[PATCH 0/2] kexec-tools/arm64: Add support to read PHYS_OFFSET from vmcoreinfo inside '/proc/kcore'

Bhupesh Sharma bhsharma at redhat.com
Wed Oct 24 14:24:52 PDT 2018

Since kernel version 4.19-rc5 (Commit 23c85094fe1895caefdd
["proc/kcore: add vmcoreinfo note to /proc/kcore"]), '/proc/kcore'
contains a new PT_NOTE which carries the VMCOREINFO information.

If the same is available, one should prefer the same to
retrieve 'PHYS_OFFSET' value exported by the kernel as this
is now the standard interface exposed by kernel for sharing
machine specific details with the user-land as per
the arm64 kernel maintainers (see [0]).

Also on certain arm64 platforms, it has been noticed that due
to a hole at the start of physical ram exposed to kernel
(i.e. it doesn't start from address 0), the kernel still
calculates the 'memstart_addr' kernel variable as 0.

Whereas the SYSTEM_RAM or IOMEM_RESERVED range in '/proc/iomem'
would carry a first entry whose start address is non-zero
(as the physical ram exposed to the kernel starts from a
non-zero address).

In such cases, if we rely on '/proc/iomem' entries to
calculate the phys_offset, then we will have mismatch
between the user-space and kernel space 'PHYS_OFFSET'
value. The present 'kexec-tools' code does the same
in 'get_memory_ranges_iomem_cb()' function when it makes
a call to 'set_phys_offset()'. This can cause the vmcore
generated via 'kexec-tools' to miss the last few bytes as
the first '/proc/iomem' starts from a non-zero address.

Details about the patches in this series:
PATCH 1/2 ->

- Tries to move the elf read functionality from 'vmcore-dmesg.c' to
  a new utility library named 'elf_info.c', so that subsequent patches
  can use the same.
- See the patch log for more details.

PATCH 2/2 ->
- Implements the actual functionality of reading the PHYS_OFFSET for arm64
  platforms from the vmcoreinfo inside '/proc/kcore' (if available).
- See the patch log for more details.

I tested this patchset on qualcomm-amberwing and huawei-taishan arm64
boards for non-KALSR boot mode. Both display the same PHYS_OFFSET in
user-space as the value used by kernel after this patch.

I will send a separate followup patch to fix the KASLR boot cases
(as the discussion on the same is in progress with the arm64
kernel maintainers).

[0] https://www.mail-archive.com/kexec@lists.infradead.org/msg20300.html

Bhupesh Sharma (2):
  util_lib: Add functionality to read elf notes
  arm64: Add support to read PHYS_OFFSET from '/proc/kcore' (if

 kexec/arch/arm64/kexec-arm64.c                     |  73 ++
 util_lib/Makefile                                  |   4 +-
 vmcore-dmesg/vmcore-dmesg.c => util_lib/elf_info.c | 157 +++--
 util_lib/include/elf_info.h                        |  30 +
 vmcore-dmesg/Makefile                              |   4 +-
 vmcore-dmesg/vmcore-dmesg.c                        | 742 +--------------------
 6 files changed, 219 insertions(+), 791 deletions(-)
 copy vmcore-dmesg/vmcore-dmesg.c => util_lib/elf_info.c (88%)
 create mode 100644 util_lib/include/elf_info.h


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