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

[Dan Raymond]

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.