On 06/09/21 at 04:32pm, Baoquan He wrote:
> Some parts of the guide are aged, hence need be updated.
>
> 1) The backup area of the 1st 640K on X86_64 has been removed
> by below commits, update the description accordingly.
>
> commit 7c321eb2b843 ("x86/kdump: Remove the backup region handling")
> commit 6f599d84231f ("x86/kdump: Always reserve the low 1M when the crashkernel option is specified")
>
> 2) Sort out the descripiton of "crashkernel syntax" part.
>
> 3) And some other minor cleanups.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
> v2->v3:
> Fix several spell checking issues pointed out by Dave.
>
> v1->v2:
> Update the obsolete descriptions about SMP and RELOCATABLE according
> to Dave's comment.
>
> Documentation/admin-guide/kdump/kdump.rst | 169 ++++++++++++++--------
> 1 file changed, 108 insertions(+), 61 deletions(-)
>
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index 75a9dd98e76e..a04fc854749d 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
> @@ -2,7 +2,7 @@
> Documentation for Kdump - The kexec-based Crash Dumping Solution
> ================================================================
>
> -This document includes overview, setup and installation, and analysis
> +This document includes overview, setup, installation, and analysis
> information.
>
> Overview
> @@ -13,9 +13,9 @@ dump of the system kernel's memory needs to be taken (for example, when
> the system panics). The system kernel's memory image is preserved across
> the reboot and is accessible to the dump-capture kernel.
>
> -You can use common commands, such as cp and scp, to copy the
> -memory image to a dump file on the local disk, or across the network to
> -a remote system.
> +You can use common commands, such as cp, scp or makedumpfile to copy
> +the memory image to a dump file on the local disk, or across the network
> +to a remote system.
>
> Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
> s390x, arm and arm64 architectures.
> @@ -26,13 +26,15 @@ the dump-capture kernel. This ensures that ongoing Direct Memory Access
> The kexec -p command loads the dump-capture kernel into this reserved
> memory.
>
> -On x86 machines, the first 640 KB of physical memory is needed to boot,
> -regardless of where the kernel loads. Therefore, kexec backs up this
> -region just before rebooting into the dump-capture kernel.
> +On x86 machines, the first 640 KB of physical memory is needed for boot,
> +regardless of where the kernel loads. For simpler handling, the whole
> +low 1M is reserved to avoid any later kernel or device driver writing
> +data into this area. Like this, the low 1M can be reused as system RAM
> +by kdump kernel without extra handling.
>
> -Similarly on PPC64 machines first 32KB of physical memory is needed for
> -booting regardless of where the kernel is loaded and to support 64K page
> -size kexec backs up the first 64KB memory.
> +On PPC64 machines first 32KB of physical memory is needed for booting
> +regardless of where the kernel is loaded and to support 64K page size
> +kexec backs up the first 64KB memory.
>
> For s390x, when kdump is triggered, the crashkernel region is exchanged
> with the region [0, crashkernel region size] and then the kdump kernel
> @@ -46,14 +48,14 @@ passed to the dump-capture kernel through the elfcorehdr= boot
> parameter. Optionally the size of the ELF header can also be passed
> when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
>
> -
> With the dump-capture kernel, you can access the memory image through
> /proc/vmcore. This exports the dump as an ELF-format file that you can
> -write out using file copy commands such as cp or scp. Further, you can
> -use analysis tools such as the GNU Debugger (GDB) and the Crash tool to
> -debug the dump file. This method ensures that the dump pages are correctly
> -ordered.
> -
> +write out using file copy commands such as cp or scp. You can also use
> +makedumpfile utility to analyze and write out filtered contents with
> +options, e.g with '-d 31' it will only write out kernel data. Further,
> +you can use analysis tools such as the GNU Debugger (GDB) and the Crash
> +tool to debug the dump file. This method ensures that the dump pages are
> +correctly ordered.
>
> Setup and Installation
> ======================
> @@ -125,9 +127,18 @@ dump-capture kernels for enabling kdump support.
