Date:	Thu, 14 Jan 2010 06:10:43 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Joshua Pincus <joshua.pincus@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>, peterz@...radead.org,
	paulus@...ba.org, acme@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: HW breakpoints perf_events request

(Adding Ingo in Cc)


On Wed, Jan 13, 2010 at 05:45:55PM -0800, Joshua Pincus wrote:
> Hi,
> 
> I have a request for an additional feature to be included
> in the recent hardware breakpoints work soon to be delivered
> in kernel 2.6.33.
> 
> Frederic Weisbecker, Prasad Krishnan and I have shared an ongoing
> private discussion on this topic.  Frederic recommended that we take
> the discussion to a wider audience.  Hence this email.  I would like
> to thank both
> of those two engineers for their patience, quick response,
> and help in this process.  They've been stellar.
> 
> I work for a company which would like to make heavy use
> of the new HW breakpoints perf_event work.  Essentially,
> we need to be able to do the following:
> 
> 1) When executed, a user-land application must be able to program 4
> pinned hardware
> breakpoint registers.  I need 1 byte granularity (address length
> specificity) and the ability
> to set RWX event triggers.
> 
> 2) All calls to clone() or fork() will propagate the
> debug register settings from parent to child(ren).
> 
> 3) When a breakpoint is triggered, the application
> thread currently running which triggered the breakpoint
> immediately stops execution and is sent a SIGTRAP.
> 
> 4) The thread transitions from the PC that triggered the breakpoint to
> the signal handler for SIGTRAP.
> 
> 5) The signal handler does some work.  (This "work" is outside the
> scope of my request, but you may have some insights.  I need to be
> able to change the PC and nPC for the thread that triggered a
> breakpoint such that when it
> returns from the signal handler it doesn't return to the instruction
> that triggered the breakpoint but to the one after it.  If I were
> using ptrace(8), I would just have the parent
> process use the ptrace(8) syscall to modify the PC and
> nPC of the child.  I'm not using ptrace(8).)
> 
> 6) The signal handler returns and the thread returns to normal
> execution at the new
> PC and nPC.
> 
> From my discussions with Frederic and Prasad, I know
> that requirements (1) and (2), as described above,
> are already being met by the current work.  I can use
> the perf_event_attr structure and the perf_event syscall
> to program breakpoints for a thread in exactly
> the way I've specified AND be able to have debug
> settings inherited from parent to child in the case of a
> fork() or clone().
> 
> However, there's nothing yet in place to allow a
> signal to be sent to the thread when a breakpoint
> has been hit.  Put another way, there's nothing here
> which affects execution flow of the user-app when the CPU
> traps due to a HW breakpoint.
> 
> Currently, the perf events infrastructure allows me
> to mmap() a block of memory and to poll() on it, waiting
> and watching for events to be described in that mmap'd
> buffer.  That will not be sufficient for what we need
> to do.  We need to have the ability to specify that
> execution of the thread should be interrupted, just as it
> would under ptrace(), and have a signal be delivered.
> Delivery of the signal must be received and processed
> by the application before the thread will be allowed to
> proceed to the nPC after the PC which caused the
> HW breakpoint event.
> 
> Is this possible?  Can we architect this feature into
> the perf_events infrastructure?
> 
> At first glance, it would seem that this is a fairly
> easy thing to do.  Right now, these same HW breakpoints
> triggers under ptrace() merely call a previously registered
> user handler which modifies the debug registers to
> record the event and to propagate a signal to the child
> process being traced.  We want to do something similar
> w/o using ptrace.  Perhaps it is as simple as providing a
> perf_event_attr setting which indicates that SIGTRAP signals
> should be sent to the thread which triggered the breakpoint
> exception.
> 
> Thanks in advance for your help,
> JP

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