# git rev-parse -q --verify b684c09f09e7a6af3794d4233ef785819e72db79^{commit}
b684c09f09e7a6af3794d4233ef785819e72db79
already have revision, skipping fetch
# git checkout -q -f -B kisskb b684c09f09e7a6af3794d4233ef785819e72db79
# git clean -qxdf
# < git log -1
# commit b684c09f09e7a6af3794d4233ef785819e72db79
# Author: Aditya Gupta <adityag@linux.ibm.com>
# Date:   Thu Jun 15 14:40:47 2023 +0530
# 
#     powerpc: update ppc_save_regs to save current r1 in pt_regs
#     
#     ppc_save_regs() skips one stack frame while saving the CPU register states.
#     Instead of saving current R1, it pulls the previous stack frame pointer.
#     
#     When vmcores caused by direct panic call (such as `echo c >
#     /proc/sysrq-trigger`), are debugged with gdb, gdb fails to show the
#     backtrace correctly. On further analysis, it was found that it was because
#     of mismatch between r1 and NIP.
#     
#     GDB uses NIP to get current function symbol and uses corresponding debug
#     info of that function to unwind previous frames, but due to the
#     mismatching r1 and NIP, the unwinding does not work, and it fails to
#     unwind to the 2nd frame and hence does not show the backtrace.
#     
#     GDB backtrace with vmcore of kernel without this patch:
#     
#     ---------
#     (gdb) bt
#      #0  0xc0000000002a53e8 in crash_setup_regs (oldregs=<optimized out>,
#         newregs=0xc000000004f8f8d8) at ./arch/powerpc/include/asm/kexec.h:69
#      #1  __crash_kexec (regs=<optimized out>) at kernel/kexec_core.c:974
#      #2  0x0000000000000063 in ?? ()
#      #3  0xc000000003579320 in ?? ()
#     ---------
#     
#     Further analysis revealed that the mismatch occurred because
#     "ppc_save_regs" was saving the previous stack's SP instead of the current
#     r1. This patch fixes this by storing current r1 in the saved pt_regs.
#     
#     GDB backtrace with vmcore of patched kernel:
#     
#     --------
#     (gdb) bt
#      #0  0xc0000000002a53e8 in crash_setup_regs (oldregs=0x0, newregs=0xc00000000670b8d8)
#         at ./arch/powerpc/include/asm/kexec.h:69
#      #1  __crash_kexec (regs=regs@entry=0x0) at kernel/kexec_core.c:974
#      #2  0xc000000000168918 in panic (fmt=fmt@entry=0xc000000001654a60 "sysrq triggered crash\n")
#         at kernel/panic.c:358
#      #3  0xc000000000b735f8 in sysrq_handle_crash (key=<optimized out>) at drivers/tty/sysrq.c:155
#      #4  0xc000000000b742cc in __handle_sysrq (key=key@entry=99, check_mask=check_mask@entry=false)
#         at drivers/tty/sysrq.c:602
#      #5  0xc000000000b7506c in write_sysrq_trigger (file=<optimized out>, buf=<optimized out>,
#         count=2, ppos=<optimized out>) at drivers/tty/sysrq.c:1163
#      #6  0xc00000000069a7bc in pde_write (ppos=<optimized out>, count=<optimized out>,
#         buf=<optimized out>, file=<optimized out>, pde=0xc00000000362cb40) at fs/proc/inode.c:340
#      #7  proc_reg_write (file=<optimized out>, buf=<optimized out>, count=<optimized out>,
#         ppos=<optimized out>) at fs/proc/inode.c:352
#      #8  0xc0000000005b3bbc in vfs_write (file=file@entry=0xc000000006aa6b00,
#         buf=buf@entry=0x61f498b4f60 <error: Cannot access memory at address 0x61f498b4f60>,
#         count=count@entry=2, pos=pos@entry=0xc00000000670bda0) at fs/read_write.c:582
#      #9  0xc0000000005b4264 in ksys_write (fd=<optimized out>,
#         buf=0x61f498b4f60 <error: Cannot access memory at address 0x61f498b4f60>, count=2)
#         at fs/read_write.c:637
#      #10 0xc00000000002ea2c in system_call_exception (regs=0xc00000000670be80, r0=<optimized out>)
#         at arch/powerpc/kernel/syscall.c:171
#      #11 0xc00000000000c270 in system_call_vectored_common ()
#         at arch/powerpc/kernel/interrupt_64.S:192
#     --------
#     
#     Nick adds:
#       So this now saves regs as though it was an interrupt taken in the
#       caller, at the instruction after the call to ppc_save_regs, whereas
#       previously the NIP was there, but R1 came from the caller's caller and
#       that mismatch is what causes gdb's dwarf unwinder to go haywire.
#     
#     Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
#     Fixes: d16a58f8854b1 ("powerpc: Improve ppc_save_regs()")
#     Reivewed-by: Nicholas Piggin <npiggin@gmail.com>
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
#     Link: https://msgid.link/20230615091047.90433-1-adityag@linux.ibm.com
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld --version
# < git log --format=%s --max-count=1 b684c09f09e7a6af3794d4233ef785819e72db79
# make -s -j 40 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_sequoia_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  44x/sequoia_defconfig
# < make -s -j 40 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_sequoia_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  help
# make -s -j 40 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_sequoia_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  olddefconfig
# make -s -j 40 ARCH=powerpc O=/kisskb/build/powerpc-next_44x_sequoia_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
Completed OK
# rm -rf /kisskb/build/powerpc-next_44x_sequoia_defconfig_powerpc-gcc5
# Build took: 0:00:27.928596