feat(tvix/store/fs): Add support for virtiofs backend r/6642

This adds a virtiofs daemon implementation which hooks into the existing
tvix-store filesystem implementation that is used for FUSE.

This allows adding the filesystem to a microvm without having to set up
FUSE inside the guest.

Change-Id: If80c36c9657f2289853e8d9a364bf4f1f7b7559c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9344
Autosubmit: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

2 files changed, 240 insertions, 0 deletions

diff --git a/tvix/store/src/fs/mod.rs b/tvix/store/src/fs/mod.rs
index 59b8f0d0854f..91adfa35f0e0 100644
--- a/tvix/store/src/fs/mod.rs
+++ b/tvix/store/src/fs/mod.rs
@@ -5,6 +5,9 @@ mod inodes;
 #[cfg(feature = "fuse")]
 pub mod fuse;
 
+#[cfg(feature = "virtiofs")]
+pub mod virtiofs;
+
 #[cfg(test)]
 mod tests;
 
diff --git a/tvix/store/src/fs/virtiofs.rs b/tvix/store/src/fs/virtiofs.rs
new file mode 100644
index 000000000000..3786a84285cd
--- /dev/null
+++ b/tvix/store/src/fs/virtiofs.rs
@@ -0,0 +1,237 @@
+use std::{
+    convert, error, fmt, io,
+    ops::Deref,
+    path::Path,
+    sync::{Arc, MutexGuard, RwLock},
+};
+
+use fuse_backend_rs::{
+    api::{filesystem::FileSystem, server::Server},
+    transport::{FsCacheReqHandler, Reader, VirtioFsWriter},
+};
+use tracing::error;
+use vhost::vhost_user::{
+    Listener, SlaveFsCacheReq, VhostUserProtocolFeatures, VhostUserVirtioFeatures,
+};
+use vhost_user_backend::{VhostUserBackendMut, VhostUserDaemon, VringMutex, VringState, VringT};
+use virtio_bindings::bindings::virtio_ring::{
+    VIRTIO_RING_F_EVENT_IDX, VIRTIO_RING_F_INDIRECT_DESC,
+};
+use virtio_queue::QueueT;
+use vm_memory::{GuestAddressSpace, GuestMemoryAtomic, GuestMemoryMmap};
+use vmm_sys_util::epoll::EventSet;
+
+const VIRTIO_F_VERSION_1: u32 = 32;
+const NUM_QUEUES: usize = 2;
+const QUEUE_SIZE: usize = 1024;
+
+#[derive(Debug)]
+enum Error {
+    /// Failed to handle non-input event.
+    HandleEventNotEpollIn,
+    /// Failed to handle unknown event.
+    HandleEventUnknownEvent,
+    /// Invalid descriptor chain.
+    InvlaidDescriptorChain,
+    /// Failed to handle filesystem requests.
+    HandleRequests(fuse_backend_rs::Error),
+    /// Failed to construct new vhost user daemon.
+    NewDaemon,
+    /// Failed to start the vhost user daemon.
+    StartDaemon,
+    /// Failed to wait for the vhost user daemon.
+    WaitDaemon,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "vhost_user_fs_error: {self:?}")
+    }
+}
+
+impl error::Error for Error {}
+
+impl convert::From<Error> for io::Error {
+    fn from(e: Error) -> Self {
+        io::Error::new(io::ErrorKind::Other, e)
+    }
+}
+
+struct VhostUserFsBackend<FS>
+where
+    FS: FileSystem + Send + Sync,
+{
+    server: Arc<Server<Arc<FS>>>,
+    event_idx: bool,
+    guest_mem: GuestMemoryAtomic<GuestMemoryMmap>,
+    cache_req: Option<SlaveFsCacheReq>,
+}
+
+impl<FS> VhostUserFsBackend<FS>
+where
+    FS: FileSystem + Send + Sync,
+{
+    fn process_queue(&mut self, vring: &mut MutexGuard<VringState>) -> std::io::Result<bool> {
+        let mut used_descs = false;
+
+        while let Some(desc_chain) = vring
+            .get_queue_mut()
+            .pop_descriptor_chain(self.guest_mem.memory())
+        {
+            let memory = desc_chain.memory();
+            let reader = Reader::from_descriptor_chain(memory, desc_chain.clone())
+                .map_err(|_| Error::InvlaidDescriptorChain)?;
+            let writer = VirtioFsWriter::new(memory, desc_chain.clone())
+                .map_err(|_| Error::InvlaidDescriptorChain)?;
+
+            self.server
+                .handle_message(
+                    reader,
+                    writer.into(),
+                    self.cache_req
+                        .as_mut()
+                        .map(|req| req as &mut dyn FsCacheReqHandler),
+                    None,
+                )
+                .map_err(Error::HandleRequests)?;
+
+            // TODO: Is len 0 correct?
+            if let Err(error) = vring
