Comment 5 for bug 1804961

Could it be related to:

commit 17ec075a651a3f9613429c2d97018fce459ed943
Author: Eric Auger <email address hidden>
Date: Fri Jun 22 13:28:37 2018 +0100

    hw/arm/virt: Use 256MB ECAM region by default

    With this patch, virt-3.0 machine uses a new 256MB ECAM region
    by default instead of the legacy 16MB one, if highmem is set
    (LPAE supported by the guest) and (!firmware_loaded || aarch64).

    Indeed aarch32 mode FW may not support this high ECAM region.

As far as I can tell the only real differences between virt-2.12 and later at the size of the ECAM region and the max number of CPUs it supports:

static void virt_machine_2_12_options(MachineClass *mc)
{
    VirtMachineClass *vmc = VIRT_MACHINE_CLASS(OBJECT_CLASS(mc));

    virt_machine_3_0_options(mc);
    SET_MACHINE_COMPAT(mc, VIRT_COMPAT_2_12);
    vmc->no_highmem_ecam = true;
    mc->max_cpus = 255;
}