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 70026426699; Tue, 20 Jan 2026 18:35:32 +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=1768934133; cv=none; b=FM7PIe5Ll5g9UyMY35BZd9T62cdXiE87taufP9zhnSwgCaPxpQObtJA5DZO+XLGNlLfDQE4CwvgZJFheRTpmjgEGLLAX6/FAZCZAx/QRaQHEJkYy74hz/V6c8yFZv1UUJnImnJTosN0Sn2GzBddjJcXLdSSUkb3prllv6N1fl/o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768934133; c=relaxed/simple; bh=5UuViMOKhctbPjtTzY+cSAg4SjcGuC20Kuka59xeaLM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=imD/F64DcIG0mLAvsGadK9VUrh7H8f4CGrQZ7sj2KlH3JxVVvQW15EW1PBnG7knd5aRs1AVaXuuAkE5qtZBd+bPnBh4zqOpal0NFLWFDr2nb52PZ4o9wRAaGr7Mp06MAYQ72MM3fiqD28AlHJHkfV8M7dJtUek+JFpryZRmrV14= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TXaAldEp; 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="TXaAldEp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 23C45C19421; Tue, 20 Jan 2026 18:35:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768934132; bh=5UuViMOKhctbPjtTzY+cSAg4SjcGuC20Kuka59xeaLM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=TXaAldEp6bxXie0YRh1TMKfNcAPUEx+KFCDkPBtoCwDUSKc93CWcWhlEKk1WlmRYW 6hw2SnPfGVkECqWVl1OcTViy+DIMqB732m365bSaSIJVALxo2duCYdBLS5C2NKP1Wd FFCBY4FU8Sx7YXbcFyU+dH+xbIEQD4l+h2q5bwp9QLdUvkUM5wds+QWAgdjuPaKnve XGFu+o10G1KWf6+kc/fo1PNZzNDpeQ3BOeNI4vs82YxE5utw0WJ3St3LYB53M8gUZk jDqVrvWWiTV0UstjDqvHgrnleljxubi8Z3sQ/OkovclSYNLvR+sBjq3m3GZPGBnFD/ P0r7tp1Yfrlng== Date: Tue, 20 Jan 2026 20:35:25 +0200 From: Mike Rapoport To: Pratyush Yadav Cc: Andrew Morton , Alexander Graf , Jason Miu , Jonathan Corbet , Pasha Tatashin , kexec@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 3/6] kho: docs: combine concepts and FDT documentation Message-ID: References: <20260105165839.285270-1-rppt@kernel.org> <20260105165839.285270-4-rppt@kernel.org> <2vxz7btce06f.fsf@kernel.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: <2vxz7btce06f.fsf@kernel.org> On Tue, Jan 20, 2026 at 04:08:56PM +0000, Pratyush Yadav wrote: > On Mon, Jan 05 2026, Mike Rapoport wrote: > > > From: "Mike Rapoport (Microsoft)" > > > > Currently index.rst in KHO documentation looks empty and sad as it only > > contains links to "Kexec Handover Concepts" and "KHO FDT" chapters. > > > > Inline contents of these chapters into index.rst to provide a single > > coherent chapter describing KHO. > > > > While on it, drop parts of the KHO FDT description that will be superseded > > by addition of KHO ABI documentation. > > > > Signed-off-by: Mike Rapoport (Microsoft) > [...] > > diff --git a/Documentation/core-api/kho/index.rst b/Documentation/core-api/kho/index.rst > > index 0c63b0c5c143..03cd9afbdb2e 100644 > > --- a/Documentation/core-api/kho/index.rst > > +++ b/Documentation/core-api/kho/index.rst > > @@ -4,10 +4,73 @@ > > Kexec Handover Subsystem > > ======================== > > > > -.. toctree:: > > - :maxdepth: 1 > > +Overview > > +======== > > I ran a test build and Sphinx complains about: > > Documentation/admin-guide/mm/kho.rst:10: WARNING: undefined label: 'kho-concepts' [ref.ref] > Documentation/admin-guide/mm/kho.rst:31: WARNING: undefined label: 'kho-finalization-phase' [ref.ref] > > I suppose you should add these labels here. It's already in Andrew's tree: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?h=mm-nonmm-unstable&id=bc1a060da2a76161ba65f1badc551de15056e398 > Otherwise LGTM. > > > > > - concepts > > - fdt > > +Kexec HandOver (KHO) is a mechanism that allows Linux to preserve memory > > +regions, which could contain serialized system states, across kexec. > > > > -.. only:: subproject and html > > +KHO uses :ref:`flattened device tree (FDT) ` to pass information about > > +the preserved state from pre-exec kernel to post-kexec kernel and :ref:`scratch > > +memory regions ` to ensure integrity of the preserved memory. > > + > > +.. _kho_fdt: > > + > > +KHO FDT > > +======= > > +Every KHO kexec