Constant MAX_STREAM_COALESCE 

pub(crate) const MAX_STREAM_COALESCE: usize = _; // 1_048_576usize

Cap on how much of a writev one coalescing buffer holds. The guest declares the iovec lengths and nothing has validated the matching pointers yet, so the buffer is sized by this rather than by what the guest claims. It also keeps a coalesced write clear of the 8 MiB frame limit that LengthDelimitedCodec imposes on remote virtual-net sockets.