linux-kernel - Re: [PATCH v1] arch/x86: port I/O tracing on x86

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Message-ID: <ac03a0cf-d699-47bc-bc14-f030fa2408d2@foxvalley.net>
Date:   Tue, 19 Sep 2023 15:31:02 -0600
From:   Dan Raymond <draymond@...valley.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com
Subject: Re: [PATCH v1] arch/x86: port I/O tracing on x86

On 9/19/2023 3:12 PM, Peter Zijlstra wrote:
>>> This means I can no longer use early_console->write() to print to my
>>> early_serial_console.
>>
>> Why not?  Did you try it?
> 
> I have tried debugging the kernel for the last 15+ years. The only
> reliable way to get something out of the machine is outb on the serial
> port. Anything else is a waste of time..
> 
> Adding tracing to it (which relies on RCU, which might not be alive at
> this point) which might itself be the problem, is a total no-go.
> 
> You do not wreck early_serial_console.

But you didn't try my patch to see if it "wrecks" early_serial_console.
I doubt it has any impact there because it does not get compiled into
boot code.  Notice the BOOT_COMPRESSED_MISC_H and BOOT_BOOT_H checks.

I don't understand your general objection.  The kernel already has
tracing for memory mapped I/O which includes serial ports.  This patch
just extends that to include port I/O.