linux-kernel - Re: [PATCH] arm64: kexec: flush log to console in nmi

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Date:   Tue, 22 Jun 2021 10:14:38 +0000
From:   Huang Shijie <shijie@...amperecomputing.com>
To:     Will Deacon <will@...nel.org>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        Catalin Marinas <catalin.marinas@....com>, tabba@...gle.com,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, patches@...erecomputing.com,
        zwang@...erecomputing.com
Subject: Re: [PATCH] arm64: kexec: flush log to console in nmi_panic()

Hi Will,
On Mon, Jun 21, 2021 at 11:08:37AM +0100, Will Deacon wrote:
> > > That sounds like something which should be done in the core code, rather
> > > than the in the architecture backend (and looks like panic() might do this
> > > already?)
> > In the non-kdump code path, the core code will take care of it, please read the
> > code in panic().
> > 
> > But in the kdump code path, the architecture code should take care of it.
> 
> Why the discrepancy? Wouldn't it make more sense to do this in panic() for
> both cases, if the prints that we want to display are coming from panic()
> itself?

In the kdump code path, code call like this:
	panic() -->__crash_kexec() --> machine_kexec();

When we reach arm64's machine_kexec(), it means we can __NOT__ return to the panic(), we will run
to the kdump linux kernel by cpu_soft_restart().

So we can not depend the panic() to print the log. :)
	
By the way, I quote part of the arm64 log after we enter __crash_kexec() in NMI context:
	1.) the log in machine_crash_shutdown()
	      ..............
		pr_crit("SMP: stopping secondary CPUs\n");
	      ..............
		pr_info("Starting crashdump kernel...\n");
	      ..............

        2.) the log in machine_kexec()

	      ..............
		WARN(in_kexec_crash && (stuck_cpus || smp_crash_stop_failed()),
			"Some CPUs may be stale, kdump will be unreliable.\n");
	      ..............
		the logs in kexec_segment_flush(kimage);
	      ..............
		pr_info("Bye!\n");


We cannot remove them all, and need to flush all the logs above to console in the NMI context.		


Thanks
Huang Shijie