Kexec today considers itself purely a boot loader: When we enter the new kernel, any state the previous kernel left behind is irrelevant and the new kernel reinitializes the system. However, there are use cases where this mode of operation is not what we actually want. In virtualization hosts for example, we want to use kexec to update the host kernel while virtual machine memory stays untouched. When we add device assignment to the mix, we also need to ensure that IOMMU and VFIO states are untouched. If we add PCIe peer to peer DMA, we need to do the same for the PCI subsystem. If we want to kexec while an SEV-SNP enabled virtual machine is running, we need to preserve the VM context pages and physical memory. See James' and my Linux Plumbers Conference 2023 presentation for details: https://lpc.events/event/17/contributions/1485/ To start us on the journey to support all the use cases above, this patch implements basic infrastructure to allow hand over of kernel state across kexec (Kexec HandOver, aka KHO). As example target, we use ftrace: With this patch set applied, you can read ftrace records from the pre-kexec environment in your post-kexec one. This creates a very powerful debugging and performance analysis tool for kexec. It's also slightly easier to reason about than full blown VFIO state preservation. == Alternatives == There are alternative approaches to (parts of) the problems above: * Memory Pools [1] - preallocated persistent memory region + allocator * PRMEM [2] - resizable persistent memory regions with fixed metadata pointer on the kernel command line + allocator * Pkernfs [3] - preallocated file system for in-kernel data with fixed address location on the kernel command line * PKRAM [4] - handover of user space pages using a fixed metadata page specified via command line All of the approaches above fundamentally have the same problem: They require the administrator to explicitly carve out a physical memory location because they have no mechanism outside of the kernel command line to pass data (including memory reservations) between kexec'ing kernels. KHO provides that base foundation. We will determine later whether we still need any of the approaches above for fast bulk memory handover of for example IOMMU page tables. But IMHO they would all be users of KHO, with KHO providing the foundational primitive to pass metadata and bulk memory reservations as well as provide easy versioning for data. == Documentation == If people are happy with the approach in this patch set, I will write up conclusive documentation including schemas for the metadata as part of its next iteration. For now, here's a rudimentary overview: We introduce a metadata file that the kernels pass between each other. How they pass it is architecture specific. The file's format is a Flattened Device Tree (fdt) which has a generator and parser already included in Linux. When the root user enables KHO through /sys/kernel/kho/active, the kernel invokes callbacks to every driver that supports KHO to serialize its state. When the actual kexec happens, the fdt is part of the image set that we boot into. In addition, we keep a "scratch region" available for kexec: A physically contiguous memory region that is guaranteed to not have any memory that KHO would preserve. The new kernel bootstraps itself using the scratch region and sets all handed over memory as in use. When drivers initialize that support KHO, they introspect the fdt and recover their state from it. This includes memory reservations, where the driver can either discard or claim reservations. == Limitations == I currently only implemented file based kexec. The kernel interfaces in the patch set are already in place to support user space kexec as well, but I have not implemented it yet. == How to Use == To use the code, please boot the kernel with the "kho_scratch=" command line parameter set: "kho_scratch=512M". KHO requires a scratch region. Make sure to fill ftrace with contents that you want to observe after kexec. Then, before you invoke file based "kexec -l", activate KHO: # echo 1 > /sys/kernel/kho/active # kexec -l Image --initrd=initrd -s # kexec -e The new kernel will boot up and contain the previous kernel's trace buffers in /sys/kernel/debug/tracing/trace. Alex [1] https://lore.kernel.org/all/169645773092.11424.7258549771090599226.stgit@skinsburskii./ [2] https://lore.kernel.org/all/20231016233215.13090-1-madvenka@xxxxxxxxxxxxxxxxxxx/ [3] https://lpc.events/event/17/contributions/1485/attachments/1296/2650/jgowans-preserving-across-kexec.pdf [4] https://lore.kernel.org/kexec/1682554137-13938-1-git-send-email-anthony.yznaga@xxxxxxxxxx/ Alexander Graf (15): mm,memblock: Add support for scratch memory memblock: Declare scratch memory as CMA kexec: Add Kexec HandOver (KHO) generation helpers kexec: Add KHO parsing support kexec: Add KHO support to kexec file loads arm64: Add KHO support x86: Add KHO support tracing: Introduce names for ring buffers tracing: Introduce names for events tracing: Introduce kho serialization tracing: Add kho serialization of trace buffers tracing: Recover trace buffers from kexec handover tracing: Add kho serialization of trace events tracing: Recover trace events from kexec handover tracing: Add config option for kexec handover Documentation/ABI/testing/sysfs-firmware-kho | 9 + Documentation/ABI/testing/sysfs-kernel-kho | 53 ++ .../admin-guide/kernel-parameters.txt | 10 + MAINTAINERS | 2 + arch/arm64/Kconfig | 12 + arch/arm64/kernel/setup.c | 2 + arch/arm64/mm/init.c | 8 + arch/x86/Kconfig | 12 + arch/x86/boot/compressed/kaslr.c | 55 ++ arch/x86/include/uapi/asm/bootparam.h | 15 +- arch/x86/kernel/e820.c | 9 + arch/x86/kernel/kexec-bzimage64.c | 39 ++ arch/x86/kernel/setup.c | 46 ++ arch/x86/mm/init_32.c | 7 + arch/x86/mm/init_64.c | 7 + drivers/of/fdt.c | 41 ++ drivers/of/kexec.c | 36 ++ include/linux/kexec.h | 56 ++ include/linux/memblock.h | 19 + include/linux/ring_buffer.h | 9 +- include/linux/trace_events.h | 1 + include/trace/trace_events.h | 2 + include/uapi/linux/kexec.h | 6 + kernel/Makefile | 2 + kernel/kexec_file.c | 41 ++ kernel/kexec_kho_in.c | 298 ++++++++++ kernel/kexec_kho_out.c | 526 ++++++++++++++++++ kernel/trace/Kconfig | 13 + kernel/trace/blktrace.c | 1 + kernel/trace/ring_buffer.c | 267 ++++++++- kernel/trace/trace.c | 76 ++- kernel/trace/trace_branch.c | 1 + kernel/trace/trace_events.c | 3 + kernel/trace/trace_functions_graph.c | 4 +- kernel/trace/trace_output.c | 106 +++- kernel/trace/trace_output.h | 1 + kernel/trace/trace_probe.c | 3 + kernel/trace/trace_syscalls.c | 29 + mm/Kconfig | 4 + mm/memblock.c | 83 ++- 40 files changed, 1901 insertions(+), 13 deletions(-) create mode 100644 Documentation/ABI/testing/sysfs-firmware-kho create mode 100644 Documentation/ABI/testing/sysfs-kernel-kho create mode 100644 kernel/kexec_kho_in.c create mode 100644 kernel/kexec_kho_out.c -- 2.40.1 Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
