call trace debugging

[preveen-stack]

Call Trace:
[  647.820144] ------------[ cut here ]------------
[  647.820848] kernel BUG at drivers/iommu/intel/iommu.c:1174!
[  647.821486] invalid opcode: 0000 [#1] PREEMPT SMP PTI
[  647.822082] CPU: 10 PID: 1181 Comm: qemu-system-x86 Not tainted 6.0.0-rc1home_lulu_2452_lulu7_vhost+ #62
[  647.823139] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-29-g6a62e0cb0dfe-prebuilt.qem4
[  647.824365] RIP: 0010:domain_unmap+0x48/0x110
[  647.825424] Code: 48 89 fb 8d 4c f6 1e 39 c1 0f 4f c8 83 e9 0c 83 f9 3f 7f 18 48 89 e8 48 d3 e8 48 85 c0 75 59
[  647.828064] RSP: 0018:ffffae5340c0bbf0 EFLAGS: 00010202
[  647.828973] RAX: 0000000000000001 RBX: ffff921793d10540 RCX: 000000000000001b
[  647.830083] RDX: 00000000080000ff RSI: 0000000000000001 RDI: ffff921793d10540
[  647.831214] RBP: 0000000007fc0100 R08: ffffae5340c0bcd0 R09: 0000000000000003
[  647.832388] R10: 0000007fc0100000 R11: 0000000000100000 R12: 00000000080000ff
[  647.833668] R13: ffffae5340c0bcd0 R14: ffff921793d10590 R15: 0000008000100000
[  647.834782] FS:  00007f772ec90640(0000) GS:ffff921ce7a80000(0000) knlGS:0000000000000000
[  647.836004] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  647.836990] CR2: 00007f02c27a3a20 CR3: 0000000101b0c006 CR4: 0000000000372ee0
[  647.838107] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  647.839283] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  647.840666] Call Trace:
[  647.841437]  <TASK>
[  647.842107]  intel_iommu_unmap_pages+0x93/0x140
[  647.843112]  __iommu_unmap+0x91/0x1b0
[  647.844003]  iommu_unmap+0x6a/0x95
[  647.844885]  vhost_vdpa_unmap+0x1de/0x1f0 [vhost_vdpa]
[  647.845985]  vhost_vdpa_process_iotlb_msg+0xf0/0x90b [vhost_vdpa]
[  647.847235]  ? _raw_spin_unlock+0x15/0x30
[  647.848181]  ? _copy_from_iter+0x8c/0x580
[  647.849137]  vhost_chr_write_iter+0xb3/0x430 [vhost]
[  647.850126]  vfs_write+0x1e4/0x3a0
[  647.850897]  ksys_write+0x53/0xd0
[  647.851688]  do_syscall_64+0x3a/0x90
[  647.852508]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[  647.853457] RIP: 0033:0x7f7734ef9f4f
[  647.854408] Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 29 76 f8 ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c8
[  647.857217] RSP: 002b:00007f772ec8f040 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
[  647.858486] RAX: ffffffffffffffda RBX: 00000000fef00000 RCX: 00007f7734ef9f4f
[  647.859713] RDX: 0000000000000048 RSI: 00007f772ec8f090 RDI: 0000000000000010
[  647.860942] RBP: 00007f772ec8f1a0 R08: 0000000000000000 R09: 0000000000000000
[  647.862206] R10: 0000000000000001 R11: 0000000000000293 R12: 0000000000000010
[  647.863446] R13: 0000000000000002 R14: 0000000000000000 R15: ffffffff01100000
[  647.864692]  </TASK>
[  647.865458] Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver nfs lockd grace fscache netfs v]
[  647.874688] ---[ end trace 0000000000000000 ]---
[  647.876013] RIP: 0010:domain_unmap+0x48/0x110
[  647.878306] Code: 48 89 fb 8d 4c f6 1e 39 c1 0f 4f c8 83 e9 0c 83 f9 3f 7f 18 48 89 e8 48 d3 e8 48 85 c0 75 59
[  647.884581] RSP: 0018:ffffae5340c0bbf0 EFLAGS: 00010202
[  647.886308] RAX: 0000000000000001 RBX: ffff921793d10540 RCX: 000000000000001b
[  647.888775] RDX: 00000000080000ff RSI: 0000000000000001 RDI: ffff921793d10540
[  647.890295] RBP: 0000000007fc0100 R08: ffffae5340c0bcd0 R09: 0000000000000003
[  647.891660] R10: 0000007fc0100000 R11: 0000000000100000 R12: 00000000080000ff
[  647.893019] R13: ffffae5340c0bcd0 R14: ffff921793d10590 R15: 0000008000100000
[  647.894506] FS:  00007f772ec90640(0000) GS:ffff921ce7a80000(0000) knlGS:0000000000000000
[  647.895963] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  647.897348] CR2: 00007f02c27a3a20 CR3: 0000000101b0c006 CR4: 0000000000372ee0
[  647.898719] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

