qemu-system-mipsel regression, Linux generated with Buildroot does not boot anymore

Host environment

• Operating system: Debian/GNU Linux 11
• OS/kernel version: 5.10
• Architecture: MIPS Little Endian
• QEMU flavor: qemu-system-mipsel
• QEMU version: 9.0.2
• QEMU command line: qemu-system-mipsel -M malta -kernel vmlinux -drive file=rootfs.ext2,format=raw -append "rootwait root=/dev/sda " -net nic,model=pcnet -net user -cpu P5600 -nographic

Emulated/Virtualized environment

• Operating system: Linux
• OS/kernel version: 6.6.18
• Architecture: MIPS Little Endian r5 (mips32r5el)

Description of problem

Buildroot Toolchain Builders try to release a new version. See a message from Thomas Petazzoni with the remaining issues: https://lore.kernel.org/buildroot/20240730223542.273693e5@windsurf/T/#u

All toolchains generate a system that fails to boot:

Run /sbin/init as init process process '/bin/busybox' started with executable stack Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004

The interesting thing is that those images boot fine with Qemu v8.2.6, but they fail to boot with Qemu v9.0.2.

I tracked it down to this commit: commit 4e999bf4 Author: Richard Henderson richard.henderson@linaro.org Date: Sun Jan 28 15:58:52 2024 +1000

target/mips: Pass ptw_mmu_idx down from mips_cpu_tlb_fill

Rather than adjust env->hflags so that the value computed
by cpu_mmu_index() changes, compute the mmu_idx that we
want directly and pass it down.

Introduce symbolic constants for MMU_{KERNEL,ERL}_IDX.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

Unfortunately just reverting this commit in 9.0.2 does not help, Qemu segfaults on Linux Kernel boot then.