The "dsb nsh; isb" sequence after "svc 0" is part of OpenBSD's mitigations for Spectre.
https://github.com/openbsd/src/commit/bbeaada4689520859307d5...
https://github.com/openbsd/src/commit/0c401ffc2a2550c32105ce...
https://github.com/openbsd/src/commit/5ecc9681133f1894e81c38...
If I'm reading the commits correctly, the OpenBSD kernel will skip two instructions after a "svc 0" when returning to userspace, on the assumption that any syscall comes from libc and therefore has "dsb nsh; isb" after it.