feat(virtio-block): Add fallocate support

resources/seccomp/aarch64-unknown-linux-musl.json

@@ -85,6 +85,10 @@
                 "syscall": "lseek",
                 "comment": "Used by the block device"
             },
+            {
+                "syscall": "fallocate",
+                "comment": "Used by the block device for discard (hole punching)"
+            },
             {
                 "syscall": "mremap",
                 "comment": "Used for re-allocating large memory regions, for example vectors"

resources/seccomp/x86_64-unknown-linux-musl.json

@@ -85,6 +85,10 @@
                 "syscall": "lseek",
                 "comment": "Used by the block device"
             },
+            {
+                "syscall": "fallocate",
+                "comment": "Used by the block device for discard (hole punching)"
+            },
             {
                 "syscall": "mremap",
                 "comment": "Used for re-allocating large memory regions, for example vectors"

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

@@ -27,7 +27,7 @@ use crate::devices::virtio::block::CacheType;
 use crate::devices::virtio::block::virtio::metrics::{BlockDeviceMetrics, BlockMetricsPerDevice};
 use crate::devices::virtio::device::{ActiveState, DeviceState, VirtioDevice};
 use crate::devices::virtio::generated::virtio_blk::{
-    VIRTIO_BLK_F_FLUSH, VIRTIO_BLK_F_RO, VIRTIO_BLK_ID_BYTES,
+    VIRTIO_BLK_F_DISCARD, VIRTIO_BLK_F_FLUSH, VIRTIO_BLK_F_RO, VIRTIO_BLK_ID_BYTES,
 };
 use crate::devices::virtio::generated::virtio_config::VIRTIO_F_VERSION_1;
 use crate::devices::virtio::generated::virtio_ids::VIRTIO_ID_BLOCK;
@@ -298,7 +298,9 @@ impl VirtioBlock {
             .map_err(VirtioBlockError::RateLimiter)?
             .unwrap_or_default();
 
-        let mut avail_features = (1u64 << VIRTIO_F_VERSION_1) | (1u64 << VIRTIO_RING_F_EVENT_IDX);
+        let mut avail_features = (1u64 << VIRTIO_F_VERSION_1)
+            | (1u64 << VIRTIO_RING_F_EVENT_IDX)
+            | (1u64 << VIRTIO_BLK_F_DISCARD);
 
         if config.cache_type == CacheType::Writeback {
             avail_features |= 1u64 << VIRTIO_BLK_F_FLUSH;

src/vmm/src/devices/virtio/block/virtio/io/async_io.rs

@@ -230,6 +230,28 @@ impl AsyncFileEngine {
         Ok(())
     }
 
+    pub fn push_discard(
+        &mut self,
+        offset: u64,
+        len: u32,
+        req: PendingRequest,
+    ) -> Result<(), RequestError<AsyncIoError>> {
+        let wrapped_user_data = WrappedRequest::new(req);
+
+        if let Err((io_uring_error, data)) =
+            self.ring
+                .push(Operation::fallocate(0, len, offset, wrapped_user_data))
+        {
+            error!(
+                "DISCARD fallocate failed (offset={}, len={}): {:?}",
+                offset, len, io_uring_error
+            );
+            return Ok(());
+        }
+
+        Ok(())
+    }
+
     fn do_pop(&mut self) -> Result<Option<Cqe<WrappedRequest>>, AsyncIoError> {
         self.ring.pop().map_err(AsyncIoError::IoUring)
     }

src/vmm/src/devices/virtio/block/virtio/io/mod.rs

@@ -6,9 +6,13 @@ pub mod sync_io;
 
 use std::fmt::Debug;
 use std::fs::File;
+use std::os::unix::io::AsRawFd;
+
+use libc::{FALLOC_FL_KEEP_SIZE, FALLOC_FL_PUNCH_HOLE, c_int, off64_t};
 
