disk_backend: add new layered disk (#404)

Add a new disk type `LayeredDisk` that is composed of multiple layers,
each of which can contribute an arbitrary subset of the sectors of the
composed disk.

This is useful for modeling differencing VHDs, the RAM diff disk, and
the upcoming sqlite-backed disk. It can be used to providing disk
snapshots and read and write caches.

In this PR, only the RAM disk and the full disk layers are defined.

Cargo.lock

@@ -1216,6 +1216,21 @@ dependencies = [
  "vmgs_broker",
 ]
 
+[[package]]
+name = "disk_layered"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "disk_backend",
+ "disk_backend_resources",
+ "futures",
+ "guestmem",
+ "inspect",
+ "scsi_buffers",
+ "thiserror 2.0.0",
+ "vm_resource",
+]
+
 [[package]]
 name = "disk_nvme"
 version = "0.0.0"
@@ -1250,15 +1265,16 @@ name = "disk_ramdisk"
 version = "0.0.0"
 dependencies = [
  "anyhow",
- "async-trait",
  "disk_backend",
  "disk_backend_resources",
+ "disk_layered",
  "event-listener",
  "guestmem",
  "inspect",
  "pal_async",
  "parking_lot",
  "scsi_buffers",
+ "test_with_tracing",
  "thiserror 2.0.0",
  "tracing",
  "vm_resource",
@@ -1992,7 +2008,6 @@ dependencies = [
  "chipset_arc_mutex_device",
  "chipset_device",
  "chipset_device_fuzz",
- "disk_backend",
  "disk_ramdisk",
  "guestmem",
  "ide",
@@ -4197,7 +4212,6 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "chipset_device",
- "disk_backend",
  "disk_ramdisk",
  "event-listener",
  "futures",
@@ -4550,6 +4564,7 @@ dependencies = [
  "disk_blob",
  "disk_crypt",
  "disk_file",
+ "disk_layered",
  "disk_prwrap",
  "disk_ramdisk",
  "disk_vhd1",
@@ -5983,7 +5998,6 @@ dependencies = [
  "anyhow",
  "async-trait",
  "criterion",
- "disk_backend",
  "disk_ramdisk",
  "event-listener",
  "fast_select",

Cargo.toml

@@ -244,6 +244,7 @@ disk_crypt = { path = "vm/devices/storage/disk_crypt" }
 disk_crypt_resources = { path = "vm/devices/storage/disk_crypt_resources" }
 disk_file = { path = "vm/devices/storage/disk_file" }
 disk_get_vmgs = { path = "vm/devices/storage/disk_get_vmgs" }
+disk_layered = { path = "vm/devices/storage/disk_layered" }
 disk_nvme = { path = "vm/devices/storage/disk_nvme" }
 disk_ramdisk = { path = "vm/devices/storage/disk_ramdisk" }
 disk_striped = { path = "vm/devices/storage/disk_striped" }

openvmm/openvmm_entry/src/lib.rs

@@ -27,6 +27,8 @@ use cli_args::EndpointConfigCli;
 use cli_args::NicConfigCli;
 use cli_args::SerialConfigCli;
 use cli_args::UefiConsoleModeCli;
+use disk_backend_resources::layer::DiskLayerHandle;
+use disk_backend_resources::layer::RamDiskLayerHandle;
 use floppy_resources::FloppyDiskConfig;
 use framebuffer::FramebufferAccess;
 use framebuffer::FRAMEBUFFER_SIZE;
@@ -843,9 +845,9 @@ fn vm_config_from_command_line(
         let vmgs_disk = if let Some(disk) = &opt.get_vmgs {
             disk_open(disk, false).context("failed to open GET vmgs disk")?
         } else {
-            disk_backend_resources::RamDiskHandle {
-                len: vmgs_format::VMGS_DEFAULT_CAPACITY,
-            }
+            disk_backend_resources::LayeredDiskHandle::single_layer(RamDiskLayerHandle {
+                len: Some(vmgs_format::VMGS_DEFAULT_CAPACITY),
+            })
             .into_resource()
         };
         vmbus_devices.extend([
@@ -1415,7 +1417,11 @@ impl NicConfig {
 
