What does --cache exactly mean?

What does --cache exactly mean? My understanding is there is no page cache in the guest kernel for virtiofs, so this option is to control the behaviour of host kernel page cache. So does it mean:

• never: each time writing to a page cache page, flush it
• always: don't flush unless the user asks to do it
• auto: flush page cache when close a file.

Do I think it in the right way? Here why do we have --cache=never? Seems it doesn't make sense when we are in the passthroughFS case.