From nobody Mon Feb 9 16:45:57 2026 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 BD2561EA7E7; Thu, 6 Feb 2025 13:29:33 +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=1738848573; cv=none; b=O4wC3mtBUwpLDUllndcwqInPvAr0VAlEQlBSfwgm7s8QLpTAUYnP93/eDXWfybk9E09iqA3uMLyRnT+M+fJGdWbHOwjVe50NpbEGB4anKR2otzDC7xppN1OTmgqJn3HqIUBTWaJVCJdULplPzkzuXO6vVNyUHXhUeG56BwkOqt4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738848573; c=relaxed/simple; bh=ke287oo305Uam86hfMT5albHkTPGceT7i4ZdUVTV7qs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ntiSy+mdjkPSxs2d/T1devIYgVXfxeVKacFbcGY9x4tjLbF16tzVStlosgQisbt0dd9HFcHG7NAniQwH3al9CTpGfvx0bAnrSfEM2vjVENPu7wAuD6jcomoJsJgjorjQGFT8Wns4ew/WuQw5hUpjRl3aLCNz+fApoJDKO7aCQgo= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LI5Cnhhe; 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="LI5Cnhhe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B66DCC4CEDD; Thu, 6 Feb 2025 13:29:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738848573; bh=ke287oo305Uam86hfMT5albHkTPGceT7i4ZdUVTV7qs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=LI5CnhheIluEf6E0yT0RtPvHaYAUSlrYt6PYxNsdXVlD1mnmeL23vT7jUPIcsCGH7 7bROZIcwG83GSfk1kdHyjFtwYBfJMvN0yFn2dVTzscgnvSojIdtykNyLALyBPdYQdU K0Bch5vpDoeXCbdMAZmaxmA+HlNug5/HmbekVQPF+a4OKOfVqgbaN0WXdL/xm10aMu 1y64lYXF1pTud4pkooN5cf/H/xqxZuWY6FVPvRyzqYSrU+K/oMXuPZwJhKinovYfIt QPlHJFDiVyKKaP5GFcG/nG7Q6qzYqH5a9SjjmsXrnhE6IsSJfon/X3ZQz2lzG9adOL g2XwZIlLqNlKw== From: Mike Rapoport To: linux-kernel@vger.kernel.org Cc: Alexander Graf , Andrew Morton , Andy Lutomirski , Anthony Yznaga , Arnd Bergmann , Ashish Kalra , Benjamin Herrenschmidt , Borislav Petkov , Catalin Marinas , Dave Hansen , David Woodhouse , Eric Biederman , Ingo Molnar , James Gowans , Jonathan Corbet , Krzysztof Kozlowski , Mark Rutland , Mike Rapoport , Paolo Bonzini , Pasha Tatashin , "H. Peter Anvin" , Peter Zijlstra , Pratyush Yadav , Rob Herring , Rob Herring , Saravana Kannan , Stanislav Kinsburskii , Steven Rostedt , Thomas Gleixner , Tom Lendacky , Usama Arif , Will Deacon , devicetree@vger.kernel.org, kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org Subject: [PATCH v4 09/14] kexec: Add documentation for KHO Date: Thu, 6 Feb 2025 15:27:49 +0200 Message-ID: <20250206132754.2596694-10-rppt@kernel.org> X-Mailer: git-send-email 2.47.2 In-Reply-To: <20250206132754.2596694-1-rppt@kernel.org> References: <20250206132754.2596694-1-rppt@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Alexander Graf With KHO in place, let's add documentation that describes what it is and how to use it. Signed-off-by: Alexander Graf Co-developed-by: Mike Rapoport (Microsoft) Signed-off-by: Mike Rapoport (Microsoft) --- Documentation/kho/concepts.rst | 80 ++++++++++++++++++++++++++++++++ Documentation/kho/index.rst | 19 ++++++++ Documentation/kho/usage.rst | 60 ++++++++++++++++++++++++ Documentation/subsystem-apis.rst | 1 + MAINTAINERS | 1 + 5 files changed, 161 insertions(+) create mode 100644 Documentation/kho/concepts.rst create mode 100644 Documentation/kho/index.rst create mode 100644 Documentation/kho/usage.rst diff --git a/Documentation/kho/concepts.rst b/Documentation/kho/concepts.rst new file mode 100644 index 000000000000..232bddacc0ef --- /dev/null +++ b/Documentation/kho/concepts.rst @@ -0,0 +1,80 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Kexec Handover Concepts +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Kexec HandOver (KHO) is a mechanism that allows Linux to preserve state - +arbitrary properties as well as memory locations - across kexec. + +It introduces multiple concepts: + +KHO Device Tree +--------------- + +Every KHO kexec carries a KHO specific flattened device tree blob that +describes the state of the system. Device drivers can register to KHO to +serialize their state before kexec. After KHO, device drivers can read +the device tree and extract previous state. + +KHO only uses the fdt container format and libfdt library, but does not +adhere to the same property semantics that normal device trees do: Propert= ies +are passed in native endianness and standardized properties like ``regs`` = and +``ranges`` do not exist, hence there are no ``#...-cells`` properties. + +KHO introduces a new concept to its device tree: ``mem`` properties. A +``mem`` property can be inside any subnode in the device tree. When presen= t, +it contains an array of physical memory ranges that the new kernel must ma= rk +as reserved on boot. It is recommended, but not required, to make these ra= nges +as physically contiguous as possible to reduce the number of array element= s :: + + struct kho_mem { + __u64 addr; + __u64 len; + }; + +After boot, drivers can call the kho subsystem to transfer ownership of me= mory +that was reserved via a ``mem`` property to themselves to continue using m= emory +from the previous execution. + +The KHO device tree follows the in-Linux schema requirements. Any element = in +the device tree is documented via device tree schema yamls that explain wh= at +data gets transferred. + +Scratch