linux-kernel - Re: [PATCH RESEND] x86/kexec: Carry forward the boot DTB on kexec

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Message-ID: <9012E729-4E7E-45F7-9816-301C0342A2DB@juniper.net>
Date: Fri, 1 Aug 2025 00:24:06 +0000
From: Brian Mak <makb@...iper.net>
To: Dave Young <dyoung@...hat.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>,
        Rob Herring <robh@...nel.org>, Saravana Kannan
	<saravanak@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] x86/kexec: Carry forward the boot DTB on kexec

On Jul 31, 2025, at 2:27 AM, Dave Young <dyoung@...hat.com> wrote:

> I worry about it since dtb is for providing boot related information,
> weird things could happen if the kernel versions are different.
> 
> About arm64 and powerpc I think maybe just nobody noticed this
> problem.  IMO it is wrong as kexec is designed to load different
> kernels not limited to current running kernels.
> 
> Otherwise the current kexec_file_load syscall only supports passing
> kernel_fd, initrd_fd and cmdline,  no extra param designed for dtb,  I
> don't know if there are other ways via attaching dtb to kernel or
> initrd.
> 
> Probably you can try to add a new flag to the kexec_file_load syscall,
> when this flag is set 1 then use the current dtb, otherwise do
> nothing.  For arm64 and power,  ideally doing the same is better,  but
> I'm not sure if we should change the old behavior, maybe they can go
> with  default to load the dtb, but users can choose not to do that.

Hi Dave,

Since we're changing the default behavior on x86 by carrying over the
current DTB, and this could potentially affect users, maybe it'd be best
to put this behavior behind a kexec_file_load flag, as you suggested.

I'll take this up in v2 if no one else has any objections to this.

For backward compatibility reasons, changing the current behavior on
ARM64 and PPC probably isn't advisable, so we'd probably want to default
to loading the boot DTB, and users can choose not to do so. I won't take
that up in this patchset though. That can be taken up in the future.

Thanks,
Brian