carries a KHO specific flattened device tree (FDT) blob that > > +describes the preserved state. The FDT includes properties describing preserved > > +memory regions and nodes that hold subsystem specific state. > > + > > +The preserved memory regions contain either serialized subsystem states, or > > +in-memory data that shall not be touched across kexec. After KHO, subsystems > > +can retrieve and restore the preserved state from KHO FDT. > > + > > +Subsystems participating in KHO can define their own format for state > > +serialization and preservation. > > + > > +.. _kho_scratch: > > + > > +Scratch Regions > > +=============== > > + > > +To boot into kexec, we need to have a physically contiguous memory range that > > +contains no handed over memory. Kexec then places the target kernel and initrd > > +into that region. The new kernel exclusively uses this region for memory > > +allocations before during boot up to the initialization of the page allocator. > > + > > +We guarantee that we always have such regions through the scratch regions: On > > +first boot KHO allocates several physically contiguous memory regions. Since > > +after kexec these regions will be used by early memory allocations, there is a > > +scratch region per NUMA node plus a scratch region to satisfy allocations > > +requests that do not require particular NUMA node assignment. > > +By default, size of the scratch region is calculated based on amount of memory > > +allocated during boot. The ``kho_scratch`` kernel command line option may be > > +used to explicitly define size of the scratch regions. > > +The scratch regions are declared as CMA when page allocator is initialized so > > +that their memory can be used during system lifetime. CMA gives us the > > +guarantee that no handover pages land in that region, because handover pages > > +must be at a static physical memory location and CMA enforces that only > > +movable pages can be located inside. > > + > > +After KHO kexec, we ignore the ``kho_scratch`` kernel command line option and > > +instead reuse the exact same region that was originally allocated. This allows > > +us to recursively execute any amount of KHO kexecs. Because we used this region > > +for boot memory allocations and as target memory for kexec blobs, some parts > > +of that memory region may be reserved. These reservations are irrelevant for > > +the next KHO, because kexec can overwrite even the original kernel. > > + > > +KHO finalization phase > > +====================== > > + > > +To enable user space based kexec file loader, the kernel needs to be able to > > +provide the FDT that describes the current kernel's state before > > +performing the actual kexec. The process of generating that FDT is > > +called serialization. When the FDT is generated, some properties > > +of the system may become immutable because they are already written down > > +in the FDT. That state is called the KHO finalization phase. > > + > > +See Also > > +======== > > + > > +- :doc:`/admin-guide/mm/kho` > > diff --git a/Documentation/core-api/liveupdate.rst b/Documentation/core-api/liveupdate.rst > > index 7960eb15a81f..e2aba13494cf 100644 > > --- a/Documentation/core-api/liveupdate.rst > > +++ b/Documentation/core-api/liveupdate.rst > > @@ -58,4 +58,4 @@ See Also > > ======== > > > > - :doc:`Live Update uAPI ` > > -- :doc:`/core-api/kho/concepts` > > > > +- :doc:`/core-api/kho/index` > > > > diff --git a/Documentation/mm/memfd_preservation.rst b/Documentation/mm/memfd_preservation.rst > > index 66e0fb6d5ef0..a8a5b476afd3 100644 > > --- a/Documentation/mm/memfd_preservation.rst > > +++ b/Documentation/mm/memfd_preservation.rst > > @@ -20,4 +20,4 @@ See Also > > ======== > > > > - :doc:`/core-api/liveupdate` > > -- :doc:`/core-api/kho/concepts` > > > > +- :doc:`/core-api/kho/index` > > -- > Regards, > Pratyush Yadav -- Sincerely yours, Mike.