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Konstantin Belousov
5bc997b6f5
SDM requires that IOPL is zero when returning to usermode with ERETU.
Do not set rflags.iopl for FreeBSD io(4) open.
This might be changed to disallow io(4) open for FRED at all with a
knob, if there is a real need.
Return ENOSYS from linux iopl(2), apparently Linux kernel can be
configured without iopl(2) support, so linux programs should be prepared
for this error.
Return EINVAL from vt(4)/syscons(4) ioctl(KDENABIO).
Fixes: 6e93f5e4d6 ("amd64: FRED support")
Sponsored by: The FreeBSD Foundation
MFC after: 1 week