linux-kernel - Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

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Date:	Sat, 24 Oct 2009 18:19:50 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
 on top of perf events

Frederic Weisbecker wrote:
> This patch rebase the implementation of the breakpoints API on top of
> perf events instances.
> 
> Each breakpoints are now perf events that handle the
> register scheduling, thread/cpu attachment, etc..
> 
> The new layering is now made as follows:
> 
>        ptrace       kgdb      ftrace   perf syscall
>           \          |          /         /
>            \         |         /         /
>                                         /
>             Core breakpoint API        /
>                                       /
>                      |               /
>                      |              /
> 
>               Breakpoints perf events
> 
>                      |
>                      |
> 
>                Breakpoints PMU ---- Debug Register constraints handling
>                                     (Part of core breakpoint API)
>                      |
>                      |
> 
>              Hardware debug registers
> 
> Reasons of this rewrite:
> 
> - Use the centralized/optimized pmu registers scheduling,
>   implying an easier arch integration
> - More powerful register handling: perf attributes (pinned/flexible
>   events, exclusive/non-exclusive, tunable period, etc...)
> 
> Impact:
> 
> - New perf ABI: the hardware breakpoints counters
> - Ptrace breakpoints setting remains tricky and still needs some per
>   thread breakpoints references.

- Broken CONFIG_KVM

> 
> Todo (in the order):
> 

- Unbreak CONFIG_KVM :)

> - Support breakpoints perf counter events for perf tools (ie: implement
>   perf_bpcounter_event())
> - Support from perf tools

Since commit 3d53c27d05, KVM uses current->thread.debugregs for
restoring the host state in case the guest played with breakpoints. We
need an equivalent interface to restore ptrace breakpoints and all
others currently in use.

Jan


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