io_uring: Add new feature gate

By adding a new io_uring feature gate, we let the user the possibility
to choose if he wants to enable the io_uring improvements or not.
Since the io_uring feature depends on the availability on recent host
kernels, it's better if we leave it off for now.

As soon as our CI will have support for a kernel 5.6 with all the
features needed from io_uring, we'll enable this feature gate
permanently.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

Cargo.toml

@@ -42,6 +42,7 @@ mmio = ["vmm/mmio_support"]
 cmos = ["vmm/cmos"]
 fwdebug = ["vmm/fwdebug"]
 kvm = ["vmm/kvm"]
+io_uring = ["vmm/io_uring"]
 
 # Integration tests require a special environment to run in
 integration_tests = []

block_util/Cargo.toml

@@ -4,6 +4,10 @@ version = "0.1.0"
 authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"
 
+[features]
+default = []
+io_uring = []
+
 [dependencies]
 io-uring = { git = "https://github.com/tokio-rs/io-uring.git", branch = "0.4" }
 libc = "0.2.74"

block_util/src/lib.rs

@@ -13,17 +13,20 @@ extern crate log;
 #[macro_use]
 extern crate serde_derive;
 
+#[cfg(feature = "io_uring")]
 use io_uring::{opcode, IoUring, Probe};
 use serde::ser::{Serialize, SerializeStruct, Serializer};
 use std::cmp;
 use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::os::linux::fs::MetadataExt;
+#[cfg(feature = "io_uring")]
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::path::PathBuf;
 use std::result;
 use virtio_bindings::bindings::virtio_blk::*;
 use vm_memory::{ByteValued, Bytes, GuestAddress, GuestMemory, GuestMemoryError, GuestMemoryMmap};
 use vm_virtio::DescriptorChain;
+#[cfg(feature = "io_uring")]
 use vmm_sys_util::eventfd::EventFd;
 
 const SECTOR_SHIFT: u8 = 9;
@@ -263,6 +266,7 @@ impl Request {
         Ok(0)
     }
 
+    #[cfg(feature = "io_uring")]
     pub fn execute_io_uring(
         &self,
         mem: &GuestMemoryMmap,
@@ -473,6 +477,7 @@ unsafe impl ByteValued for VirtioBlockGeometry {}
 
 /// Check if io_uring for block device can be used on the current system, as
 /// it correctly supports the expected io_uring features.
+#[cfg(feature = "io_uring")]
 pub fn block_io_uring_is_supported() -> bool {
     let error_msg = "io_uring not supported:";
 

virtio-devices/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2018"
 default = []
 pci_support = ["pci"]
 mmio_support = []
+io_uring = ["block_util/io_uring"]
 
 [dependencies]
 anyhow = "1.0"

virtio-devices/src/lib.rs

@@ -31,6 +31,7 @@ use std::io;
 mod device;
 pub mod balloon;
 pub mod block;
+#[cfg(feature = "io_uring")]
 pub mod block_io_uring;
 mod console;
 pub mod epoll_helper;
@@ -46,6 +47,7 @@ pub mod vsock;
 
 pub use self::balloon::*;
 pub use self::block::*;
+#[cfg(feature = "io_uring")]
 pub use self::block_io_uring::*;
 pub use self::console::*;
 pub use self::device::*;

vmm/Cargo.toml

@@ -12,6 +12,7 @@ mmio_support = ["virtio-devices/mmio_support"]
 cmos = ["devices/cmos"]
 fwdebug = ["devices/fwdebug"]
 kvm = ["hypervisor/kvm"]
+io_uring = ["virtio-devices/io_uring"]
 
 [dependencies]
 arc-swap = ">=0.4.4"

vmm/src/device_manager.rs

@@ -32,6 +32,7 @@ use arch::layout;
 use arch::layout::{APIC_START, IOAPIC_SIZE, IOAPIC_START};
 #[cfg(target_arch = "aarch64")]
 use arch::DeviceType;
+#[cfg(feature = "io_uring")]
 use block_util::block_io_uring_is_supported;
 #[cfg(target_arch = "aarch64")]
 use devices::gic;
@@ -1669,6 +1670,8 @@ impl DeviceManager {
                 .map_err(DeviceManagerError::DetectImageType)?;
             let (virtio_device, migratable_device) = match image_type {
                 ImageType::Raw => {
+                    #[cfg(feature = "io_uring")]
+                    {
                         // Use asynchronous backend relying on io_uring if the
                         // syscalls are supported.
                         if block_io_uring_is_supported() {
@@ -1717,6 +1720,32 @@ impl DeviceManager {
                             )
                         }
                     }
+
+                    #[cfg(not(feature = "io_uring"))]
+                    {
+                        let dev = Arc::new(Mutex::new(
+                            virtio_devices::Block::new(
+                                id.clone(),
+                                raw_img,
+                                disk_cfg
+                                    .path
+                                    .as_ref()
+                                    .ok_or(DeviceManagerError::NoDiskPath)?
+                                    .clone(),
+                                disk_cfg.readonly,
+                                disk_cfg.iommu,
+                                disk_cfg.num_queues,
+                                disk_cfg.queue_size,
+                            )
+                            .map_err(DeviceManagerError::CreateVirtioBlock)?,
+                        ));
+
+                        (
+                            Arc::clone(&dev) as VirtioDeviceArc,
+                            dev as Arc<Mutex<dyn Migratable>>,
+                        )
+                    }
+                }
                 ImageType::Qcow2 => {
                     let qcow_img =
                         QcowFile::from(raw_img).map_err(DeviceManagerError::QcowDeviceCreate)?;