Regions +--------------- + +To boot into kexec, we need to have a physically contiguous memory range t= hat +contains no handed over memory. Kexec then places the target kernel and in= itrd +into that region. The new kernel exclusively uses this region for memory +allocations before during boot up to the initialization of the page alloca= tor. + +We guarantee that we always have such regions through the scratch regions:= On +first boot KHO allocates several physically contiguous memory regions. Sin= ce +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 particilar NUMA node assignment. +By default, size of the scratch region is calculated based on amount of me= mory +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 pag= es +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 al= lows +us to recursively execute any amount of KHO kexecs. Because we used this r= egion +for boot memory allocations and as target memory for kexec blobs, some par= ts +of that memory region may be reserved. These reservations are irrenevant f= or +the next KHO, because kexec can overwrite even the original kernel. + +KHO active phase +---------------- + +To enable user space based kexec file loader, the kernel needs to be able = to +provide the device tree that describes the previous kernel's state before +performing the actual kexec. The process of generating that device tree is +called serialization. When the device tree is generated, some properties +of the system may become immutable because they are already written down +in the device tree. That state is called the KHO active phase. diff --git a/Documentation/kho/index.rst b/Documentation/kho/index.rst new file mode 100644 index 000000000000..5e7eeeca8520 --- /dev/null +++ b/Documentation/kho/index.rst @@ -0,0 +1,19 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Kexec Handover Subsystem +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +.. toctree:: + :maxdepth: 1 + + concepts + usage + +.. only:: subproject and html + + + Indices + =3D=3D=3D=3D=3D=3D=3D + + * :ref:`genindex` diff --git a/Documentation/kho/usage.rst b/Documentation/kho/usage.rst new file mode 100644 index 000000000000..e7300fbb309c --- /dev/null +++ b/Documentation/kho/usage.rst @@ -0,0 +1,60 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Kexec Handover Usage +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Kexec HandOver (KHO) is a mechanism that allows Linux to preserve state - +arbitrary properties as well as memory locations - across kexec. + +This document expects that you are familiar with the base KHO +:ref:`Documentation/kho/concepts.rst `. If you have not read +them yet, please do so now. + +Prerequisites +------------- + +KHO is available when the ``CONFIG_KEXEC_HANDOVER`` config option is set t= o y +at compile time. Every KHO producer may have its own config option that you +need to enable if you would like to preserve their respective state across +kexec. + +To use KHO, please boot the kernel with the ``kho=3Don`` command line +parameter. You may use ``kho_scratch`` parameter to define size of the +scratch regions. For example ``kho_scratch=3D512M,512M`` will reserve a 512 +MiB for a global scratch region and 512 MiB per NUMA node scratch regions +on boot. + +Perform a KHO kexec +------------------- + +Before you can perform a KHO kexec, you need to move the system into the +:ref:`Documentation/kho/concepts.rst ` :: + + $ echo 1 > /sys/kernel/kho/active + +After this command, the KHO device tree is available in ``/sys/kernel/kho/= dt``. + +Next, load the target payload and kexec into it. It is important that you +use the ``-s`` parameter to use the in-kernel kexec file loader, as user +space kexec tooling currently has no support for KHO with the user space +based file loader :: + + # kexec -l Image --initrd=3Dinitrd -s + # kexec -e + +The new kernel will boot up and contain some of the previous kernel's stat= e. + +For example, if you used ``reserve_mem`` command line parameter to create +an early memory reservation, the new kernel will have that memory at the +same physical address as the old kernel. + +Abort a KHO exec +---------------- + +You can move the system out of KHO active phase again by calling :: + + $ echo 1 > /sys/kernel/kho/active + +After this command, the KHO device tree is no longer available in +``/sys/kernel/kho/dt``. diff --git a/Documentation/subsystem-apis.rst b/Documentation/subsystem-api= s.rst index b52ad5b969d4..5fc69d6ff9f0 100644 --- a/Documentation/subsystem-apis.rst +++ b/Documentation/subsystem-apis.rst @@ -90,3 +90,4 @@ Other subsystems peci/index wmi/index tee/index + kho/index diff --git a/MAINTAINERS b/MAINTAINERS index e1e01b2a3727..82c2ef421c00 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -12828,6 +12828,7 @@ S: Maintained W: http://kernel.org/pub/linux/utils/kernel/kexec/ F: Documentation/ABI/testing/sysfs-firmware-kho F: Documentation/ABI/testing/sysfs-kernel-kho +F: Documentation/kho/ F: include/linux/kexec.h F: include/uapi/linux/kexec.h F: kernel/kexec* --=20 2.47.2