[preveen-stack]

A stack trace is a list of function calls that have been executed leading up to an error or exception. It typically includes the name of the function, the file name, and line number where the function was called, and information about the error or exception that was raised.

Here are some tips for reading a stack trace:

1. Start at the top of the stack trace and work your way down. The top of the stack trace will show the most recent function call, while the bottom of the stack trace will show the initial function call.

2. Look for the line number and file name where the error or exception occurred. This information can help you narrow down where the problem is occurring in your code.

3. Look for any error messages or exception types that are displayed in the stack trace. These messages can provide clues about the cause of the problem.

4. Look for any additional information that is included in the stack trace, such as the values of variables or parameters that were passed to the functions.

5. Pay attention to the indentation of the stack trace. The indentation indicates the level of nested function calls.

6. Finally, use the information in the stack trace to debug your code. Once you have identified the location of the error, you can go back to your code and try to figure out what is causing the problem.

[preveen-stack]

This is a kernel panic trace which is generated when the Linux kernel detects an internal error that it cannot recover from. The call trace shows the sequence of function calls that led to the error.

Here is a breakdown of the information provided in the stack trace:

• The first line shows the message "kernel BUG at drivers/iommu/intel/iommu.c:1174!", which indicates that a kernel bug was encountered in the iommu driver at line 1174 of the iommu.c file.

• The second line shows the error code (in this case, an invalid opcode) and the error type (in this case, a #1, which means a hardware error).

• The third line shows the process ID (PID), the name of the process (in this case, qemu-system-x86), and whether or not the kernel has been modified or "tainted."

• The fourth line shows the hardware name and the BIOS version.

• The fifth line shows the instruction pointer (RIP) at the point where the error occurred, along with the function name (domain_unmap) and the offset within the function (0x48/0x110).

• The following lines show the values of various registers (RAX, RBX, RCX, etc.) and flags (EFLAGS).

• The last part of the trace shows the call stack, which is a list of functions that were called leading up to the error. The most recent function call is at the top, and the earlier calls are listed below it. The function names are shown along with the offset within the function.

In this case, the kernel panic was caused by a bug in the Intel IOMMU driver, specifically in the domain_unmap function. The call stack shows that the error occurred while trying to unmap pages in the IOMMU, and that the error was triggered by a call to the vhost_vdpa_unmap function.

[preveen-stack]

The code section in the trace refers to the specific code location where the kernel encountered a bug and crashed. In this particular trace, the code section is shown as:

kernel BUG at drivers/iommu/intel/iommu.c:1174!

This indicates that the bug occurred in the intel/iommu.c file at line 1174. The kernel encountered an invalid opcode, which caused the crash. The subsequent lines in the trace provide more information about the state of the system at the time of the crash, such as the CPU, process ID, and register values.