 fn disk_open(disk_cli: &DiskCliKind, read_only: bool) -> anyhow::Result<Resource<DiskHandleKind>> {
     let disk_type = match disk_cli {
-        &DiskCliKind::Memory(len) => Resource::new(disk_backend_resources::RamDiskHandle { len }),
+        &DiskCliKind::Memory(len) => {
+            Resource::new(disk_backend_resources::LayeredDiskHandle::single_layer(
+                RamDiskLayerHandle { len: Some(len) },
+            ))
+        }
         DiskCliKind::File(path) => open_disk_type(path, read_only)
             .with_context(|| format!("failed to open {}", path.display()))?,
         DiskCliKind::Blob { kind, url } => Resource::new(disk_backend_resources::BlobDiskHandle {
@@ -1426,8 +1432,13 @@ fn disk_open(disk_cli: &DiskCliKind, read_only: bool) -> anyhow::Result<Resource
             },
         }),
         DiskCliKind::MemoryDiff(inner) => {
-            Resource::new(disk_backend_resources::RamDiffDiskHandle {
-                lower: disk_open(inner, true)?,
+            Resource::new(disk_backend_resources::LayeredDiskHandle {
+                layers: vec![
+                    RamDiskLayerHandle { len: None }.into_resource().into(),
+                    DiskLayerHandle(disk_open(inner, true)?)
+                        .into_resource()
+                        .into(),
+                ],
             })
         }
         DiskCliKind::PersistentReservationsWrapper(inner) => Resource::new(
@@ -2367,7 +2378,9 @@ async fn run_control(driver: &DefaultDriver, mesh: &VmmMesh, opt: Options) -> an
                                 .with_context(|| format!("failed to open {}", path.display()))?
                         }
                         Some(size) => {
-                            Resource::new(disk_backend_resources::RamDiskHandle { len: size })
+                            Resource::new(disk_backend_resources::LayeredDiskHandle::single_layer(
+                                RamDiskLayerHandle { len: Some(size) },
+                            ))
                         }
                     };
 

openvmm/openvmm_resources/Cargo.toml

@@ -34,6 +34,7 @@ serial_socket.workspace = true
 disk_blob = { workspace = true, optional = true }
 disk_crypt = { workspace = true, optional = true }
 disk_file.workspace = true
+disk_layered.workspace = true
 disk_prwrap.workspace = true
 disk_ramdisk.workspace = true
 disk_vhd1.workspace = true

openvmm/openvmm_resources/src/lib.rs

@@ -42,6 +42,7 @@ vm_resource::register_static_resolvers! {
     net_dio::resolver::DioResolver,
 
     // Disks
+    disk_layered::resolver::LayeredDiskResolver,
     #[cfg(feature = "disk_crypt")]
     disk_crypt::resolver::DiskCryptResolver,
     disk_ramdisk::resolver::RamDiskResolver,

