Enable DIO

[IridiumXOR]

How is it possible to enable direct I/O?
Obviously, I enable the flag dio=1 when I load the module however, if I check using a debug build the module always returns Direct IO Disabled. I have used a USB3 drive on a real machine but I have also tested the option on a QEMU virtual machine with SCSI, SATA and USB virtual disks with ext4 and xfs but no results (also mounting partitions with sync option).
Can you explain how to use this feature?

[IridiumXOR]

I have investigated a little bit and seems that starting from kernel 4.14 (when vfs_write was replaced by kernel_write) DIO is not usable anymore, because kernel_write always returns -EFAULT if used with O_DIRECT (it is hardcoded in the code)

EDIT: I have discovered that this behavior is specific of EXT4 if you use a VFAT partition DIO works