linux-kernel - Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

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Message-ID: <beec5651-5c23-4f5e-a0a3-d1cc01a8490a@kernel.org>
Date: Fri, 28 Nov 2025 10:30:15 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Borislav Petkov <bp@...en8.de>, Kiryl Shutsemau <kas@...nel.org>
Cc: "Pratik R. Sampat" <prsampat@....com>, linux-mm@...ck.org,
 linux-coco@...ts.linux.dev, linux-efi@...r.kernel.org, x86@...nel.org,
 linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
 dave.hansen@...ux.intel.com, ardb@...nel.org, akpm@...ux-foundation.org,
 osalvador@...e.de, thomas.lendacky@....com, michael.roth@....com
Subject: Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

On 11/27/25 19:12, Borislav Petkov wrote:
> On Thu, Nov 27, 2025 at 05:35:57PM +0000, Kiryl Shutsemau wrote:
>>> ABI break for kexec? Is that a thing?
>>>
>>> Since when do we enforce ABI compatibility for kexec and where are we
>>> documenting that?
> 
> I'll take that as a "no".
> 
>> The whole purpose of kexec() is to switch between kernel versions. This
>> struct defines format we communicate information about unaccepted memory
>> between kernels. The mismatch will lead to boot failure.
>>
>> The structure is versioned. Ideally, we should know the format of the
>> structure the next kernel supports and act accordingly in the first
>> kernel. Like, we can accept all memory before kexec on mismatch.
> 
> None of that matters if you kexec the same kernels.
> 
> IOW, for some reason you want to be able to kexec different kernels. The
> question is why do we care?

kexecing the same kernel is typically used for kdump purposes.

kexecing different kernels is used for all sorts of things 
(live-upgrade, grub-emu come to mind). It's quite common to kexec 
different kernels, or maybe I misunderstood the question here?

-- 
Cheers

David