Okay, I'm trying to figure out whether this bug report is about user-mode or system-mode in QEMU. The user provided a lot of details about their issue. Let me break it down. First, the problem occurs when running a "hello world" program using QEMU-system-aarch64. They noticed that after a certain commit, the behavior changed—it stopped working correctly. Specifically, they're using thevirt machine type with a cortex-a53 CPU and secure=on. The output from their hello world gets lost at a particular instruction in the disassembly. They built QEMU with several options: --disable-user, --disable-linux-user, etc., which suggests they're not compiling user-mode emulation support. They're using system mode because that's what -kernel option is for—it boots an OS or kernel image. The hello world seems to be a bare-metal program since it's linked as a flat binary (aarch64-none-elf). It directly uses printf, which implies they have a minimal C runtime setup, maybe with newlib or something similar. The fact that the output is lost suggests issues in how the system handles I/O, perhaps related to device emulation or interrupt handling. Looking at the disassembly, after str q0, [sp, #80], it goes into a series of udf instructions. UDF means undefined, which usually indicates an exception. This might be happening because QEMU isn't correctly emulating some system-level feature required for printf to work, like certain device registers or interrupt controllers. The bisecting shows that the bug started after commit 59754f85ed in QEMU. That commit could have changed how the virt machine or the GIC (Generic Interrupt Controller) is handled, which might affect how the kernel or application interacts with devices for output. Since they're using a bare-metal setup and relying on QEMU's system emulation to provide the necessary hardware environment, this seems like a system-mode issue. The problem likely lies in how QEMU emulates certain peripherals or handles interrupts required for printf to send data to the console. In summary, because they're running a kernel image with system-mode QEMU and the issue relates to device interaction (serial output), it's classified as a system-mode bug. system