 pub use self::async_io::{AsyncFileEngine, AsyncIoError};
 pub use self::sync_io::{SyncFileEngine, SyncIoError};
+use crate::device_manager::mmio::MMIO_LEN;
 use crate::devices::virtio::block::virtio::PendingRequest;
 use crate::devices::virtio::block::virtio::device::FileEngineType;
 use crate::vstate::memory::{GuestAddress, GuestMemoryMmap};
@@ -172,6 +176,30 @@ impl FileEngine {
             FileEngine::Sync(engine) => engine.flush().map_err(BlockIoError::Sync),
         }
     }
+
+    pub fn discard(
+        &mut self,
+        offset: u64,
+        count: u32,
+        req: PendingRequest,
+    ) -> Result<FileEngineOk, RequestError<BlockIoError>> {
+        match self {
+            FileEngine::Async(engine) => match engine.push_discard(offset, count, req) {
+                Ok(_) => Ok(FileEngineOk::Submitted),
+                Err(err) => Err(RequestError {
+                    req: err.req,
+                    error: BlockIoError::Async(err.error),
+                }),
+            },
+            FileEngine::Sync(engine) => match engine.discard(offset, count) {
+                Ok(count) => Ok(FileEngineOk::Executed(RequestOk { req, count })),
+                Err(err) => Err(RequestError {
+                    req,
+                    error: BlockIoError::Sync(err),
+                }),
+            },
+        }
+    }
 }
 
 #[cfg(test)]

src/vmm/src/devices/virtio/block/virtio/io/sync_io.rs

@@ -3,7 +3,9 @@
 
 use std::fs::File;
 use std::io::{Seek, SeekFrom, Write};
+use std::os::unix::io::AsRawFd;
 
+use libc::{FALLOC_FL_KEEP_SIZE, FALLOC_FL_PUNCH_HOLE, c_int, off64_t};
 use vm_memory::{GuestMemoryError, ReadVolatile, WriteVolatile};
 
 use crate::vstate::memory::{GuestAddress, GuestMemory, GuestMemoryMmap};
@@ -18,6 +20,8 @@ pub enum SyncIoError {
     SyncAll(std::io::Error),
     /// Transfer: {0}
     Transfer(GuestMemoryError),
+    /// Discard: {0}
+    Discard(std::io::Error),
 }
 
 #[derive(Debug)]
@@ -81,4 +85,40 @@ impl SyncFileEngine {
         // Sync data out to physical media on host.
         self.file.sync_all().map_err(SyncIoError::SyncAll)
     }
+
+    pub fn discard(&mut self, offset: u64, len: u32) -> Result<u32, SyncIoError> {
+        // Do checked conversion to avoid possible wrap/cast issues on 64-bit systems.
+        let off_i64 = i64::try_from(offset).map_err(|_| {
+            SyncIoError::Discard(std::io::Error::new(
+                std::io::ErrorKind::InvalidInput,
+                "offset overflow",
+            ))
+        })?;
+
+        let len_i64: i64 = len.into();
+
+        // # Safety: calling libc::fallocate is safe here because:
+        // - `self.file.as_raw_fd()` is a valid file descriptor owned by this struct,
+        // - `off_i64` and `len_i64` are validated copies of the incoming unsigned values converted
+        //   to the C `off64_t` type, and
+        // - the syscall is properly checked for an error return value.
+        unsafe {
+            let ret = libc::fallocate(
+                self.file.as_raw_fd(),
+                libc::FALLOC_FL_PUNCH_HOLE | libc::FALLOC_FL_KEEP_SIZE,
+                off_i64,
+                len_i64,
+            );
+            if ret != 0 {
+                error!(
+                    "DISCARD fallocate failed (offset={}, len={}): {:?}",
+                    offset,
+                    len,
+                    std::io::Error::last_os_error()
+                );
+                return Ok(len);
+            }
+        }
+        Ok(len)
+    }
 }

