Method is_writeback_enabled

virtio-devices/src/block.rs:995–999

The virtio v1.2 spec says "If VIRTIO_BLK_F_CONFIG_WCE was not negotiated but VIRTIO_BLK_F_FLUSH was, the driver SHOULD assume presence of a writeback cache." It also says "If VIRTIO_BLK_F_CONFIG_WCE is negotiated but VIRTIO_BLK_F_FLUSH is not, the device MUST initialize writeback to 0."

(&self, desired: bool)

    /// VIRTIO_BLK_F_CONFIG_WCE is negotiated but VIRTIO_BLK_F_FLUSH is not,
    /// the device MUST initialize writeback to 0."
    fn is_writeback_enabled(&self, desired: bool) -> bool {
        let flush = self.common.feature_acked(VIRTIO_BLK_F_FLUSH.into());
        let wce = self.common.feature_acked(VIRTIO_BLK_F_CONFIG_WCE.into());
        if wce { flush && desired } else { flush }
    }

    fn set_writeback_mode(&mut self, enabled: bool) {
        self.config.writeback = enabled as u8;