From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 377F94F881 for ; Tue, 27 Jan 2026 07:31:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769499120; cv=none; b=hQHeeVEGJRQsunu/pSYBQiCiIR/gRwp4Un4MVbDcbfWRiZuuxytXxocJDoZ3SOrjDDOiFPFG7KfOhRyoO0BVaeaYYJS+rnwhCxluTRqsx22vgc1toPq3KF2iPCR56N2fP1Hb9beTEBvE9f1beE6iqm65kbL0Adx6St29qJdOuP8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769499120; c=relaxed/simple; bh=RlBJGa1qa6Olo8efDWcRWJLbj1v7PDFyAEadNj37KbQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UTMiZt6mZC33zvTMfJ3JxH8FrJUrXre/JF+oxyXqrcH5V5B+aRIbPic1wGfH0Carv8V/Fk5HVeY+KlGTKVRGOwhUhD46dwXd54+nMttVcZQGu3z1OxYFbjAjFIE2a6zl05yzwV0SvoM6ahXqTkZOfRi/Y6Kj5kxvT5j8oCy4V4I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=e6woip7n; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="e6woip7n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7EC3CC116C6; Tue, 27 Jan 2026 07:31:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769499119; bh=RlBJGa1qa6Olo8efDWcRWJLbj1v7PDFyAEadNj37KbQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=e6woip7ntNeXOmzDiTKg3LLAfc1enMO4l8ZkLD2IKmLhRIaGTQOQvYA4CiXPXZjgt DKWYmoaCFW2rn8hQv4qzVYcYbjcxYtm8BG6fUH9hG4h6J0lUe4qAC/FXmwMbZz+oCC ElqCSjF48IHd2ul9DumyOGhjWSUQ35xOnKmcJ42LGz0iRulL+3QdTsqWqnPYyKapgo 5ZRWCtHBfmzCk3skHSDMXACnzFSoAnMo89jW0zHKUdNqIhG3oiQ6SEpjP+jaeGQipI H7XuBMPcU/m+fredxnl0Xo8advRkI/i9Ks8cVRNHAWeh9R8M27F1Nkx2Y5Idvo+0C+ e2gk+E5OQwEvw== Date: Tue, 27 Jan 2026 09:31:49 +0200 From: Mike Rapoport To: Breno Leitao Cc: Alexander Graf , Pasha Tatashin , Pratyush Yadav , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org, usamaarif642@gmail.com, rmikey@meta.com, clm@fb.com, riel@surriel.com, SeongJae Park , kernel-team@meta.com Subject: Re: [PATCH v5 3/4] kho: kexec-metadata: track previous kernel chain Message-ID: References: <20260126-kho-v5-0-7cd0f69ab204@debian.org> <20260126-kho-v5-3-7cd0f69ab204@debian.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260126-kho-v5-3-7cd0f69ab204@debian.org> On Mon, Jan 26, 2026 at 08:07:25AM -0800, 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: > > * eb2266312507 ("x86/boot: Fix page table access in 5-level to 4-level paging transition") > * 77d48d39e991 ("efistub/tpm: Use ACPI reclaim memory for event log to avoid corruption") > * 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_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. > > Signed-off-by: Breno Leitao > Acked-by: SeongJae Park Reviewed-by: Mike Rapoport (Microsoft) with a small nit below > --- > include/linux/kho/abi/kexec_handover.h | 31 ++++++++++++++ > kernel/liveupdate/kexec_handover.c | 74 ++++++++++++++++++++++++++++++++++ > 2 files changed, 105 insertions(+) > > diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h > index 2201a0d2c159a..1f017756e9991 100644 > --- a/include/linux/kho/abi/kexec_handover.h > +++ b/include/linux/kho/abi/kexec_handover.h > @@ -11,6 +11,7 @@ > #define _LINUX_KHO_ABI_KEXEC_HANDOVER_H > > #include > +#include > > /** > * DOC: Kexec Handover ABI > @@ -84,6 +85,36 @@ > /* The FDT property for sub-FDTs. */ > #define KHO_FDT_SUB_TREE_PROP_NAME "fdt" > > +/** > + * DOC: Kexec Metadata ABI > + * > + * The "kexec-metadata" subtree stores optional metadata about the kexec chain. > + * It is registered via kho_add_subtree(), keeping it independent from the core > + * KHO ABI. This allows the metadata format to evolve without affecting other > + * KHO consumers. > + * > + * The metadata is stored as a plain C struct rather than FDT format for > + * simplicity and direct field access. > + */ > + > +/** > + * struct kho_kexec_metadata - Kexec metadata passed between kernels > + * @previous_release: Kernel version string that initiated the kexec > + * @kexec_count: Number of kexec boots since last cold boot > + * > + * This structure is preserved across kexec and allows the new kernel to > + * identify which kernel it was booted from and how many kexec reboots > + * have occurred. > + * > + * __NEW_UTS_LEN is part of UAPI, so it safe to use it in here. Nit: it's usually referred to as uABI. > + */ > +struct kho_kexec_metadata { > + char previous_release[__NEW_UTS_LEN + 1]; > + u32 kexec_count; > +} __packed; > + > +#define KHO_METADATA_NODE_NAME "kexec-metadata" > + > /** > * DOC: Kexec Handover ABI for vmalloc Preservation > * -- Sincerely yours, Mike.