[preveen-stack]

In a Linux kernel trace, the "Code" section refers to the machine code instructions that were executed by the processor at the point where the trace was generated. In the example you provided, the code section is:

48 89 fb 8d 4c f6 1e 39 c1 0f 4f c8 83 e9 0c 83 f9 3f 7f 18 48 89 e8 48 d3 e8 48 85 c0 75 59

This is a sequence of hexadecimal values that represents the actual instructions executed by the processor. The meaning of these instructions depends on the specific processor architecture and the context in which they were executed. In order to understand what this code does, one would need to disassemble it and interpret the resulting assembly instructions.

[preveen-stack]

This is a line from a Linux kernel stack trace. The code section in the trace represents the machine code instructions that were being executed when the kernel encountered an error.

Here's a breakdown of the code section you provided:

48 89 fb 8d 4c f6 1e 39 c1 0f 4f c8 83 e9 0c 83 f9 3f 7f 18 48 89 e8 48 d3 e8 48 85 c0 75 59

Each pair of characters represents a single byte of machine code in hexadecimal notation. Here's what each byte represents:

48 89 fb             mov    %rdi,%rbx
8d 4c f6 1e          lea    (%rsi,%rdx,8),%r9
39 c1                cmp    %eax,%ecx
0f 4f c8             cmovg  %eax,%ecx
83 e9 0c             sub    $0xc,%ecx
83 f9 3f             cmp    $0x3f,%ecx
7f 18                jg     24 <function_name+0x24>
48 89 e8             mov    %rbp,%rax
48 d3 e8             shr    %cl,%rax
48 85 c0             test   %rax,%rax
75 59                jne    91 <function_name+0x91>

This is x86-64 machine code, which is the instruction set architecture used by most modern desktop and server CPUs.

Without more context, it's difficult to say exactly what this code is doing or what the error might be. However, we can see that it's performing some arithmetic operations, comparing values, and jumping to different parts of the code based on the results of those comparisons. The mov instructions are moving values between registers, while the lea instruction is performing a memory address calculation. The jg and jne instructions are conditional jumps based on the results of the cmp instructions.

If you have more information about the error or the context in which this code is running, I might be able to provide more specific insights.

[preveen-stack]

Register values can provide useful information when troubleshooting software or hardware issues. Depending on the context, reading register values can help us:

• Identify the current state of the system: Register values can give us an idea of what the system is doing at a particular point in time. For example, the program counter (PC) register can tell us which instruction the processor is currently executing.

• Diagnose issues: Register values can provide clues as to what might be causing a particular issue. For example, if a program crashes, examining the contents of the registers can help identify where the program failed.

• Optimize performance: Register values can help identify performance bottlenecks in software. By analyzing how often certain registers are accessed and how long they are held for, we can identify areas where optimizations can be made.

Overall, reading register values is a valuable tool for software and hardware engineers when trying to understand the behavior of a system. However, it's important to have a deep understanding of the system's architecture and how registers are used to fully interpret the information provided by register values.

[preveen-stack]

Yes, we can understand some information by looking at the register values in the provided trace. Here's what we can understand:

• RSP (stack pointer): Points to the current position in the stack where the next value will be pushed. It is currently pointing to ffffae5340c0bbf0.
• EFLAGS: A register that contains the current status of the processor. In this case, the value is 00010202.
• RAX, RBX, RCX, RDX, RSI, RDI, RBP, R8-R15: These are general-purpose registers that are used to store data and perform operations. The values in these registers at this particular moment can provide some insight into what the program is doing, but without context, it is difficult to draw conclusions.
• FS, GS, and knlGS: These are segment registers that are used to access memory segments in protected mode. The values of these registers can provide information about which segment the program is currently accessing.
• CS, DS, ES: These are segment registers that are used to access code, data, and stack segments. The values of these registers at this particular moment can provide some insight into what the program is doing, but again, without context, it is difficult to draw conclusions.
• CR0, CR2, CR3, and CR4: These are control registers that are used to control various aspects of the processor, such as paging and memory protection. The values of these registers at this particular moment can provide some insight into the memory management setup of the system.
• DR0-DR7: These are debug registers that are used to set breakpoints and perform other debugging tasks. The values of these registers at this particular moment are all set to 0, indicating that no debug registers are currently being used.

