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 B511942B737; Tue, 20 Jan 2026 16:08:59 +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=1768925339; cv=none; b=BkaDWPVdoRkfMPyrk3D9QPDH1GCnDkyzhEoav7rOt/0uk6GDpnvpATyp+TXYJIOomqiaFsRJF45lPej4GzvJTYKuax4CJDs9JejMd3Rg/MtqcdlbTVBa1cGxZz0RswcJ/L+wEK+vhQ1Eu979bo4iXq8iM+VvGx/y6ypZawzviQs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768925339; c=relaxed/simple; bh=os1EwFT5nvnw99r6n/Mg19gaCUrjgTrNjKTqCnEbqzs=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=l2ior0WqhsKgup4k4ldvSF+4c9rRrb+85BcQq6oeHqZFJtblcAPhks4OQbui4lWovA2lWyQyTPKVVBMCISCDt1QRQ0/jEO0Kyg4DacOwVz4proMKtJvnlIUIIxcLI/vcFOaAjriPLFPZVk5ZJFF2THSFrG4PUuvg8wmoZtPniIE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=g9Tg7woK; 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="g9Tg7woK" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 81BFBC16AAE; Tue, 20 Jan 2026 16:08:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768925339; bh=os1EwFT5nvnw99r6n/Mg19gaCUrjgTrNjKTqCnEbqzs=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=g9Tg7woKpvI/KsMGSfnoNo0YwIS4G+sKz+BWpN2P7V68wE1XLyYkmeEbL6WPngR5d WkiLXBu2+bGWI55uOYf7LogQY9MXJl2Q91vZ9qcSfQkn0PVRmsQ7BfQzTlmLho6Ng1 GDrlV4HyJL2I2MFnEE9rlwIV6tQkKxrSWljL3OWyynWhc6XZa6EjvKauM+iD1qzfjW eJb+IPurbeVcPvU0DGnYpcmYkd8JJHXXLCq1igkdDqD7zeW+OLcwAt9gAXf6li+nWi 9HqJe2cEhrvP/9TKUOs1F5w8QzNZ0rAnyWvZk5H74XHpR1m63VP9FGvXP+9GqjxMih 1L8/mL8J3iHcQ== From: Pratyush Yadav To: Mike Rapoport Cc: Andrew Morton , Alexander Graf , Jason Miu , Jonathan Corbet , Pasha Tatashin , Pratyush Yadav , 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 In-Reply-To: <20260105165839.285270-4-rppt@kernel.org> (Mike Rapoport's message of "Mon, 5 Jan 2026 18:58:36 +0200") References: <20260105165839.285270-1-rppt@kernel.org> <20260105165839.285270-4-rppt@kernel.org> Date: Tue, 20 Jan 2026 16:08:56 +0000 Message-ID: <2vxz7btce06f.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain 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. 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