qemu seems to lack support for pid namespace.

# Version

qemu-4.0.0

# commands used to launch qemu-aarch64 in user mode.

printf '%s\n' ':qemu-aarch64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xb7\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-aarch64:'"${QEMU_BINFMT_FLAGS}" >/proc/sys/fs/binfmt_misc/register

> sudo cp /usr/bin/qemu-aarch64 $RPI/usr/bin
> sudo chroot $RPI /bin/ksh -l

# host

Gentoo Linux amd64

# Guest

Gentoo Linux aarch64

# The problem that I have

"emerge" program fails due to the error, "qemu: qemu_thread_create: Invalid argument".
"emerge" is Gentoo's package manager that compiles and installs packages.

# How to reproduce the issue

Execute

unshare --pid -- echo hello world

or

python -c "import portage.process; portage.process.spawn(['echo', 'hello', 'world'], unshare_pid=True)"