Date: Mon, 8 Jun 2009 10:39:47 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Jaswinder Singh Rajput <jaswinder@kernel.org>,
       "H. Peter Anvin" <hpa@zytor.com>
Cc: x86 maintainers <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [GIT-PULL -tip] x86: xpu_debug patches
Message-ID: <20090608083947.GC6372@elte.hu>
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* Jaswinder Singh Rajput <jaswinder@kernel.org> wrote:

> Ingo,
> 
> Please pull cpu_debug patches :
> 
> The following changes since commit 5095f59bda6793a7b8f0856096d6893fe98e0e51:
>   Jaswinder Singh Rajput (1):
>         x86: cpu_debug: Remove model information to reduce encoding-decoding
> 
> are available in the git repository at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/jaswinder/linux-2.6-cpu.git x86/cpu
> 
> Jaswinder Singh Rajput (5):
>       x86: cpu_debug update Kconfig entry
>       x86: cpu_debug.c remove some not required header files
>       x86: cpu_debug.c use a WARN_ONCE() instead of a pr_err()
>       x86: cpu_debug make room to support more categories
>       x86: cpu_debug update MSR list to support new architectures
> 
>  arch/x86/Kconfig                 |   12 ++++-
>  arch/x86/include/asm/cpu_debug.h |  102 +++++++++++++-------------------------
>  arch/x86/kernel/cpu/cpu_debug.c  |   45 +++++++++-------
>  3 files changed, 70 insertions(+), 89 deletions(-)
> 
> Complete diff:
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index a6efe0a..cd2debd 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -955,9 +955,19 @@ config X86_CPUID
>  
>  config X86_CPU_DEBUG
>  	tristate "/sys/kernel/debug/x86/cpu/* - CPU Debug support"
> +	select DEBUG_FS
>  	---help---
>  	  If you select this option, this will provide various x86 CPUs
> -	  information through debugfs.
> +	  information through debugfs. Any user can read these file but writing
> +	  needs root privilege.
> +
> +	  Note: 1. If you compile cpu_debug as a module, it will _not_ be loaded
> +	  automatically (like usual drivers). You will need to load it manually
> +	  (or add it to list of modules loaded during boot).
> +
> +	  2. You need debugfs, if you want to mount debugfs automatically
> +	  append this line in /etc/fstab:
> +	  debugfs	/sys/kernel/debug	debugfs	defaults	0 0

Those look like good fixes - but before we start extending on all 
this - a more general question has to be raised: do people find this 
useful?

I had a look at the current /debug/x86/cpu*/ layout, and a fair 
portion of it looks completely meaningless at the moment: there's a 
lot of hexa fields with absolutely zero symbolic information about 
what the fields actually mean.

There's a few good exceptions like /debug/x86/cpu/cpu0/apic/apic or 
/debug/x86/cpu/cpu0/cr/cr, although even those should probably be 
renamed to:

  /debug/x86/cpu/cpu0/apic/state
  /debug/x86/cpu/cpu0/cr/state

as 'apic/apic' and 'cr/cr' is meaningless.

Or better yet, why isnt there a directory structure to known values:

  /debug/x86/cpu/cpu0/apic/LVTERR/value

?

This would allow the changing/reading of all known values.

The 'total state' could still be accessed easily as well, via:

  cat /debug/x86/cpu/cpu0/apic/*/value

Paradoxically, all the _unknown_ values have a split out per 
register layout. I.e. the namespace is bloated with a lot of 
'unknown' state - while the 'known' (and printed/interpreted) 
registers are hidden and collected into meaninglessly named files. 
It should be exactly the other way around.

All in one, this code either needs to become useful in practice, 
needs to provide a nice, structured, intuitive 'view' of the CPU's 
current state (as known to the kernel) - or it should not be done at 
all.

If we want a quirky interface we can use msr-tools and /sbin/rdmsr 
just fine. So /debug/x86/cpu/ really has to become a strikingly 
intuitive big step forward in terms of CPU state enumeration and 
visualization, for it to make sense to be carried in the kernel.

	Ingo
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