Ubuntu builds its riscv64 port against the RVA23 profile (since 25.10), and QEMU's default rv64 CPU does not implement all profile extensions: Ubuntu binaries die with SIGILL during init and the kernel panics with 'Attempted to kill init'. Select the rva23s64 profile CPU (QEMU 10.1+) for riscv64 under TCG; profiles are supersets, so baseline rv64gc rootfses run unchanged on it. KVM mode keeps -cpu host.