[preveen-stack]

Yes, typically in a call trace, the latest call is displayed at the top, and the previous calls are listed below it in a stack-like format. This is because the most recent function call is the one currently being executed, and it calls the previous function, which calls the one before it, and so on. When an error or crash occurs, the call trace is generated to help identify which function caused the error or crash, and it shows the function call hierarchy in reverse order, starting from the most recent function call that led to the error or crash.

[preveen-stack]

The call trace shows the sequence of function calls that led to the current point in the code execution. The call trace is listed in reverse order, with the most recent function call at the top and the initial function call at the bottom.

In this case, the call trace suggests that the error occurred while processing a write operation in the vhost_vdpa driver. The call stack shows that the write operation led to a call to the function vhost_vdpa_process_iotlb_msg, which in turn called vhost_vdpa_unmap, iommu_unmap, __iommu_unmap, and finally intel_iommu_unmap_pages.

[preveen-stack]

[ 647.847235] ? _raw_spin_unlock+0x15/0x30 This line is part of the call trace and indicates a possible function call that was made during the execution of the code. In this case, the function being referenced is _raw_spin_unlock, which is part of the kernel's spinlock implementation. The numbers after the function name, 0x15/0x30, indicate the offset of the instruction within the function. This information can be helpful in identifying the specific point in the code where the function call was made, and in determining what might have caused the issue that led to the trace being generated.

[preveen-stack]

In the context of the call trace, the ? before the function name indicates that this function call may not be directly related to the current issue being traced. It means that the current function was not called by the previous function in the trace, but instead was likely called by a different code path. This is denoted by the question mark and is used to indicate that the relationship between this function and the previous one in the trace is not completely clear.

[preveen-stack]

In the call trace, some lines may start with a question mark "?" followed by the function name. This indicates that the function is a weak function or a function whose address cannot be determined by the kernel. The question mark indicates that the function is not guaranteed to be present or accessible during runtime, and its absence does not necessarily indicate a problem.

Additionally, some lines may have a "+" symbol followed by an offset value. This indicates the offset within the function where the call was made. For example, "intel_iommu_unmap_pages+0x93/0x140" means that the call was made at an offset of 0x93 within the "intel_iommu_unmap_pages" function, which has a total size of 0x140 bytes.

Finally, some lines may have the module name enclosed in square brackets, such as "[vhost_vdpa]". This indicates the module or driver where the function is defined.

[preveen-stack]

Some other topics you may want to explore regarding analyzing a call trace include:

1. Kernel debugging: Understanding how to use kernel debugging tools such as gdb, kdb, kgdb, or SystemTap can help you troubleshoot issues and analyze call traces in more detail.

2. Symbol resolution: In order to understand what functions are being called in the call trace, you need to know how to resolve symbol names from their memory addresses.

3. Stack tracing: Call traces are a type of stack trace, which can provide valuable information about the execution path of a program or system. Understanding how to generate and analyze stack traces can help you diagnose performance issues, memory leaks, and other problems.

4. Timing analysis: In addition to function calls, call traces can also provide timing information about how long each function call takes to execute. Analyzing this timing data can help you identify performance bottlenecks and other issues.

5. Profiling: Profiling tools such as perf, oprofile, or gprof can provide detailed information about the performance of a program or system, including function call frequencies, execution times, and memory usage. Analyzing profiling data can help you optimize your code and improve system performance.

Overall, analyzing call traces is a complex and specialized task that requires a deep understanding of the operating system, computer architecture, and software development. However, with the right tools and knowledge, you can use call traces to gain valuable insights into the behavior of your system and optimize its performance.