linux-kernel - Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

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Date:	Tue, 12 Nov 2013 11:53:37 +0530
From:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"steve.capper@...aro.org" <steve.capper@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"Vijaya.Kumar@...iumnetworks.com" <Vijaya.Kumar@...iumnetworks.com>,
	Jiang Liu <liuj97@...il.com>,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

>>> OK, I've ensured that the hw_breakpoint (from perf) can work
>>> with kprobes (from ftrace) at the same address on x86.
>>> So if arm64 already support hw_breakpoint on perf, kprobes should
>>> work with it.
>>
>> Single-stepping on x86 is different to the step behaviour on arm64 afaik. On
>> ARM, we have to manually remove the breakpoint, perform a single-step, then
>> add the breakpoint again. If we re-enable debug exceptions in the kprobe
>> handler, the step will complete early and we'll never step off the
>> breakpoint.
>
> I'm unsure about arm64's debug feature behavior, what does happen when
> it performs a single-step on sw-breakpoint?
>
>> Sandeepa: I think you need to retry Masami's test on the arm64 model, since
>> I'm fairly sure it won't work as expected without some additional code.
>
> OK, anyway, for testing same one, we need to port ftrace first. So the next
> plan is to make a kprobe module to put a probe (which just printk something)
> on a specific function (e.g. vfs_symlink), and run perf record with
> hw-breakpoint as below
>
> $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo
>
> Note that 0xXXXXXX is the address of vfs_symlink.
>
> After that, you can see the message in dmesg and also check the perf result
> with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
> it works)
Thanks for steps, ARM64 ftrace patches are under review on arm mailing
list, I can contact the (linaro) developer implementing ftrace on
what's supported and then figure-out a way to test this concurrency of
kprobes breakpoint and hardware breakpoint.

Thanks,
Sandeepa
>
> Thank you,
>
> --
> Masami HIRAMATSU
> IT Management Research Dept. Linux Technology Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@...achi.com
>
>
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