On Mon, Jul 30, 2012 at 09:31:06AM +0200, Paolo Bonzini wrote:

You only need to add REQ_FLUSH support.  The virtio-blk protocol does
not support REQ_FUA, because there's no easy way to do it in userspace.

A bio-based driver needs to handle both REQ_FLUSH and REQ_FUA as it does
not get the sequencing of REQ_FUA into REQ_FLUSH that request based drivers
can request.  To what the REQ_FUA request gets translated is a different story.