the 
> docs]
> it can't do breakpoints or any of that. Just to pour over the data after a 
> crash.
> Ah, you can resume the hypervisor after this. OK, definitly not for field
> analysis.
> 
> And kexec/kdump is much more powerful than this.

yeah I wondered that myself as well and came to the conclusion it was
limited. Good point about kexec/kdump. I should definitely update the
description to be more verbose.

> 
> 
>> +
>> +config CRASH_DEBUG
>> +    bool "Crash Debugging Support"
>> +    depends on X86
>> +    ---help---
>> +      If you want to be able to attach gdb to Xen to be able to debug
>> +      Xen if it crashes then say Y.
> 
> Should it have a link to the docs ? On how to use it?

yeah I think maybe just pointing someone to docs/misc/crashdb.txt cause
its not immediately obvious that's the doc file to go with crash_debug.

> 
> And maybe mention that kexec/kdump is better suited for capturing the whole
> machine and one can do post-mortem analysis much more intensly?

Yeah that's a really good point. I didn't think of that.


-- 
Doug Goldstein

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