+                .get_queue_mut()
+                .add_used(memory, desc_chain.head_index(), 0)
+            {
+                error!(?error, "failed to add desc back to ring");
+            }
+
+            // TODO: What happens if we error out before here?
+            used_descs = true;
+        }
+
+        let needs_notification = if self.event_idx {
+            match vring
+                .get_queue_mut()
+                .needs_notification(self.guest_mem.memory().deref())
+            {
+                Ok(needs_notification) => needs_notification,
+                Err(error) => {
+                    error!(?error, "failed to check if queue needs notification");
+                    true
+                }
+            }
+        } else {
+            true
+        };
+
+        if needs_notification {
+            if let Err(error) = vring.signal_used_queue() {
+                error!(?error, "failed to signal used queue");
+            }
+        }
+
+        Ok(used_descs)
+    }
+}
+
+impl<FS> VhostUserBackendMut<VringMutex> for VhostUserFsBackend<FS>
+where
+    FS: FileSystem + Send + Sync,
+{
+    fn num_queues(&self) -> usize {
+        NUM_QUEUES
+    }
+
+    fn max_queue_size(&self) -> usize {
+        QUEUE_SIZE
+    }
+
+    fn features(&self) -> u64 {
+        1 << VIRTIO_F_VERSION_1
+            | 1 << VIRTIO_RING_F_INDIRECT_DESC
+            | 1 << VIRTIO_RING_F_EVENT_IDX
+            | VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits()
+    }
+
+    fn protocol_features(&self) -> VhostUserProtocolFeatures {
+        VhostUserProtocolFeatures::MQ | VhostUserProtocolFeatures::SLAVE_REQ
+    }
+
+    fn set_event_idx(&mut self, enabled: bool) {
+        self.event_idx = enabled;
+    }
+
+    fn update_memory(&mut self, _mem: GuestMemoryAtomic<GuestMemoryMmap>) -> std::io::Result<()> {
+        // This is what most the vhost user implementations do...
+        Ok(())
+    }
+
+    fn set_slave_req_fd(&mut self, cache_req: SlaveFsCacheReq) {
+        self.cache_req = Some(cache_req);
+    }
+
+    fn handle_event(
+        &mut self,
+        device_event: u16,
+        evset: vmm_sys_util::epoll::EventSet,
+        vrings: &[VringMutex],
+        _thread_id: usize,
+    ) -> std::io::Result<bool> {
+        if evset != EventSet::IN {
+            return Err(Error::HandleEventNotEpollIn.into());
+        }
+
+        let mut queue = match device_event {
+            // High priority queue
+            0 => vrings[0].get_mut(),
+            // Regurlar priority queue
+            1 => vrings[1].get_mut(),
+            _ => {
+                return Err(Error::HandleEventUnknownEvent.into());
+            }
+        };
+
+        if self.event_idx {
+            loop {
+                queue
+                    .get_queue_mut()
+                    .enable_notification(self.guest_mem.memory().deref())
+                    .map_err(|e| io::Error::new(io::ErrorKind::Other, e.to_string()))?;
+                if !self.process_queue(&mut queue)? {
+                    break;
+                }
+            }
+        } else {
+            self.process_queue(&mut queue)?;
+        }
+
+        Ok(false)
+    }
+}
+
+pub fn start_virtiofs_daemon<FS, P>(fs: FS, socket: P) -> io::Result<()>
+where
+    FS: FileSystem + Send + Sync + 'static,
+    P: AsRef<Path>,
+{
+    let guest_mem = GuestMemoryAtomic::new(GuestMemoryMmap::new());
+
+    let server = Arc::new(fuse_backend_rs::api::server::Server::new(Arc::new(fs)));
+
+    let backend = Arc::new(RwLock::new(VhostUserFsBackend {
+        server,
+        guest_mem: guest_mem.clone(),
+        event_idx: false,
+        cache_req: None,
+    }));
+
+    let listener = Listener::new(socket, true).unwrap();
+
+    let mut fs_daemon =
+        VhostUserDaemon::new(String::from("vhost-user-fs-tvix-store"), backend, guest_mem)
+            .map_err(|_| Error::NewDaemon)?;
+
+    fs_daemon.start(listener).map_err(|_| Error::StartDaemon)?;
+
+    fs_daemon.wait().map_err(|_| Error::WaitDaemon)?;
+
+    Ok(())
+}