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diff --git a/Documentation/arch/arm64/kdump.rst b/Documentation/arch/arm64/kdump.rst new file mode 100644 index 000000000000..56a89f45df28 --- /dev/null +++ b/Documentation/arch/arm64/kdump.rst @@ -0,0 +1,92 @@ +======================================= +crashkernel memory reservation on arm64 +======================================= + +Author: Baoquan He <bhe@redhat.com> + +Kdump mechanism is used to capture a corrupted kernel vmcore so that +it can be subsequently analyzed. In order to do this, a preliminarily +reserved memory is needed to pre-load the kdump kernel and boot such +kernel if corruption happens. + +That reserved memory for kdump is adapted to be able to minimally +accommodate the kdump kernel and the user space programs needed for the +vmcore collection. + +Kernel parameter +================ + +Through the kernel parameters below, memory can be reserved accordingly +during the early stage of the first kernel booting so that a continuous +large chunk of memomy can be found. The low memory reservation needs to +be considered if the crashkernel is reserved from the high memory area. + +- crashkernel=size@offset +- crashkernel=size +- crashkernel=size,high crashkernel=size,low + +Low memory and high memory +========================== + +For kdump reservations, low memory is the memory area under a specific +limit, usually decided by the accessible address bits of the DMA-capable +devices needed by the kdump kernel to run. Those devices not related to +vmcore dumping can be ignored. On arm64, the low memory upper bound is +not fixed: it is 1G on the RPi4 platform but 4G on most other systems. +On special kernels built with CONFIG_ZONE_(DMA|DMA32) disabled, the +whole system RAM is low memory. Outside of the low memory described +above, the rest of system RAM is considered high memory. + +Implementation +============== + +1) crashkernel=size@offset +-------------------------- + +The crashkernel memory must be reserved at the user-specified region or +fail if already occupied. + + +2) crashkernel=size +------------------- + +The crashkernel memory region will be reserved in any available position +according to the search order: + +Firstly, the kernel searches the low memory area for an available region +with the specified size. + +If searching for low memory fails, the kernel falls back to searching +the high memory area for an available region of the specified size. If +the reservation in high memory succeeds, a default size reservation in +the low memory will be done. Currently the default size is 128M, +sufficient for the low memory needs of the kdump kernel. + +Note: crashkernel=size is the recommended option for crashkernel kernel +reservations. The user would not need to know the system memory layout +for a specific platform. + +3) crashkernel=size,high crashkernel=size,low +--------------------------------------------- + +crashkernel=size,(high|low) are an important supplement to +crashkernel=size. They allows the user to specify how much memory needs +to be allocated from the high memory and low memory respectively. On +many systems the low memory is precious and crashkernel reservations +from this area should be kept to a minimum. + +To reserve memory for crashkernel=size,high, searching is first +attempted from the high memory region. If the reservation succeeds, the +low memory reservation will be done subsequently. + +If reservation from the high memory failed, the kernel falls back to +searching the low memory with the specified size in crashkernel=,high. +If it succeeds, no further reservation for low memory is needed. + +Notes: + +- If crashkernel=,low is not specified, the default low memory + reservation will be done automatically. + +- if crashkernel=0,low is specified, it means that the low memory + reservation is omitted intentionally. |