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Peter Anvin" , Andy Lutomirski , Peter Zijlstra , "Rob Herring" , Steven Rostedt , "Andrew Morton" , Mark Rutland , "Tom Lendacky" , Ashish Kalra , James Gowans , Stanislav Kinsburskii , , , , Anthony Yznaga , Usama Arif , David Woodhouse , Benjamin Herrenschmidt Subject: [PATCH v2 07/17] kexec: Add documentation for KHO Date: Fri, 22 Dec 2023 19:35:57 +0000 Message-ID: <20231222193607.15474-8-graf@amazon.com> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20231222193607.15474-1-graf@amazon.com> References: <20231222193607.15474-1-graf@amazon.com> MIME-Version: 1.0 X-Originating-IP: [10.253.83.51] X-ClientProxiedBy: EX19D037UWB001.ant.amazon.com (10.13.138.123) To EX19D020UWC004.ant.amazon.com (10.13.138.149) X-Rspamd-Queue-Id: 3402240013 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: wrrsyr635z8xgpzwndjgmfyd8q6z4xjj X-HE-Tag: 1703273853-488189 X-HE-Meta: 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 dCzgdOPc ONRgD2euNZSxFOxIoGfS6+JXDoGJggXE0EFM5GuB7fdvUTziXeDnqsDSDigj72ll4KYe8+CDPksonb+0HG+S1eInGh0jlYOhrhPIMKmV/BzvOxqEKa/YQ75TlkGr5KCnPk+jqD4fvRvigA4e/jZEfDvM+UFNJoYcU2EfYP/BtjXkOrbMUjGPDbfNIF4Yawb1H8NDXBg6OJz8/qIAkKSjyMOQVKIcn6TTs9MYoyrnbNcotxZcB9WncKW5lYCSSOExgaPkOca+8inBH4f8mtKEWG7MJshNpLtvu06PV X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: With KHO in place, let's add documentation that describes what it is and how to use it. Signed-off-by: Alexander Graf --- Documentation/kho/concepts.rst | 88 ++++++++++++++++++++++++++++++++ Documentation/kho/index.rst | 19 +++++++ Documentation/kho/usage.rst | 57 +++++++++++++++++++++ Documentation/subsystem-apis.rst | 1 + 4 files changed, 165 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..8e4fe8c57865 --- /dev/null +++ b/Documentation/kho/concepts.rst @@ -0,0 +1,88 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +======================= +Kexec Handover Concepts +======================= + +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: Properties +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 inside any subnode in the device tree. When present, +it contains an array of physical memory ranges that the new kernel must mark +as reserved on boot. It is recommended, but not required, to make these ranges +as physically contiguous as possible to reduce the number of array elements :: + + struct kho_mem { + __u64 addr; + __u64 len; + }; + +After boot, drivers can call the kho subsystem to transfer ownership of memory +that was reserved via a ``mem`` property to themselves to continue using memory +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 what +data gets transferred. + +Mem cache +--------- + +The new kernel needs to know about all memory reservations, but is unable to +parse the device tree yet in early bootup code because of memory limitations. +To simplify the initial memory reservation flow, the old kernel passes a +preprocessed array of physically contiguous reserved ranges to the new kernel. + +These reservations have to be separate from architectural memory maps and +reservations because they differ on every kexec, while the architectural ones +get passed directly between invocations. + +The less entries this cache contains, the faster the new kernel will boot. + +Scratch Region +-------------- + +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 it ingests the mem cache. + +We guarantee that we always have such a region through the scratch region: On +first boot, you can pass the ``kho_scratch`` kernel command line option. When +it is set, Linux allocates a CMA region of the given size. 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 irrenevant for +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 + +======================== +Kexec Handover Subsystem +======================== + +.. toctree:: + :maxdepth: 1 + + concepts + usage + +.. only:: subproject and html + + + Indices + ======= + + * :ref:`genindex` diff --git a/Documentation/kho/usage.rst b/Documentation/kho/usage.rst new file mode 100644 index 000000000000..5efa2a58f9c3 --- /dev/null +++ b/Documentation/kho/usage.rst @@ -0,0 +1,57 @@ +.. SPDX-License-Identifier: GPL-2.0-or-later + +==================== +Kexec Handover Usage +==================== + +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_KHO`` config option is set to y +at compile team. Every KHO producer has 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_scratch`` command +line parameter set to allocate a scratch region. For example +``kho_scratch=512M`` will reserve a 512 MiB scratch region 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=initrd -s + # kexec -e + +The new kernel will boot up and contain some of the previous kernel's state. + +For example, if you enabled ``CONFIG_FTRACE_KHO``, the new kernel will contain +the old kernel's trace buffers in ``/sys/kernel/debug/tracing/trace``. + +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-apis.rst index 930dc23998a0..8207b6514d87 100644 --- a/Documentation/subsystem-apis.rst +++ b/Documentation/subsystem-apis.rst @@ -86,3 +86,4 @@ Storage interfaces misc-devices/index peci/index wmi/index + kho/index