From 3480e69ff5cde1c253beaf535bcdffc4596f672f Mon Sep 17 00:00:00 2001
From: Rob Bradford <robert.bradford@intel.com>
Date: Fri, 12 Nov 2021 10:38:51 +0000
Subject: [PATCH] vmm: Cache whether io_uring is supported in DeviceManager

Probing for whether the io_uring is supported is time consuming so cache
this value if it is known to reduce the cost for secondary block devices
that are added.

Before:

cloud-hypervisor: 3.988896ms: <vmm> INFO:vmm/src/device_manager.rs:1901 -- Creating virtio-block device: DiskConfig { path: Some("/home/rob/workloads/focal-server-cloudimg-amd64-custom-20210609-0.raw"), readonly: false, direct: false, iommu: false, num_queues: 1, queue_size: 128, vhost_user: false, vhost_socket: None, poll_queue: true, rate_limiter_config: None, id: Some("_disk0"), disable_io_uring: false, pci_segment: 0 }
cloud-hypervisor: 14.129591ms: <vmm> INFO:vmm/src/device_manager.rs:1983 -- Using asynchronous RAW disk file (io_uring)
cloud-hypervisor: 14.159853ms: <vmm> INFO:vmm/src/device_manager.rs:1901 -- Creating virtio-block device: DiskConfig { path: Some("/tmp/disk"), readonly: false, direct: false, iommu: false, num_queues: 1, queue_size: 128, vhost_user: false, vhost_socket: None, poll_queue: true, rate_limiter_config: None, id: Some("_disk1"), disable_io_uring: false, pci_segment: 0 }
cloud-hypervisor: 22.110281ms: <vmm> INFO:vmm/src/device_manager.rs:1983 -- Using asynchronous RAW disk file (io_uring)

After:

cloud-hypervisor: 4.880411ms: <vmm> INFO:vmm/src/device_manager.rs:1916 -- Creating virtio-block device: DiskConfig { path: Some("/home/rob/workloads/focal-server-cloudimg-amd64-custom-20210609-0.raw"), readonly: false, direct: false, iommu: false, num_queues: 1, queue_size: 128, vhost_user: false, vhost_socket: None, poll_queue: true, rate_limiter_config: None, id: Some("_disk0"), disable_io_uring: false, pci_segment: 0 }
cloud-hypervisor: 14.105123ms: <vmm> INFO:vmm/src/device_manager.rs:1998 -- Using asynchronous RAW disk file (io_uring)
cloud-hypervisor: 14.134837ms: <vmm> INFO:vmm/src/device_manager.rs:1916 -- Creating virtio-block device: DiskConfig { path: Some("/tmp/disk"), readonly: false, direct: false, iommu: false, num_queues: 1, queue_size: 128, vhost_user: false, vhost_socket: None, poll_queue: true, rate_limiter_config: None, id: Some("_disk1"), disable_io_uring: false, pci_segment: 0 }
cloud-hypervisor: 14.221869ms: <vmm> INFO:vmm/src/device_manager.rs:1998 -- Using asynchronous RAW disk file (io_uring)

Signed-off-by: Rob Bradford <robert.bradford@intel.com>
---
 vmm/src/device_manager.rs | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/vmm/src/device_manager.rs b/vmm/src/device_manager.rs
index a2149a617..b43ef71e8 100644
--- a/vmm/src/device_manager.rs
+++ b/vmm/src/device_manager.rs
@@ -906,6 +906,9 @@ pub struct DeviceManager {
 
     // Helps identify if the VM is currently being restored
     restoring: bool,
+
+    // io_uring availability if detected
+    io_uring_supported: Option<bool>,
 }
 
 impl DeviceManager {
@@ -1043,6 +1046,7 @@ impl DeviceManager {
             gpio_device: None,
             force_iommu,
             restoring,
+            io_uring_supported: None,
         };
 
         let device_manager = Arc::new(Mutex::new(device_manager));
@@ -1886,6 +1890,17 @@ impl DeviceManager {
         Ok(devices)
     }
 
+    // Cache whether io_uring is supported to avoid probing for very block device
+    fn io_uring_is_supported(&mut self) -> bool {
+        if let Some(supported) = self.io_uring_supported {
+            return supported;
+        }
+
+        let supported = block_io_uring_is_supported();
+        self.io_uring_supported = Some(supported);
+        supported
+    }
+
     fn make_virtio_block_device(
         &mut self,
         disk_cfg: &mut DiskConfig,
@@ -1962,7 +1977,7 @@ impl DeviceManager {
                 ImageType::FixedVhd => {
                     // Use asynchronous backend relying on io_uring if the
                     // syscalls are supported.
-                    if block_io_uring_is_supported() && !disk_cfg.disable_io_uring {
+                    if self.io_uring_is_supported() && !disk_cfg.disable_io_uring {
                         info!("Using asynchronous fixed VHD disk file (io_uring)");
                         Box::new(
                             FixedVhdDiskAsync::new(file)
@@ -1979,7 +1994,7 @@ impl DeviceManager {
                 ImageType::Raw => {
                     // Use asynchronous backend relying on io_uring if the
                     // syscalls are supported.
-                    if block_io_uring_is_supported() && !disk_cfg.disable_io_uring {
+                    if self.io_uring_is_supported() && !disk_cfg.disable_io_uring {
                         info!("Using asynchronous RAW disk file (io_uring)");
                         Box::new(RawFileDisk::new(file)) as Box<dyn DiskFile>
                     } else {