Re: [PATCH v4 0/2] Add capability to dump fdt blob for arm64 platforms

[Bhupesh Sharma <bhsharma@redhat.com>]

Hi Akashi,

On 07/09/2018 12:41 PM, AKASHI Takahiro wrote:
> Bhupesh, Simon,
> 
> #I'm afraid that I gonna rehash the old discussion.
> 
> Looking into ppc's kexec-tools code, I found that ppc version of
> this tool has a similar feature and it saves a new dtb into a file,
> named "debug.dtb."
> See save_fixed_up_dtb() in kexec/arch/ppc/fixup_dtb.c
> 
> Even if it is a debug feature, we'd better go in the same way
> (if possible).

Thanks for sharing the same.

I had a look earlier also at the ppc - "debug.dtb" code in kexec-tools, 
but this is again fails to fulfill the same purpose that we are trying 
to resolve for early primary kernel crash(es) on arm64 machines, because 
of which we are not able to reach the command prompt with the primary 
kernel itself.

We would need to either reach the command prompt to dump out/save the 
"debug.dtb" file when we are running the primary kernel (which is 
unfortunately not always possible wit latest kernels on some of the 
arm64 machines), or use some initramfs scriptware to dump out the same 
to some secondary storage device (nfs server or usb stick).

So, I would still suggest to stick with the approach I proposed for now 
(and we can take a similar approach as ppc for arm64 later on) once we 
have kexec, kdump and other user-space utilities (which rely on the 
kexec kernel framework) working stably on arm64 machines (with latest 
upstream kernels).

Thanks,
Bhupesh

> On Thu, Jun 21, 2018 at 03:54:36PM +0530, Bhupesh Sharma wrote:
>> Changes since v3:
>> ----------------
>>   - Addressed comments from Akashi regarding:
>>     ~ Using sensible names for variables and pointers.
>>     ~ Removing usage of local pointer variable in 'dump_fdt()' function.
>>     ~ Tried addressing a comment regarding converting GET_CELL macro to
>>       inline function, but the final implementation became more tricky, so
>>       dropped the same and decided to go ahead with the macro for
>>       simplicity.
>>   - v3 can be viewed here: https://www.spinics.net/lists/kexec/msg20544.html
>>
>> Changes since v2:
>> ----------------
>>   - Added ascii prints for printing bootargs.
>>   - v2 can be viewed here: http://lists.infradead.org/pipermail/kexec/2018-April/020532.html
>>
>> Changes since v1:
>> ----------------
>>   - No functional changes: Just added a cover letter to explain the
>>     background better and also capture some details on where I found this
>>     patchset handy. Also added some dtb dumps logs from 'kexec -p -d' for
>>     reference (with this patchset applied) for clarity.
>>   - v1 can be viewed here: http://lists.infradead.org/pipermail/kexec/2018-April/020407.html
>>
>> While working on a couple of issues related to primary kernel crash on freescale
>> and huawei arm64 boards, I noticed that the primary kernel crashed before it could reach
>> the command prompt but was able to launch some early initramfs scriptware.
>>
>> In the initial initramfs scriptware crashkernel loading was automated along
>> with auto load of other userspace applications (for e.g. on the freescale board
>> there are networking applications like ODP/DPDK which can be launched automatically via
>> scriptware).
>>
>> I was hoping that the crashkernel would be able to load when the primary kernel crashes,
>> and using the crash core dump thus obtained, I would be able to debug the problem which
>> caused the primary kernel to crash late in the boot flow (before reaching the boot prompt).
>>
>> Unfortunately currently we can experience an early crash in crashkernel itself
>> (on such example is the 'acpi table access' issue in the arm64 crashkernel
>> which we discussed some time back upstream
>> <https://www.spinics.net/lists/arm-kernel/msg616632.html>):
>>
>> In such cases, we have no opportunity to obtain the crash core dump which can be
>> used to debug the primary kernel crash.
>>
>> Now, looking at just the panic messages from the crashkernel in such cases is sometimes
>> not very useful in debugging what might have caused it to crash when the primary kernel
>> is able to atleast boot past that point on the same hardware platform.
>>
>> Debugging the issue closer (especially on the request for help on the freescale board), I
>> realized that the crashkernel crash may be caused by improper/buggy fixing of 'dtb'
>> being passed to the crashkernel - especially the 'linux,usable-memory-range' property.
>>
>> For such cases, I found that dumping the dtb blob entries from kexec-tools is
>> a useful debugging tip as I could identify the 'linux,usable-memory-range'
>> property did not contain ACPI RECLAIM region entries.
>>
>> Please note that since the primary kernel crashes before the command prompt
>> can be reached, it is not possible to run a dtc interpreter there (and it
>> also adds the requirement for an additional 'dtc' tool to be present in the initramfs).
>>
>> Also, it might not be possible to always correctly time the 'dtc' interpreter loading
>> via the initramfs scriptware and store the binary/hex output to a storage device
>> just after the crashkernel is loaded via 'kexec -p' as the storage driver itself
>> might have panick'ed during the meanwhile.
>>
>> In view of the above, it would be useful to dump the fdt blob being passed to the second
>> (kexec/kdump) kernel when '-d' flag is specified while invoking kexec/kdump. This allows
>>   one to look at the device-tree fields that is being passed to the secondary
>> kernel and accordingly debug issues.
>>
>> This can be specially useful for the arm64 case, where we are still fixing up some issues
>> with the upstream kexec-tools/arm64 kernel.
>>
>> I loathe to keep this patch locally and apply it locally on top of the upstream 'kexec-tools'
>> patches when debugging such issues, so it would be probably good to have this feature
>> available in upstream itself.
>>
>> Here is an example output of the dtb dump(on an arm64 board), on serial console with
>> the patchset applied and 'kexec -p' launched used with a '-d' flag using initramfs scriptware:
>>
>> <..snip..>
>>
>> setup_2nd_dtb: found /sys/firmware/fdt
>>   / {
>>      #size-cells = <0x00000002>;
>>      #address-cells = <0x00000002>;
>>      chosen {
>>          linux,usable-memory-range = <0x00000000 0xdfe00000 0x00000000 0x20000000>;
>>          linux,elfcorehdr = <0x00000000 0xffdf0000 0x00000000 0x00001400>;
>>          kaslr-seed = <0x00000000 0x00000000>;
>>          linux,uefi-mmap-desc-ver = <0x00000001>;
>>          linux,uefi-mmap-desc-size = <0x00000030>;
>>          linux,uefi-mmap-size = <0x000020a0>;
>>          linux,uefi-mmap-start = <0x00000000 0x07a81018>;
>>          linux,uefi-system-table = <0x00000000 0x17fc0018>;
>> 	bootargs = "root=/dev/mapper/rhel_qualcomm--amberwing--rep--15-root ro rd.lvm.lv=rhel_qualcomm-amberwing-rep-15/root rd.lvm.lv=rhel_qualcomm-amberwing-rep-15/swap";
>>          linux,initrd-end = <0x00000000 0x05e8a7a1>;
>>          linux,initrd-start = <0x00000000 0x04b49000>;
>>      };
>>   };
>>
>> <..snip..>
>>
>> Bhupesh Sharma (2):
>>    dt-ops: Add helper API to dump fdt blob
>>    kexec-arm64: Add functionality to dump 2nd dtb
>>
>>   kexec/arch/arm64/kexec-arm64.c |   2 +
>>   kexec/dt-ops.c                 | 141 +++++++++++++++++++++++++++++++++++++++++
>>   kexec/dt-ops.h                 |   1 +
>>   3 files changed, 144 insertions(+)
>>
>> -- 
>> 2.7.4
>>


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