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![Mukesh Ojha](/assets/img/avatar_default.png)
There could be a scenario where we define some region in normal memory and use them store to logs which is later retrieved by bootloader during warm reset. In this scenario, we wanted to treat this memory as normal cacheable memory instead of default behaviour which is an overhead. Making it cacheable could improve performance. This commit gives control to change mem_type from Device tree, and also documents the value for normal memory. Signed-off-by: Mukesh Ojha <mojha@codeaurora.org> Signed-off-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/1616438537-13719-1-git-send-email-mojha@codeaurora.org
164 lines
6.2 KiB
ReStructuredText
164 lines
6.2 KiB
ReStructuredText
Ramoops oops/panic logger
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=========================
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Sergiu Iordache <sergiu@chromium.org>
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Updated: 10 Feb 2021
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Introduction
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------------
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Ramoops is an oops/panic logger that writes its logs to RAM before the system
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crashes. It works by logging oopses and panics in a circular buffer. Ramoops
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needs a system with persistent RAM so that the content of that area can
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survive after a restart.
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Ramoops concepts
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----------------
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Ramoops uses a predefined memory area to store the dump. The start and size
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and type of the memory area are set using three variables:
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* ``mem_address`` for the start
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* ``mem_size`` for the size. The memory size will be rounded down to a
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power of two.
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* ``mem_type`` to specify if the memory type (default is pgprot_writecombine).
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Typically the default value of ``mem_type=0`` should be used as that sets the pstore
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mapping to pgprot_writecombine. Setting ``mem_type=1`` attempts to use
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``pgprot_noncached``, which only works on some platforms. This is because pstore
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depends on atomic operations. At least on ARM, pgprot_noncached causes the
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memory to be mapped strongly ordered, and atomic operations on strongly ordered
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memory are implementation defined, and won't work on many ARMs such as omaps.
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Setting ``mem_type=2`` attempts to treat the memory region as normal memory,
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which enables full cache on it. This can improve the performance.
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The memory area is divided into ``record_size`` chunks (also rounded down to
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power of two) and each kmesg dump writes a ``record_size`` chunk of
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information.
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Limiting which kinds of kmsg dumps are stored can be controlled via
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the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
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``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
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``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
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``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
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(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
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``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
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otherwise KMSG_DUMP_MAX.
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The module uses a counter to record multiple dumps but the counter gets reset
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on restart (i.e. new dumps after the restart will overwrite old ones).
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Ramoops also supports software ECC protection of persistent memory regions.
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This might be useful when a hardware reset was used to bring the machine back
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to life (i.e. a watchdog triggered). In such cases, RAM may be somewhat
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corrupt, but usually it is restorable.
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Setting the parameters
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----------------------
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Setting the ramoops parameters can be done in several different manners:
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A. Use the module parameters (which have the names of the variables described
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as before). For quick debugging, you can also reserve parts of memory during
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boot and then use the reserved memory for ramoops. For example, assuming a
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machine with > 128 MB of memory, the following kernel command line will tell
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the kernel to use only the first 128 MB of memory, and place ECC-protected
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ramoops region at 128 MB boundary::
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mem=128M ramoops.mem_address=0x8000000 ramoops.ecc=1
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B. Use Device Tree bindings, as described in
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``Documentation/devicetree/bindings/reserved-memory/ramoops.txt``.
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For example::
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reserved-memory {
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#address-cells = <2>;
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#size-cells = <2>;
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ranges;
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ramoops@8f000000 {
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compatible = "ramoops";
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reg = <0 0x8f000000 0 0x100000>;
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record-size = <0x4000>;
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console-size = <0x4000>;
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};
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};
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C. Use a platform device and set the platform data. The parameters can then
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be set through that platform data. An example of doing that is:
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.. code-block:: c
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#include <linux/pstore_ram.h>
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[...]
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static struct ramoops_platform_data ramoops_data = {
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.mem_size = <...>,
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.mem_address = <...>,
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.mem_type = <...>,
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.record_size = <...>,
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.max_reason = <...>,
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.ecc = <...>,
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};
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static struct platform_device ramoops_dev = {
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.name = "ramoops",
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.dev = {
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.platform_data = &ramoops_data,
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},
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};
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[... inside a function ...]
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int ret;
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ret = platform_device_register(&ramoops_dev);
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if (ret) {
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printk(KERN_ERR "unable to register platform device\n");
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return ret;
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}
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You can specify either RAM memory or peripheral devices' memory. However, when
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specifying RAM, be sure to reserve the memory by issuing memblock_reserve()
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very early in the architecture code, e.g.::
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#include <linux/memblock.h>
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memblock_reserve(ramoops_data.mem_address, ramoops_data.mem_size);
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Dump format
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-----------
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The data dump begins with a header, currently defined as ``====`` followed by a
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timestamp and a new line. The dump then continues with the actual data.
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Reading the data
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----------------
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The dump data can be read from the pstore filesystem. The format for these
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files is ``dmesg-ramoops-N``, where N is the record number in memory. To delete
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a stored record from RAM, simply unlink the respective pstore file.
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Persistent function tracing
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---------------------------
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Persistent function tracing might be useful for debugging software or hardware
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related hangs. The functions call chain log is stored in a ``ftrace-ramoops``
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file. Here is an example of usage::
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# mount -t debugfs debugfs /sys/kernel/debug/
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# echo 1 > /sys/kernel/debug/pstore/record_ftrace
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# reboot -f
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[...]
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# mount -t pstore pstore /mnt/
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# tail /mnt/ftrace-ramoops
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0 ffffffff8101ea64 ffffffff8101bcda native_apic_mem_read <- disconnect_bsp_APIC+0x6a/0xc0
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0 ffffffff8101ea44 ffffffff8101bcf6 native_apic_mem_write <- disconnect_bsp_APIC+0x86/0xc0
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0 ffffffff81020084 ffffffff8101a4b5 hpet_disable <- native_machine_shutdown+0x75/0x90
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0 ffffffff81005f94 ffffffff8101a4bb iommu_shutdown_noop <- native_machine_shutdown+0x7b/0x90
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0 ffffffff8101a6a1 ffffffff8101a437 native_machine_emergency_restart <- native_machine_restart+0x37/0x40
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0 ffffffff811f9876 ffffffff8101a73a acpi_reboot <- native_machine_emergency_restart+0xaa/0x1e0
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0 ffffffff8101a514 ffffffff8101a772 mach_reboot_fixups <- native_machine_emergency_restart+0xe2/0x1e0
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0 ffffffff811d9c54 ffffffff8101a7a0 __const_udelay <- native_machine_emergency_restart+0x110/0x1e0
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0 ffffffff811d9c34 ffffffff811d9c80 __delay <- __const_udelay+0x30/0x40
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0 ffffffff811d9d14 ffffffff811d9c3f delay_tsc <- __delay+0xf/0x20
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