linux-kernel - Re: [RFC PATCH 3/3] dynamic_debug: Add support for dynamic register trace

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Date:   Tue, 7 Aug 2018 17:57:14 +0100
From:   Will Deacon <will.deacon@....com>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Laura Abbott <labbott@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Jason Baron <jbaron@...mai.com>,
        Tony Luck <tony.luck@...el.com>, Arnd Bergmann <arnd@...db.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Joe Perches <joe@...ches.com>,
        Jim Cromie <jim.cromie@...il.com>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [RFC PATCH 3/3] dynamic_debug: Add support for dynamic register
 trace

On Fri, Aug 03, 2018 at 07:58:44PM +0530, Sai Prakash Ranjan wrote:
> Introduce dynamic debug filtering mechanism to register
> tracing as dynamic_rtb() which will reduce a lot of
> overhead otherwise of tracing all the register reads/writes
> in all files.
> 
> Now we can just specify the file name or any wildcard pattern
> as any other dynamic debug facility in bootargs and dynamic rtb
> will just trace them and the output can be seen in pstore.
> 
> TODO: Now we use same 'p' flag but will add a separate flag for register trace
> later.
> 
> Example for tracing all register reads/writes in drivers/soc/qcom/* below:
> 
>   # dyndbg="file drivers/soc/qcom/* +p" in bootargs
>   # reboot -f
>   # mount -t pstore pstore /sys/fs/pstore
>   # cat /sys/fs/pstore/rtb-ramoops-0
>     [LOGK_WRITEL ] ts:1373030419  data:ffff00000d5065a4  <ffff00000867cb44>  qcom_smsm_probe+0x51c/0x668
>     [LOGK_WRITEL ] ts:1373360576  data:ffff00000d506608  <ffff00000867cb44>  qcom_smsm_probe+0x51c/0x668
> 
> Also we add uncached_logk api to readl/writel definitions for arm64
> as of now. This can be extended to arm as well later for tracing.
> 
> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> ---
>  arch/arm64/include/asm/io.h   | 93 +++++++++++++++++++++++++++++++++++

Putting all of this in the arch code, which basically duplicates everything,
feels very wrong to me. Perhaps take a look at the ongoing work for
instrumenting the atomics and take some inspiration from there?

Ideally, the architecture just needs to provide the low-level primivites
(which it already does) and the core can generate instruments versions if
required.

Will