> System kernel config options
> ----------------------------
>
> -1) Enable "kexec system call" in "Processor type and features."::
> +1) Enable "kexec system call" or "kexec file based system call" in
> + "Processor type and features."::
> +
> + CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y
> +
> + And both of them will select KEXEC_CORE::
>
> - CONFIG_KEXEC=y
> + CONFIG_KEXEC_CORE=y
> +
> + Subsequently, CRASH_CORE is selected by KEXEC_CORE::
> +
> + CONFIG_CRASH_CORE=y
>
> 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
> filesystems." This is usually enabled by default::
> @@ -175,17 +186,19 @@ Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
>
> CONFIG_HIGHMEM4G
>
> -2) On i386 and x86_64, disable symmetric multi-processing support
> - under "Processor type and features"::
> +2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
> + command line when loading the dump-capture kernel because one
> + CPU is enough for kdump kernel to dump vmcore on most of systems.
>
> - CONFIG_SMP=n
> + However, you can also specify nr_cpus=X to enable multiple processors
> + in kdump kernel. In this case, "disable_cpu_apicid=" is needed to
> + tell kdump kernel which cpu is 1st kernel's BSP. Please refer to
> + admin-guide/kernel-parameters.txt for more details.
>
> - (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
> - when loading the dump-capture kernel, see section "Load the Dump-capture
> - Kernel".)
> + With CONFIG_SMP=n, the above things are not related.
>
> -3) If one wants to build and use a relocatable kernel,
> - Enable "Build a relocatable kernel" support under "Processor type and
> +3) A relocatable kernel is suggested to be built by default. If not yet,
> + enable "Build a relocatable kernel" support under "Processor type and
> features"::
>
> CONFIG_RELOCATABLE=y
> @@ -232,7 +245,7 @@ Dump-capture kernel config options (Arch Dependent, ia64)
> as a dump-capture kernel if desired.
>
> The crashkernel region can be automatically placed by the system
> - kernel at run time. This is done by specifying the base address as 0,
> + kernel at runtime. This is done by specifying the base address as 0,
> or omitting it all together::
>
> crashkernel=256M@0
> @@ -241,10 +254,6 @@ Dump-capture kernel config options (Arch Dependent, ia64)
>
> crashkernel=256M
>
> - If the start address is specified, note that the start address of the
> - kernel will be aligned to 64Mb, so if the start address is not then
> - any space below the alignment point will be wasted.
> -
> Dump-capture kernel config options (Arch Dependent, arm)
> ----------------------------------------------------------
>
> @@ -260,46 +269,81 @@ Dump-capture kernel config options (Arch Dependent, arm64)
> on non-VHE systems even if it is configured. This is because the CPU
> will not be reset to EL2 on panic.
>
> -Extended crashkernel syntax
> +crashkernel syntax
> ===========================
> +1) crashkernel=size@offset
>
> -While the "crashkernel=size[@offset]" syntax is sufficient for most
> -configurations, sometimes it's handy to have the reserved memory dependent
> -on the value of System RAM -- that's mostly for distributors that pre-setup
> -the kernel command line to avoid a unbootable system after some memory has
> -been removed from the machine.
> + Here 'size' specifies how much memory to reserve for the dump-capture kernel
> + and 'offset' specifies the beginning of this reserved memory. For example,
> + "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
> + starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
>
> -The syntax is::
> + The crashkernel region can be automatically placed by the system
> + kernel at run time. This is done by specifying the base address as 0,
> + or omitting it all together::
>
> - crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
> - range=start-[end]
> + crashkernel=256M@0
> + or::
>
> -For example::
> + crashkernel=256M
>
> - crashkernel=512M-2G:64M,2G-:128M
> + If the start address is specified, note that the start address of the
> + kernel will be aligned to a value (which is Arch dependent), so if the
> + start address is not then any space below the alignment point will be
> + wasted.
