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Wed, 3 Dec 2025 09:46:24 -0500 (EST) Date: Wed, 3 Dec 2025 14:46:23 +0000 From: Kiryl Shutsemau To: "David Hildenbrand (Red Hat)" , ardb@kernel.org Cc: Borislav Petkov , "Pratik R. Sampat" , linux-mm@kvack.org, linux-coco@lists.linux.dev, linux-efi@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, akpm@linux-foundation.org, osalvador@suse.de, thomas.lendacky@amd.com, michael.roth@amd.com, torvalds@linux-foundation.org Subject: Re: [RFC PATCH 2/4] mm: Add support for unaccepted memory hotplug Message-ID: References: <20251127181233.GBaSiUkaLzwANS_6WT@fat_crate.local> <20251128113411.GAaSmIs0kSWGhCYkaA@fat_crate.local> <47927c25-a317-488a-823f-ac0588f4eee4@kernel.org> <20251201111201.GAaS14AX18qeHN20xf@fat_crate.local> <052d7f47-edb6-4978-bc9a-c7eae469720f@kernel.org> <20251201191036.GEaS3oLBY8PEuE91Ap@fat_crate.local> <20251201202507.GFaS35o7WtLJOM0_jh@fat_crate.local> <420865fb-34cc-43a8-820c-b15b5f24a27c@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <420865fb-34cc-43a8-820c-b15b5f24a27c@kernel.org> X-Rspamd-Queue-Id: 90A9E14000A X-Rspamd-Server: rspam02 X-Stat-Signature: qa7hbs9dimjp8udcx96eydy8arq4emqs X-Rspam-User: X-HE-Tag: 1764773187-591419 X-HE-Meta: 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 sGQ67lD2 n9dVUG9iGwo8AWb3hdBCbEvFtdv+7YUVO7z7jyTXtIP0+FlFTZ9QSuTaLR2F6GXlYx5fCBE4jxFwqEY1h35gpvuo38b7tA5a6OTtwjqPBC7AibSw16iTra0d8MmwMkgDdzn+KsOCOQsgt8KgVagmt92NflCtUiE5YzI6s6mwN65uBKGc+JY0G8X9k9sEa/ul1Tb+20pWZHMaymFFfhK2ZWrE0FNlSY7M2CkARJevZHE0FKTszz4wtWmEwQ9HfQHAmoXt13afXYyVNFLhG+geErgyKNw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Dec 01, 2025 at 09:36:58PM +0100, David Hildenbrand (Red Hat) wrote: > On 12/1/25 21:25, Borislav Petkov wrote: > > On Mon, Dec 01, 2025 at 09:10:26PM +0100, David Hildenbrand (Red Hat) wrote: > > > Just to be clear, I don't think it exist and also I don't think that it > > > should exist. > > > > By that logic if it doesn't exist and someone sends a patch, I should simply > > ignore a review comment about that patch breaking some non-existent ABI and > > simply take it. > > Well, we can always discuss and see if there is a way to not break a > specific use case, independent of any ABI stability guarantees. There is also the #1 Kernel Rule: "we do not break users." 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. > > > > Well, it certainly works for me. > > > > Unless you folks come-a-runnin' later screaming it broke some use case of > > yours. > > Heh, not me, but likely some of the CoCo folks regarding this specific use > case (kexec in a confidential VM). > > > And then we're back to what I've been preaching on this thread from the > > very beginning: having a common agreement on what ABI Linux enforces. > > Right. Maybe Kiryl knows more about this specific case as he brought up that > these structures are versioned. 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? I believe the answer is "yes," at least for some of them, like LINUX_EFI_INITRD_MEDIA_GUID. I also think LINUX_EFI_UNACCEPTED_MEM_TABLE_GUID should be considered in the same way. Ard, do you have any comments on this? -- Kiryl Shutsemau / Kirill A. Shutemov