could not set breakpoint on _start or arm_reset

[wooshifu]

project: rpi3-test note: build with macro WITH_KERNEL_VM=1 issue: could not set breakpoint on _start or arm_reset steps:

1. run qemu-system-aarch64 -s -S -M raspi3 -kernel lk.elf -semihosting -serial null -serial mon:stdio -nographic to start the debug session
2. run aarch64-elf-gdb lk.elf to start debugging
3. target remote :1234
4. b _start
5. b arm_reset
6. si shows 0x0000000000080004 in ?? ()
7. disassemble arm_reset shows Cannot access memory at address 0xffff000000080000
8. b lk_main
9. c then gdb can break at lk_main as extecpted

questiones:

• why gdb says Cannot access memory at address 0xffff000000080000?
• why gdb command b _start or b arm_reset not working?
• why gdb command b lk_main works?

config.h

#pragma once
#define LK 1
#define ARM_ARCH_WAIT_FOR_SECONDARIES 1
#define MEMBASE 0x00000000
#define MEMSIZE 0x40000000
#define MMU_WITH_TRAMPOLINE 1
#define BCM2837 1
#define ARM64_CPU_CORTEX_A53 1
#define ARM_ISA_ARMV8 1
#define IS_64BIT 1
#define ARCH_DEFAULT_STACK_SIZE 4096
#define WITH_SMP 1
#define SMP_MAX_CPUS 4
#define SMP_CPU_CLUSTER_SHIFT 8
#define SMP_CPU_ID_BITS 24
#define KERNEL_ASPACE_BASE 0xffff000000000000
#define KERNEL_ASPACE_SIZE 0x0001000000000000
#define USER_ASPACE_BASE 0x0000000001000000
#define USER_ASPACE_SIZE 0x0000fffffe000000
#define KERNEL_BASE 0xffff000000000000
#define KERNEL_LOAD_OFFSET 0x00080000
#define MEMBASE 0x00000000
#define MEMSIZE 0x40000000
#define PLATFORM_HAS_DYNAMIC_TIMER 1
#define LK_HEAP_IMPLEMENTATION miniheap
#define PROJECT_RPI3_TEST 1
#define PROJECT "rpi3-test"
#define TARGET_RPI3 1
#define TARGET "rpi3"
#define PLATFORM_BCM28XX 1
#define PLATFORM "bcm28xx"
#define ARCH_ARM64 1
#define ARCH "arm64"
#define WITH_APP 1
#define WITH_APP_SHELL 1
#define WITH_APP_STRINGTESTS 1
#define WITH_APP_TESTS 1
#define WITH_ARCH 1
#define WITH_DEV 1
#define WITH_DEV_TIMER_ARM_GENERIC 1
#define WITH_KERNEL 1
#define WITH_KERNEL_VM 1
#define WITH_LIB_CBUF 1
#define WITH_LIB_CKSUM 1
#define WITH_LIB_CONSOLE 1
#define WITH_LIB_DEBUG 1
#define WITH_LIB_DEBUGCOMMANDS 1
#define WITH_LIB_FDT 1
#define WITH_LIB_FIXED_POINT 1
#define WITH_LIB_FONT 1
#define WITH_LIB_GFX 1
#define WITH_LIB_GFXCONSOLE 1
#define WITH_LIB_HEAP 1
#define WITH_LIB_HEAP_MINIHEAP 1
#define WITH_LIB_IO 1
#define WITH_LIB_LIBC 1
#define WITH_PLATFORM 1
#define WITH_TARGET 1
#define LK_DEBUGLEVEL 2
#define GLOBAL_INCLUDES "-I./build-rpi3-test_-I./include_-Iexternal/include_-Itarget/rpi3/include_-Iplatform/bcm28xx/include_-Iarch/arm64/include_-Itop/include_-Iapp/include_-Iapp/shell/include_-Iapp/stringtests/include_-Iapp/tests/include_-Iarch/include_-Idev/include_-Idev/timer/arm_generic/include_-Ikernel/include_-Ilib/cbuf/include_-Iexternal/lib/cksum/include_-Ilib/debugcommands/include_-Iexternal/lib/fdt/include_-Ilib/gfx/include_-Ilib/gfxconsole/include_-Iplatform/include_-Itarget/include_-Ikernel/vm/include_-Ilib/console/include_-Ilib/debug/include_-Ilib/fixed_point/include_-Ilib/font/include_-Ilib/heap/include_-Ilib/heap/include_-Ilib/libc/include_-Ilib/heap/miniheap/include_-Ilib/heap/miniheap/include_-Ilib/io/include"
#define GLOBAL_COMPILEFLAGS "-g_-include_./build-rpi3-test/config.h_-v_-Wextra_-Wall_-Werror=return-type_-Wshadow_-Wdouble-promotion_-Wno-multichar_-Wno-unused-parameter_-Wno-unused-function_-Wno-unused-label_-Wno-nonnull-compare_-fno-common"
#define GLOBAL_OPTFLAGS "-O0_-g_-ggdb"
#define GLOBAL_CFLAGS "--std=gnu11_-Werror-implicit-function-declaration_-Wstrict-prototypes_-Wwrite-strings"
#define GLOBAL_CPPFLAGS "--std=c++11_-fno-exceptions_-fno-rtti_-fno-threadsafe-statics"
#define GLOBAL_ASMFLAGS "-DASSEMBLY"
#define GLOBAL_LDFLAGS "_-L._-Lexternal"
#define ARCH_COMPILEFLAGS "__-fno-omit-frame-pointer"
#define ARCH_CFLAGS ""
#define ARCH_CPPFLAGS ""
#define ARCH_ASMFLAGS ""
#define ARCH_LDFLAGS "_-z_max-page-size=4096"

[travisg]

The problem here is the cpu hasn't turned on the MMU at this point, and the address you're trying to set the breakpoint on is the virtual address once the mmu is enabled. The first bit of start.S is running with mmu disabled, so the address the cpu is running at is a physical address.

If you set the breakpoint a bit farther on, once the mmu is enabled it probably will have a better chance of working.

[wooshifu]

got it. thanks a lot😁