petri/src/vm/construct.rs

@@ -18,7 +18,9 @@ use crate::BOOT_NVME_NSID;
 use crate::SCSI_INSTANCE;
 use crate::SIZE_1_GB;
 use anyhow::Context;
-use disk_backend_resources::RamDiffDiskHandle;
+use disk_backend_resources::layer::DiskLayerHandle;
+use disk_backend_resources::layer::RamDiskLayerHandle;
+use disk_backend_resources::LayeredDiskHandle;
 use framebuffer::Framebuffer;
 use framebuffer::FramebufferAccess;
 use framebuffer::FRAMEBUFFER_SIZE;
@@ -753,7 +755,13 @@ impl PetriVmConfigSetupCore<'_> {
                     PcatGuest::Vhd(_) => GuestMedia::Disk {
                         read_only: false,
                         disk_parameters: None,
-                        disk_type: (RamDiffDiskHandle { lower: inner_disk }).into_resource(),
+                        disk_type: LayeredDiskHandle {
+                            layers: vec![
+                                RamDiskLayerHandle { len: None }.into_resource().into(),
+                                DiskLayerHandle(inner_disk).into_resource().into(),
+                            ],
+                        }
+                        .into_resource(),
                     },
                     PcatGuest::Iso(_) => GuestMedia::Dvd(
                         SimpleScsiDvdHandle {
@@ -793,8 +801,13 @@ impl PetriVmConfigSetupCore<'_> {
                             device: SimpleScsiDiskHandle {
                                 read_only: false,
                                 parameters: Default::default(),
-                                disk: RamDiffDiskHandle {
-                                    lower: open_disk_type(&path, true)?,
+                                disk: LayeredDiskHandle {
+                                    layers: vec![
+                                        RamDiskLayerHandle { len: None }.into_resource().into(),
+                                        DiskLayerHandle(open_disk_type(&path, true)?)
+                                            .into_resource()
+                                            .into(),
+                                    ],
                                 }
                                 .into_resource(),
                             }
@@ -820,8 +833,13 @@ impl PetriVmConfigSetupCore<'_> {
                         msix_count: 64,
                         namespaces: vec![NamespaceDefinition {
                             nsid: BOOT_NVME_NSID,
-                            disk: RamDiffDiskHandle {
-                                lower: open_disk_type(&path, true)?,
+                            disk: LayeredDiskHandle {
+                                layers: vec![
+                                    RamDiskLayerHandle { len: None }.into_resource().into(),
+                                    DiskLayerHandle(open_disk_type(&path, true)?)
+                                        .into_resource()
+                                        .into(),
+                                ],
                             }
                             .into_resource(),
                             read_only: false,
@@ -928,9 +946,9 @@ impl PetriVmConfigSetupCore<'_> {
             vmbus_redirection: false,
             vtl2_settings: None, // Will be added at startup to allow tests to modify
             vmgs_disk: Some(
-                disk_backend_resources::RamDiskHandle {
-                    len: vmgs_format::VMGS_DEFAULT_CAPACITY,
-                }
+                LayeredDiskHandle::single_layer(RamDiskLayerHandle {
+                    len: Some(vmgs_format::VMGS_DEFAULT_CAPACITY),
+                })
                 .into_resource(),
             ),
             framebuffer: framebuffer.then(|| SharedFramebufferHandle.into_resource()),

vm/devices/get/guest_emulation_device/src/test_utilities.rs

@@ -6,7 +6,6 @@ use crate::GedChannel;
 use crate::GuestConfig;
 use crate::GuestEmulationDevice;
 use crate::GuestFirmwareConfig;
-use disk_backend::Disk;
 use get_protocol::test_utilities::TEST_VMGS_CAPACITY;
 use get_protocol::HostNotifications;
 use get_protocol::HostRequests;
@@ -264,10 +263,7 @@ pub fn create_host_channel(
         None,
         recv,
         None,
-        Some(
-            Disk::new(disk_ramdisk::RamDisk::new(TEST_VMGS_CAPACITY as u64, false).unwrap())
-                .unwrap(),
-        ),
+        Some(disk_ramdisk::ram_disk(TEST_VMGS_CAPACITY as u64, false).unwrap()),
     );
 
     if let Some(ged_responses) = ged_responses {

vm/devices/storage/disk_backend/src/lib.rs

@@ -7,19 +7,14 @@
 //! (such as the VMGS file system).
 //!
 //! `Disk`s are backed by a [`DiskIo`] implementation. Specific disk
-//! backends should be in their own crates. The exceptions that prove the rule
-//! is [`ZeroDisk`][], which is small enough to be in this crate and serve as an
-//! example.
-//!
-//! [`ZeroDisk`]: crate::zerodisk::ZeroDisk
+//! backends should be in their own crates.
 
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]
 
 pub mod pr;
 pub mod resolve;
 pub mod sync_wrapper;
-pub mod zerodisk;
 
 use guestmem::AccessError;
 use inspect::Inspect;