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<think>
Okay, so I'm trying to figure out whether this bug in QEMU is user-mode or system-mode related. Let's see what the problem is.
The issue is that when running certain applications under QEMU using x86_64, the signal handlers cause a crash because of misaligned stack addresses leading to a `movaps` instruction failing. The user provided some debug output from GDB showing that the stack pointer (RSP) isn't aligned properly. According to the x86_64 psABI, when transferring control to a function entry point, `(RSP - 8)` should be a multiple of 16. But in this case, it's not, causing the crash.
The user also provided some kernel code that aligns the stack pointer for signal handlers. They mentioned that QEMU is jumping to the signal handler with RSP aligned to 16B. So I'm thinking about where the misalignment could be happening.
QEMU emulates the CPU and handles system calls, including signals. In user-mode emulation, QEMU would set up the stack correctly according to the ABI when invoking signal handlers. But if it's not doing that properly, then the issue is in how QEMU sets up the stack for the signal handler.
Looking at the kernel code snippet, it seems like the kernel aligns the stack pointer based on whether it's an IA32 frame or not. For x86_64, it rounds down and subtracts 8 to ensure alignment. If QEMU isn't correctly applying this logic when setting up the stack for signal delivery, that could cause misalignment.
Since the problem occurs in userspace applications (like Ruby) running under QEMU, but the issue is with how the stack is aligned when entering a signal handler, it's likely related to how QEMU handles the setup of the stack pointer before invoking the signal handler. That would be part of user-mode emulation because it's about how user-space processes are managed and their execution environment is set up.
So, putting this together, the bug is in how QEMU (as a user-mode emulator) sets up the stack for signal handlers, not properly aligning RSP according to the ABI. Therefore, it's a user-mode issue.
</think>
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