id = 2916
title = "qemu-system-arm hangs when attempting to enable MMU on Cortex-A7"
state = "opened"
created_at = "2025-04-07T11:27:39.291Z"
closed_at = "n/a"
labels = ["target: arm", "workflow::Needs Info"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2916"
host-os = "Ubuntu 22.04.5 LTS"
host-arch = "x86_64"
qemu-version = "QEMU emulator version 9.2.3"
guest-os = "bare-metal/?"
guest-arch = "ARM"
description = """QEMU 9.x.x+ hangs when attempting to do enable the MMU from SCTLRL - M bit: https://developer.arm.com/documentation/ddi0601/2025-03/AArch32-Registers/SCTLR--System-Control-Register

The instruction that hangs is the writing of the SCTLR register:

```
mrc     p15, 0, r0, c1, c0, 0
orr     r0, r0, 1
mcr     p15, 0, r0, c1, c0, 0
```

I am attempting to enable unaligned accesses and SCTLR-A bit doesn't seem to have any effect if the SCTLR-M is not enabled. Doing an unaligned access on cortex-a7 should be supported but it always trigger a Fault."""
reproduce = """1. add the mrc/orr/mcr instruction sequence in the ResetHandler
2. link the elf
3. attempt to execute it"""
additional = """The unaligned access looked like it was working in QEMU 8.x.x but it might not have been emulated(?). I also am facing the same issues with MCR hanging and unaligned access not supported with latest 10.0.0-RC2.

When it hangs, QEMU has to be killed and terminal reset.

There might be two separate issues here:

1. writing SCTLR register
2. emulated cortex-a7 not supporting unaligned access (hardware supports it)"""