[RFC PATCH v7 01/10] fadump: Add documentation for firmware-assisted dump.
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From: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxxxxxxx> Documentation for firmware-assisted dump. This document is based on the original documentation written for phyp assisted dump by Linas Vepstas and Manish Ahuja, with few changes to reflect the current implementation. Signed-off-by: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxxxxxxx> --- Documentation/powerpc/firmware-assisted-dump.txt | 270 ++++++++++++++++++++++ 1 files changed, 270 insertions(+), 0 deletions(-) create mode 100644 Documentation/powerpc/firmware-assisted-dump.txt diff --git a/Documentation/powerpc/firmware-assisted-dump.txt b/Documentation/powerpc/firmware-assisted-dump.txt new file mode 100644 index 0000000..3007bc9 --- /dev/null +++ b/Documentation/powerpc/firmware-assisted-dump.txt @@ -0,0 +1,270 @@ + + Firmware-Assisted Dump + ------------------------ + July 2011 + +The goal of firmware-assisted dump is to enable the dump of +a crashed system, and to do so from a fully-reset system, and +to minimize the total elapsed time until the system is back +in production use. + +- Firmware assisted dump (fadump) infrastructure is intended to replace + the existing phyp assisted dump. +- Fadump uses the same firmware interfaces and memory reservation model + as phyp assisted dump. +- Unlike phyp dump, fadump exports the memory dump through /proc/vmcore + in the ELF format in the same way as kdump. This helps us reuse the + kdump infrastructure for dump capture and filtering. +- Unlike phyp dump, userspace tool does not need to refer any sysfs + interface while reading /proc/vmcore. +- Unlike phyp dump, fadump allows user to release all the memory reserved + for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem. +- Once enabled through kernel boot parameter, fadump can be + started/stopped through /sys/kernel/fadump_registered interface (see + sysfs files section below) and can be easily integrated with kdump + service start/stop init scripts. + +Comparing with kdump or other strategies, firmware-assisted +dump offers several strong, practical advantages: + +-- Unlike kdump, the system has been reset, and loaded + with a fresh copy of the kernel. In particular, + PCI and I/O devices have been reinitialized and are + in a clean, consistent state. +-- Once the dump is copied out, the memory that held the dump + is immediately available to the running kernel. And therefore, + unlike kdump, fadump doesn't need a 2nd reboot to get back + the system to the production configuration. + +The above can only be accomplished by coordination with, +and assistance from the Power firmware. The procedure is +as follows: + +-- The first kernel registers the sections of memory with the + Power firmware for dump preservation during OS initialization. + These registered sections of memory are reserved by the first + kernel during early boot. + +-- When a system crashes, the Power firmware will save + the low memory (boot memory of size larger of 5% of system RAM + or 256MB) of RAM to the previous registered region. It will + also save system registers, and hardware PTE's. + + NOTE: The term 'boot memory' means size of the low memory chunk + that is required for a kernel to boot successfully when + booted with restricted memory. By default, the boot memory + size will be the larger of 5% of system RAM or 256MB. + Alternatively, user can also specify boot memory size + through boot parameter 'fadump_reserve_mem=' which will + override the default calculated size. Use this option + if default boot memory size is not sufficient for second + kernel to boot successfully. + +-- After the low memory (boot memory) area has been saved, the + firmware will reset PCI and other hardware state. It will + *not* clear the RAM. It will then launch the bootloader, as + normal. + +-- The freshly booted kernel will notice that there is a new + node (ibm,dump-kernel) in the device tree, indicating that + there is crash data available from a previous boot. During + the early boot OS will reserve rest of the memory above + boot memory size effectively booting with restricted memory + size. This will make sure that the second kernel will not + touch any of the dump memory area. + +-- User-space tools will read /proc/vmcore to obtain the contents + of memory, which holds the previous crashed kernel dump in ELF + format. The userspace tools may copy this info to disk, or + network, nas, san, iscsi, etc. as desired. + +-- Once the userspace tool is done saving dump, it will echo + '1' to /sys/kernel/fadump_release_mem to release the reserved + memory back to general use, except the memory required for + next firmware-assisted dump registration. + + e.g. + # echo 1 > /sys/kernel/fadump_release_mem + +Please note that the firmware-assisted dump feature +is only available on Power6 and above systems with recent +firmware versions. + +Implementation details: +---------------------- + +During boot, a check is made to see if firmware supports +this feature on that particular machine. If it does, then +we check to see if an active dump is waiting for us. If yes +then everything but boot memory size of RAM is reserved during +early boot (See Fig. 2). This area is released once we finish +collecting the dump from user land scripts (e.g. kdump scripts) +that are run. If there is dump data, then the +/sys/kernel/fadump_release_mem file is created, and the reserved +memory is held. + +If there is no waiting dump data, then only the memory required +to hold CPU state, HPTE region, boot memory dump and elfcore +header, is reserved at the top of memory (see Fig. 1). This area +is *not* released: this region will be kept permanently reserved, +so that it can act as a receptacle for a copy of the boot memory +content in addition to CPU state and HPTE region, in the case a +crash does occur. + + o Memory Reservation during first kernel + + Low memory Top of memory + 0 boot memory size | + | | |<--Reserved dump area -->| + V V | Permanent Reservation V + +-----------+----------/ /----------+---+----+-----------+----+ + | | |CPU|HPTE| DUMP |ELF | + +-----------+----------/ /----------+---+----+-----------+----+ + | ^ + | | + \ / + ------------------------------------------- + Boot memory content gets transferred to + reserved area by firmware at the time of + crash + Fig. 1 + + o Memory Reservation during second kernel after crash + + Low memory Top of memory + 0 boot memory size | + | |<------------- Reserved dump area ----------- -->| + V V V + +-----------+----------/ /----------+---+----+-----------+----+ + | | |CPU|HPTE| DUMP |ELF | + +-----------+----------/ /----------+---+----+-----------+----+ + | | + V V + Used by second /proc/vmcore + kernel to boot + Fig. 2 + +Currently the dump will be copied from /proc/vmcore to a +a new file upon user intervention. The dump data available through +/proc/vmcore will be in ELF format. Hence the existing kdump +infrastructure (kdump scripts) to save the dump works fine with +minor modifications. + +The tools to examine the dump will be same as the ones +used for kdump. + +How to enable firmware-assisted dump (fadump): +------------------------------------- + +1. Set config option CONFIG_FA_DUMP=y and build kernel. +2. Boot into linux kernel with 'fadump=on' kernel cmdline option. +3. Optionally, user can also set 'fadump_reserve_mem=' kernel cmdline + to specify size of the memory to reserve for boot memory dump + preservation. + +NOTE: If firmware-assisted dump fails to reserve memory then it will + fallback to existing kdump mechanism if 'crashkernel=' option + is set at kernel cmdline. + +Sysfs/debugfs files: +------------ + +Firmware-assisted dump feature uses sysfs file system to hold +the control files and debugfs file to display memory reserved region. + +Here is the list of files under kernel sysfs: + + /sys/kernel/fadump_enabled + + This is used to display the fadump status. + 0 = fadump is disabled + 1 = fadump is enabled + + This interface can be used by kdump init scripts to identify if + fadump is enabled in the kernel and act accordingly. + + /sys/kernel/fadump_registered + + This is used to display the fadump registration status as well + as to control (start/stop) the fadump registration. + 0 = fadump is not registered. + 1 = fadump is registered and ready to handle system crash. + + To register fadump echo 1 > /sys/kernel/fadump_registered and + echo 0 > /sys/kernel/fadump_registered for un-register and stop the + fadump. Once the fadump is un-registered, the system crash will not + be handled and vmcore will not be captured. This interface can be + easily integrated with kdump service start/stop. + + /sys/kernel/fadump_release_mem + + This file is available only when fadump is active during + second kernel. This is used to release the reserved memory + region that are held for saving crash dump. To release the + reserved memory echo 1 to it: + + echo 1 > /sys/kernel/fadump_release_mem + + After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region + file will change to reflect the new memory reservations. + + The existing userspace tools (kdump infrastructure) can be easily + enhanced to use this interface to release the memory reserved for + dump and continue without 2nd reboot. + +Here is the list of files under powerpc debugfs: +(Assuming debugfs is mounted on /sys/kernel/debug directory.) + + /sys/kernel/debug/powerpc/fadump_region + + This file shows the reserved memory regions if fadump is + enabled otherwise this file is empty. The output format + is: + <region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size> + + e.g. + Contents when fadump is registered during first kernel + + # cat /sys/kernel/debug/powerpc/fadump_region + CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0 + HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0 + DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0 + + Contents when fadump is active during second kernel + + # cat /sys/kernel/debug/powerpc/fadump_region + CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020 + HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000 + DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000 + : [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000 + +NOTE: Please refer to Documentation/filesystems/debugfs.txt on + how to mount the debugfs filesystem. + + +TODO: +----- + o Need to come up with the better approach to find out more + accurate boot memory size that is required for a kernel to + boot successfully when booted with restricted memory. + o The fadump implementation introduces a fadump crash info structure + in the scratch area before the ELF core header. The idea of introducing + this structure is to pass some important crash info data to the second + kernel which will help second kernel to populate ELF core header with + correct data before it gets exported through /proc/vmcore. The current + design implementation does not address a possibility of introducing + additional fields (in future) to this structure without affecting + compatibility. Need to come up with the better approach to address this. + The possible approaches are: + 1. Introduce version field for version tracking, bump up the version + whenever a new field is added to the structure in future. The version + field can be used to find out what fields are valid for the current + version of the structure. + 2. Reserve the area of predefined size (say PAGE_SIZE) for this + structure and have unused area as reserved (initialized to zero) + for future field additions. + The advantage of approach 1 over 2 is we don't need to reserve extra space. +--- +Author: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxxxxxxx> +This document is based on the original documentation written for phyp +assisted dump by Linas Vepstas and Manish Ahuja. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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