>
> -This would mean:
> +2) range1:size1[,range2:size2,...][@offset]
>
> - 1) if the RAM is smaller than 512M, then don't reserve anything
> - (this is the "rescue" case)
> - 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
> - 3) if the RAM size is larger than 2G, then reserve 128M
> + While the "crashkernel=size[@offset]" syntax is sufficient for most
> + configurations, sometimes it's handy to have the reserved memory dependent
> + on the value of System RAM -- that's mostly for distributors that pre-setup
> + the kernel command line to avoid a unbootable system after some memory has
> + been removed from the machine.
>
> + The syntax is::
>
> + crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
> + range=start-[end]
>
> -Boot into System Kernel
> -=======================
> + For example::
> +
> + crashkernel=512M-2G:64M,2G-:128M
> +
> + This would mean:
>
> + 1) if the RAM is smaller than 512M, then don't reserve anything
> + (this is the "rescue" case)
> + 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
> + 3) if the RAM size is larger than 2G, then reserve 128M
> +
> +3) crashkernel=size,high and crashkernel=size,low
> +
> + If memory above 4G is preferred, crashkernel=size,high can be used to
> + fulfill that. With it, physical memory is allowed to be allocated from top,
> + so could be above 4G if system has more than 4G RAM installed. Otherwise,
> + memory region will be allocated below 4G if available.
> +
> + When crashkernel=X,high is passed, kernel could allocate physical memory
> + region above 4G, low memory under 4G is needed in this case. There are
> + three ways to get low memory:
> +
> + 1) Kernel will allocate at least 256M memory below 4G automatically
> + if crashkernel=Y,low is not specified.
> + 2) Let user specify low memory size instead.
> + 3) Specified value 0 will disable low memory allocation::
> +
> + crashkernel=0,low
> +
> +Boot into System Kernel
> +-----------------------
> 1) Update the boot loader (such as grub, yaboot, or lilo) configuration
> files as necessary.
>
> -2) Boot the system kernel with the boot parameter "crashkernel=Y@X",
> - where Y specifies how much memory to reserve for the dump-capture kernel
> - and X specifies the beginning of this reserved memory. For example,
> - "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
> - starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
> +2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
>
> - On x86 and x86_64, use "crashkernel=64M@16M".
> + On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the
> + start address 'X' is not necessary, kernel will search a suitable
> + area. Unless an explicit start address is expected.
>
> On ppc64, use "crashkernel=128M@32M".
>
> @@ -331,8 +375,8 @@ of dump-capture kernel. Following is the summary.
>
> For i386 and x86_64:
>
> - - Use vmlinux if kernel is not relocatable.
> - Use bzImage/vmlinuz if kernel is relocatable.
> + - Use vmlinux if kernel is not relocatable.
>
> For ppc64:
>
> @@ -392,7 +436,7 @@ loading dump-capture kernel.
>
> For i386, x86_64 and ia64:
>
> - "1 irqpoll maxcpus=1 reset_devices"
> + "1 irqpoll nr_cpus=1 reset_devices"
>
> For ppc64:
>
> @@ -400,7 +444,7 @@ For ppc64:
>
> For s390x:
>
> - "1 maxcpus=1 cgroup_disable=memory"
> + "1 nr_cpus=1 cgroup_disable=memory"
>
> For arm:
>
> @@ -408,7 +452,7 @@ For arm:
>
> For arm64:
>
> - "1 maxcpus=1 reset_devices"
> + "1 nr_cpus=1 reset_devices"
>
> Notes on loading the dump-capture kernel:
>
> @@ -488,6 +532,10 @@ the following command::
>
> cp /proc/vmcore <dump-file>
>
> +You can also use makedumpfile utility to write out the dump file
> +with specified options to filter out unwanted contents, e.g::
> +
> + makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
>
> Analysis
> ========
> @@ -535,8 +583,7 @@ This will cause a kdump to occur at the add_taint()->panic() call.
> Contact
> =======
>
> -- Vivek Goyal (vgoyal@xxxxxxxxxx)
> -- Maneesh Soni (maneesh@xxxxxxxxxx)
> +- kexec@xxxxxxxxxxxxxxxxxxx
>
> GDB macros
> ==========
> --
> 2.17.2
>
Acked-by: Dave Young <dyoung@xxxxxxxxxx>
Thanks
Dave
