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

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Message-ID: <20251203155855.GCaTBeP8fQDLx3T0X8@fat_crate.local>
Date: Wed, 3 Dec 2025 16:58:55 +0100
From: Borislav Petkov <bp@...en8.de>
To: Kiryl Shutsemau <kas@...nel.org>
Cc: "David Hildenbrand (Red Hat)" <david@...nel.org>, ardb@...nel.org,
	"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,
	akpm@...ux-foundation.org, osalvador@...e.de,
	thomas.lendacky@....com, michael.roth@....com,
	torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug

On Wed, Dec 03, 2025 at 02:46:23PM +0000, Kiryl Shutsemau wrote:
> There is also the #1 Kernel Rule: "we do not break users."

Do I need to point you to that too:

Documentation/process/stable-api-nonsense.rst

?

> Booting a different version of the kernel is a core functionality of
> kexec. It is widely used to deploy new kernels or revert to older ones.
> Breaking this functionality is a show-stopper for most, if not all,
> hyperscalers.
> 
> This specific change may not be a show-stopper as CoCo deployment is not
> widespread enough to be noticed yet.
> 
> The notion that nobody promised that you can kexec into a different kernel
> is absurd. It is used everywhere.

Dude, can you please stop handwaving and say what you really wanna say: you
want different kernels to kexec. And it has worked so far but nothing
guarantees that. And we should all agree on some strategy going forward and
enforce it.

I don't care if different kernels can kexec or not. If I need to kexec, then
I simply build the same kernel.

So I'd take a patch which breaks that and when the submitter gets stopped by
you or someone else, I'll go tell him: "well, actually, I can't take your
patch because Kiryl said so but that's his opinion."

Do you see how absurd this is?!

Geez, I'm tired of typing the same shit over and over again on this thread.

Feel free to propose to make kexec'ing different kernels a rule and let's all
discuss it but let's stop this nonsense of what worked and so on. The kernel
gets complicated constantly, grows things here and there and without such
a rule, are you going to sit around and guard that kexec works?

Pfff.

> I am not involved in the deployment of CoCo VMs, but I don't believe it
> is specifically about CoCo or the kexec ABI. I think it is more about
> the boot protocol. Kexec is one way to boot the kernel.
> 
> Should we consider the EFI configuration tables format as part of the
> boot protocol?

You're basically proving my point: this needs to be discussed and agreed upon.
It doesn't matter if it used to work implicitly in the past.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette