rng-seed addition causing test_loongarch64_virt.py to hang in EFI startup

Host environment

• Operating system: Fedora 41
• OS/kernel version: 6.11.7-300.fc41.x86_64
• Architecture: x86_64
• QEMU flavor: qemu-system-loongarch64
• QEMU version: git d9bd1ccb
• QEMU command line:

Description of problem

Since the rng-seed addition, the test_loongarch64_virt.py test will periodically hang.

git bisect blames this

commit d9bd1ccbf1d84d872aed684c65fec33814b8ac1b
Author: Jason A. Donenfeld <Jason@zx2c4.com>
Date:   Thu Sep 5 17:33:16 2024 +0200

    hw/loongarch: virt: pass random seed to fdt
    
    If the FDT contains /chosen/rng-seed, then the Linux RNG will use it to
    initialize early. Set this using the usual guest random number
    generation function.
    
    This is the same procedure that's done in b91b6b5a2c ("hw/microblaze:
    pass random seed to fdt"), e4b4f0b71c ("hw/riscv: virt: pass random seed
    to fdt"), c6fe3e6b4c ("hw/openrisc: virt: pass random seed to fdt"),
    67f7e426e5 ("hw/i386: pass RNG seed via setup_data entry"), c287941a4d
    ("hw/rx: pass random seed to fdt"), 5e19cc68fb ("hw/mips: boston: pass
    random seed to fdt"), 6b23a67916 ("hw/nios2: virt: pass random seed to fdt")
    c4b075318e ("hw/ppc: pass random seed to fdt"), and 5242876f37
    ("hw/arm/virt: dt: add rng-seed property").
    
    These earlier commits later were amended to rerandomize the RNG seed on
    snapshot load, but the LoongArch code somehow already does that, despite
    not having this patch here, presumably due to some lucky copy and
    pasting.
    
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Reviewed-by: Song Gao <gaosong@loongson.cn>
    Message-Id: <20240905153316.2038769-1-Jason@zx2c4.com>
    Signed-off-by: Song Gao <gaosong@loongson.cn>

When it hangs, test_loongarch64_virt.py will get stuck waiting for serial console output from the guest.

Looking at the console.log file shows it to be completely empty.

This appears to indicate it has hung before EDK has even initialized, as it has not even printed the 'Entering C environment' message

Steps to reproduce

1. ./configure --target-list=loongarch64-softmmu
2. make -j 20
3. n=0 ; while true ; do n=$(expr $n + 1); echo $n ; QEMU_TEST_QEMU_BINARY=./build/qemu-system-loongarch64 PYTHONPATH=./python ./tests/functional/test_loongarch64_virt.py ; done

Most commonly it will hang within 10 iterations, very occasionally needing upto 25