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b=gU5ih5qsMO7SmY/P80P2pCZSA3Ljhr1wgGtOkuXRGDoq+rIz+QcjTSlK/sPrEsKCm oUL1goaNVxqbB4MMLX6uhuNJZfpz6RzoX3T5bn4RqecI2ztDVlbm9faKl3jI+Pdp1s eC1QyUQOoK9P3iGg7daPKtkTPBmsX4p6HZwM93ETSmtNunlKFMB+4TP1u6mkuFVyIk e9xcN65pIQFA3w+isBLRUffhruy8GD2yLh9kdmTEJyI+0odbknXoz6nNhaii8KOtjj Zq7R9XrNn4LTkoGTTBbB+bDTaBRT0QYPk4ZB3QNlqmyenTSj2bf1sEpBJS67MaPF3a Y8RhyYHey0ZlA== From: Pratyush Yadav To: Breno Leitao Cc: Alexander Graf , Mike Rapoport , Pasha Tatashin , Pratyush Yadav , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, usama.arif@linux.dev, SeongJae Park , kernel-team@meta.com Subject: Re: [PATCH v9 5/6] kho: kexec-metadata: track previous kernel chain In-Reply-To: <20260316-kho-v9-5-ed6dcd951988@debian.org> (Breno Leitao's message of "Mon, 16 Mar 2026 04:54:35 -0700") References: <20260316-kho-v9-0-ed6dcd951988@debian.org> <20260316-kho-v9-5-ed6dcd951988@debian.org> Date: Fri, 03 Apr 2026 11:29:08 +0000 Message-ID: <2vxzika8cmez.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: 3B8BD1C0007 X-Stat-Signature: 7euxmssm84835zg8z7aebzzbe8rnif4u X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1775215753-911293 X-HE-Meta: U2FsdGVkX19AejNrdpTMa2P9CkiGlTdBH3Nh619hQJdSI/u5foqruudgAL/XPDKMyZIZyLRoKJ0F1YNzRGTxdD9wJNhV6Y38F6KVVm+VYjoBcoOi5SSPKnEv4I/PJGkSxgoFNOfda5BOY0GNmplgsXF0XEkZnLd/fhBCSYzd67og9IidI3houyodCYJmiDMtbaIWUuanY2NX2FRmXjA1BqqttsWcmdf0jtZcJZD5uBG8yrJyHJk/yqjkHcB8gCNZ/PQDRxEeuydy+WCDzL5QELG27uYMnmg9Cz5Nywm9iMVBBSvaQ+uUi2bcl5LbUAK9s8P4VZNRkkp/OpA+4I22+wN0sS5DbcyLsuYxA+TyRVK1CUibaD2O1qsT7T5zR3kFc4OcQMtXZY4DP206/JnElc3WuFCcuRvXEBbBA2EEuj+EQ3euk9iZizfeMiVzOmxhQywj3Vi/+Ac+/epuWhrKHPcADThAifplog495Jb3zPAg8cUByO6YdaufzKCvcN9n7BsdCq8c3+ODjlRgtD7BbAyHEDzI4OGG2MksQgJ6G1g6Hc2R/Oa756zTPINZHOjNlxQ8li40/x/e79b6NqRZjGK++IM8h9wA4bLlEfmjj5UtFBDgRCbh5eAeyy90tXWzPNXE7dITEiIZpCEzITMJNZ1vlVUeCh4ApCNN+70s+y5nRDssKie6wrsvdtu3N5QCJDRBCPAhi8blIeGBxPYev9lX4rmbqMoAmOapif5y0kPGlQlrEvQthJIltGjYbeBCbRg+5Y4aNJIdoIsm3mzBEWuZoqIMxA27gz2mjuRCm+Po5v830zsb4ve9vzSeiikybH1hWjE65h55kbXCfvPhTHrb3BR4gX2xRDv1cOCqK6i2dZXrpsteWLJ9IiEppEsXhrlV/qsQM01DtZrR4S5zPu4SDZVv7TVeF7z//JolsHCAk3DEN5oNPToY4I2S+ShhdDwAh5M2uqAHAzlB14V u/TLNtJK RS3LDp2uKuMyf+f/q4nWpSv67eFzrDrm6ELUgiR9BI2HWdHKJyIZCed5tuyL6u7nhhygnb3y+D4PQVmyft9mswdpAMYu/q7EVUk/Vhnj1dQM6h4leXVJoeO+IyNRPyFJekIGnBbHU+t2hw1fmph+smNRrYZccYLWbYeyUMkh/Idq1SDGTyQ7pgAGL68Apd2g44XDMiJ2aE3qq8gpFpqhuHlG6dw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Mar 16 2026, Breno Leitao wrote: > Use Kexec Handover (KHO) to pass the previous kernel's version string > and the number of kexec reboots since the last cold boot to the next > kernel, and print it at boot time. > > Example output: > [ 0.000000] KHO: exec from: 6.19.0-rc4-next-20260107 (count 1) > > Motivation > ========== > > Bugs that only reproduce when kexecing from specific kernel versions > are difficult to diagnose. These issues occur when a buggy kernel > kexecs into a new kernel, with the bug manifesting only in the second > kernel. > > Recent examples include the following commits: > > * commit eb2266312507 ("x86/boot: Fix page table access in > 5-level to 4-level paging transition") > * commit 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory > for event log to avoid corruption") > * commit 64b45dd46e15 ("x86/efi: skip memattr table on kexec > boot") > > As kexec-based reboots become more common, these version-dependent bugs > are appearing more frequently. At scale, correlating crashes to the > previous kernel version is challenging, especially when issues only > occur in specific transition scenarios. > > Implementation > ============== > > The kexec metadata is stored as a plain C struct (struct kho_kexec_metadata) > rather than FDT format, for simplicity and direct field access. It is > registered via kho_add_subtree() as a separate subtree, keeping it > independent from the core KHO ABI. This design choice: > > - Keeps the core KHO ABI minimal and stable > - Allows the metadata format to evolve independently > - Avoids requiring version bumps for all KHO consumers (LUO, etc.) > when the metadata format changes > > The struct kho_kexec_metadata contains two fields: > - previous_release: The kernel version that initiated the kexec > - kexec_count: Number of kexec boots since last cold boot > > On cold boot, kexec_count starts at 0 and increments with each kexec. > The count helps identify issues that only manifest after multiple > consecutive kexec reboots. > > Acked-by: SeongJae Park > Reviewed-by: Mike Rapoport (Microsoft) > Signed-off-by: Breno Leitao Reviewed-by: Pratyush Yadav [...] -- Regards, Pratyush Yadav