linux-kernel - Re: [PATCHv4 1/2] tracing: Add register read/write tracing support

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Date:   Fri, 19 Nov 2021 20:49:53 +0530
From:   Sai Prakash Ranjan <quic_saipraka@...cinc.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Will Deacon <will@...nel.org>, <rostedt@...dmis.org>,
        Catalin Marinas <catalin.marinas@....com>,
        <quic_psodagud@...cinc.com>, <gregkh@...uxfoundation.org>,
        <arnd@...db.de>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        <mingo@...hat.com>, Prasad Sodagudi <psodagud@...eaurora.org>
Subject: Re: [PATCHv4 1/2] tracing: Add register read/write tracing support

On 11/19/2021 7:47 PM, Marc Zyngier wrote:
> On Fri, 19 Nov 2021 14:07:09 +0000,
> Sai Prakash Ranjan <quic_saipraka@...cinc.com> wrote:
>> On 11/19/2021 7:13 PM, Marc Zyngier wrote:
>>> On Mon, 15 Nov 2021 11:33:29 +0000,
>>> Sai Prakash Ranjan <quic_saipraka@...cinc.com> wrote:
>>>> From: Prasad Sodagudi <psodagud@...eaurora.org>
>>>>
> [...]
>
>>>> Reason why we wouldn't need value along with mmio write log is
>>>> that value can be easily deduced from the caller_name+offset which is
>>>> printed already by the rwmmio trace events which gives the exact
>>>> location of mmio writes and the value is easily known from the driver.
>>> That's a very narrow view of what can be written in an MMIO
>>> registers. We write dynamic values at all times, and if we are able to
>>> trace MMIO writes, then the value written out must be part of the trace.
>>>
>>> I'd rather you try and get to the bottom of this issue rather than
>>> paper over it.
>>>
>>> Thanks,
>>>
>>> 	M.
>>>
>> Sure, idea was to put it out in the open if anyone has any idea as
>> to what might be happening there since the version where directly
>> instrumenting the raw read/write accessors in arm64/asm/io.h was
>> working fine casting doubts if this has to do something with
>> inlining as Arnd mentioned before.
> Yup. I wouldn't be surprised if MMIO accessors were getting directly
> inlined at the wrong location and creating havoc. For example:
>
> 	writel(readl(addr1) | 1, addr2);
>
> If you're not careful about capturing the result of the read rather
> than the read itself, you can end-up with something really funky. No
> idea if that's what is happening, but a disassembly of the generated
> code could tell you.
>
> 	M.
>

I did that initially (compare the disassembly in working and non-working 
case) but didn't find
anything noticeable, maybe I need to look some more. Thanks for the 
suggestion.

Thanks,
Sai