linux-kernel - Re: [rfc] x86, bts: improve X86_PTRACE

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Message-ID: <ada1vyhc9ik.fsf@cisco.com>
Date:	Wed, 15 Oct 2008 23:57:23 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Markus Metzger <markus.t.metzger@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	markus.t.metzger@...il.com
Subject: Re: [rfc] x86, bts: improve X86_PTRACE_BTS help text

Thanks for improving the help text; it looks much better.

One comment:

 >  config X86_DS
 > -	bool "Debug Store support"
 > -	default y
 > -	help
 > -	  Add support for Debug Store.
 > -	  This allows the kernel to provide a memory buffer to the hardware
 > -	  to store various profiling and tracing events.
 > +	def_bool X86_PTRACE_BTS
 > +	depends on X86_DEBUGCTLMSR

using def_bool here to get this turned on if X86_PTRACE_BTS is set
doesn't seem scalable to more uses of X86_DS.  I would have thought that
having X86_PTRACE_BTS select X86_DS would make more sense.

Or are other users of X86_DS not planned?  In which case should there be
a separate X86_DS option at all?

 > +	  This adds a ptrace interface to the hardware's branch trace store.
 > +
 > +	  Debuggers may use it to collect an execution trace of the debugged
 > +	  application in order to answer the question 'how did I get here?'.
 > +	  Debuggers may trace user mode as well as kernel mode.
 > +
 > +	  Say Y unless there is no application development on this machine
 > +	  and you want to save a small amount of code size.

This looks much better -- someone like me could read this and make an
informed decision about the config option.

Thanks,
  Roland
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