include, block: add krun_enable_relaxed_sync_for_disk API

Add an API that allows the user to explicitly specify whether they want
to enable the relaxed sync behavior on macOS to match the
Virtualization.Framework's behavior.

Signed-off-by: Jake Correnti <[email protected]>

Author: jakecorrenti

include/libkrun.h

@@ -1034,6 +1034,27 @@ int32_t krun_add_serial_console_default(uint32_t ctx_id,
  */
 int32_t krun_set_root_disk_remount(uint32_t ctx_id, const char *device, const char *fstype, const char *options);
 
+/**
+ * On macOS, `std::fs::File::flush()` uses `F_FULLFSYNC`, which syncs the buffers to the drive an asks
+ * the drive to wait for flushing all the data to the permanent storage. In contrast, Apple
+ * Virtualization.Framework only issues an `fsync()`, which doesn't ask the drive to flush it's buffered
+ * data.
+ *
+ * Configure whether to match the Virtualization.Framework behavior.
+ *
+ * Arguments:
+ *  "ctx_id" - the configuration context ID.
+ *  "block_id"  - a null-terminated string representing the partition to configure.
+ *  "enable" - whether or not to enable relaxed sync behavior.
+ *
+ * Returns:
+ *  Zero on success or a negative error number on failure.
+ *
+ * Notes:
+ *  This is a macOS only API.
+ */
+int32_t krun_enable_relaxed_sync_for_disk(uint32_t ctx_id, const char *block_id, bool enable);
+
 /**
  * Starts and enters the microVM with the configured parameters. The VMM will attempt to take over
  * stdin/stdout to manage them on behalf of the process running inside the isolated environment,

src/devices/src/virtio/block/device.rs

@@ -208,6 +208,7 @@ impl Block {
         disk_image_path: String,
         disk_image_format: ImageType,
         is_disk_read_only: bool,
+        #[cfg(target_os = "macos")] enable_relaxed_sync: bool,
     ) -> io::Result<Block> {
         let disk_image = OpenOptions::new()
             .read(true)
@@ -220,7 +221,7 @@ impl Block {
             .write(!is_disk_read_only)
             .filename(disk_image_path);
         #[cfg(target_os = "macos")]
-        let file_opts = file_opts.relaxed_sync(true);
+        let file_opts = file_opts.relaxed_sync(enable_relaxed_sync);
         let file = ImagoFile::open_sync(file_opts)?;
 
         let disk_image = match disk_image_format {

src/libkrun/src/lib.rs

@@ -656,6 +656,8 @@ pub unsafe extern "C" fn krun_add_disk(
                 disk_image_path: disk_path.to_string(),
                 disk_image_format: ImageType::Raw,
                 is_disk_read_only: read_only,
+                #[cfg(target_os = "macos")]
+                enable_relaxed_sync: false,
             };
             cfg.add_block_cfg(block_device_config);
         }
@@ -703,6 +705,8 @@ pub unsafe extern "C" fn krun_add_disk2(
                 disk_image_path: disk_path.to_string(),
                 disk_image_format: format,
                 is_disk_read_only: read_only,
+                #[cfg(target_os = "macos")]
+                enable_relaxed_sync: false,
             };
             cfg.add_block_cfg(block_device_config);
         }
@@ -730,6 +734,8 @@ pub unsafe extern "C" fn krun_set_root_disk(ctx_id: u32, c_disk_path: *const c_c
                 disk_image_path: disk_path.to_string(),
                 disk_image_format: ImageType::Raw,
                 is_disk_read_only: false,
+                #[cfg(target_os = "macos")]
+                enable_relaxed_sync: false,
             };
             cfg.set_root_block_cfg(block_device_config);
         }
@@ -757,6 +763,8 @@ pub unsafe extern "C" fn krun_set_data_disk(ctx_id: u32, c_disk_path: *const c_c
                 disk_image_path: disk_path.to_string(),
                 disk_image_format: ImageType::Raw,
                 is_disk_read_only: false,
+                #[cfg(target_os = "macos")]
+                enable_relaxed_sync: false,
             };
             cfg.set_data_block_cfg(block_device_config);
         }
@@ -2275,6 +2283,34 @@ pub unsafe extern "C" fn krun_set_kernel_console(ctx_id: u32, console_id: *const
     KRUN_SUCCESS
 }
 
+#[allow(clippy::missing_safety_doc)]
+#[no_mangle]
+#[cfg(target_os = "macos")]
+pub unsafe extern "C" fn krun_enable_relaxed_sync(
+    ctx_id: u32,
+    c_block_id: *const c_char,
+    enable: bool,
+) -> i32 {
+    let block_id = match CStr::from_ptr(c_block_id).to_str() {
+        Ok(block_id) => block_id.to_string(),
+        Err(_) => return -libc::EINVAL,
+    };
+    match CTX_MAP.lock().unwrap().entry(ctx_id) {
+        Entry::Occupied(mut ctx_cfg) => {
+            let cfg = ctx_cfg.get_mut();
+            for blk in cfg.block_cfgs.iter_mut() {
+                if blk.block_id == block_id {
+                    blk.enable_relaxed_sync = enable;
+                    return KRUN_SUCCESS;
+                }
+            }
+            error!("no block device configured matching the provided block ID");
+            -libc::EINVAL
+        }
+        Entry::Vacant(_) => -libc::ENOENT,
+    }
+}
+
 #[no_mangle]
 #[allow(unreachable_code)]
 pub extern "C" fn krun_start_enter(ctx_id: u32) -> i32 {

src/vmm/src/vmm_config/block.rs

@@ -28,6 +28,8 @@ pub struct BlockDeviceConfig {
     pub disk_image_path: String,
     pub disk_image_format: ImageType,
     pub is_disk_read_only: bool,
+    #[cfg(target_os = "macos")]
+    pub enable_relaxed_sync: bool,
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
@@ -63,6 +65,8 @@ impl BlockBuilder {
             config.disk_image_path,
             config.disk_image_format,
             config.is_disk_read_only,
+            #[cfg(target_os = "macos")]
+            config.enable_relaxed_sync,
         )
         .map_err(BlockConfigError::CreateBlockDevice)
     }