block: make available VIRTIO_BLK_F_SEG_MAX

This allows the guest to put in more than one segment per request. It
can improve the throughput of the system.

Introduce a new check to make sure the queue size configured by the user
is large enough to hold at least one segment.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

virtio-devices/src/block.rs

@@ -56,6 +56,8 @@ const RATE_LIMITER_EVENT: u16 = EPOLL_HELPER_EVENT_LAST + 3;
 // latency scale, for reduce precision loss in calculate.
 const LATENCY_SCALE: u64 = 10000;
 
+pub const MINIMUM_BLOCK_QUEUE_SIZE: u16 = 2;
+
 #[derive(Error, Debug)]
 pub enum Error {
     #[error("Failed to parse the request: {0}")]
@@ -625,6 +627,7 @@ impl Block {
                     | (1u64 << VIRTIO_BLK_F_CONFIG_WCE)
                     | (1u64 << VIRTIO_BLK_F_BLK_SIZE)
                     | (1u64 << VIRTIO_BLK_F_TOPOLOGY)
+                    | (1u64 << VIRTIO_BLK_F_SEG_MAX)
                     | (1u64 << VIRTIO_RING_F_EVENT_IDX)
                     | (1u64 << VIRTIO_RING_F_INDIRECT_DESC);
                 if iommu {
@@ -660,6 +663,7 @@ impl Block {
                     physical_block_exp,
                     min_io_size: (topology.minimum_io_size / logical_block_size) as u16,
                     opt_io_size: (topology.optimal_io_size / logical_block_size) as u32,
+                    seg_max: (queue_size - MINIMUM_BLOCK_QUEUE_SIZE) as u32,
                     ..Default::default()
                 };