src/vmm/src/devices/virtio/block/virtio/metrics.rs

@@ -157,6 +157,8 @@ pub struct BlockDeviceMetrics {
     pub invalid_reqs_count: SharedIncMetric,
     /// Number of flushes operation triggered on this block device.
     pub flush_count: SharedIncMetric,
+    /// Number of discard operation triggered on this block device.
+    pub discard_count: SharedIncMetric,
     /// Number of events triggered on the queue of this block device.
     pub queue_event_count: SharedIncMetric,
     /// Number of events ratelimiter-related.
@@ -210,6 +212,7 @@ impl BlockDeviceMetrics {
         self.invalid_reqs_count
             .add(other.invalid_reqs_count.fetch_diff());
         self.flush_count.add(other.flush_count.fetch_diff());
+        self.discard_count.add(other.discard_count.fetch_diff());
         self.queue_event_count
             .add(other.queue_event_count.fetch_diff());
         self.rate_limiter_event_count

src/vmm/src/devices/virtio/block/virtio/mod.rs

@@ -63,4 +63,12 @@ pub enum VirtioBlockError {
     RateLimiter(std::io::Error),
     /// Persistence error: {0}
     Persist(crate::devices::virtio::persist::PersistError),
+    /// Sector overflow in discard segment
+    SectorOverflow,
+    /// Discard segment exceeds disk size
+    BeyondDiskSize,
+    /// Invalid flags in discard segment
+    InvalidDiscardFlags,
+    /// Invalid discard request (e.g., empty segments)
+    InvalidDiscardRequest,
 }

src/vmm/src/devices/virtio/block/virtio/request.rs

@@ -9,17 +9,19 @@ use std::convert::From;
 
 use vm_memory::GuestMemoryError;
 
+use super::io::{BlockIoError, SyncIoError};
 use super::{SECTOR_SHIFT, SECTOR_SIZE, VirtioBlockError, io as block_io};
 use crate::devices::virtio::block::virtio::device::DiskProperties;
 use crate::devices::virtio::block::virtio::metrics::BlockDeviceMetrics;
 pub use crate::devices::virtio::generated::virtio_blk::{
     VIRTIO_BLK_ID_BYTES, VIRTIO_BLK_S_IOERR, VIRTIO_BLK_S_OK, VIRTIO_BLK_S_UNSUPP,
-    VIRTIO_BLK_T_FLUSH, VIRTIO_BLK_T_GET_ID, VIRTIO_BLK_T_IN, VIRTIO_BLK_T_OUT,
+    VIRTIO_BLK_T_DISCARD, VIRTIO_BLK_T_FLUSH, VIRTIO_BLK_T_GET_ID, VIRTIO_BLK_T_IN,
+    VIRTIO_BLK_T_OUT,
 };
 use crate::devices::virtio::queue::DescriptorChain;
 use crate::logger::{IncMetric, error};
 use crate::rate_limiter::{RateLimiter, TokenType};
-use crate::vstate::memory::{ByteValued, Bytes, GuestAddress, GuestMemoryMmap};
+use crate::vstate::memory::{Address, ByteValued, Bytes, GuestAddress, GuestMemoryMmap};
 
 #[derive(Debug, derive_more::From)]
 pub enum IoErr {
@@ -34,6 +36,7 @@ pub enum RequestType {
     Out,
     Flush,
     GetDeviceID,
+    Discard,
     Unsupported(u32),
 }
 
@@ -44,6 +47,7 @@ impl From<u32> for RequestType {
             VIRTIO_BLK_T_OUT => RequestType::Out,
             VIRTIO_BLK_T_FLUSH => RequestType::Flush,
             VIRTIO_BLK_T_GET_ID => RequestType::GetDeviceID,
+            VIRTIO_BLK_T_DISCARD => RequestType::Discard,
             t => RequestType::Unsupported(t),
         }
     }
@@ -176,6 +180,12 @@ impl PendingRequest {
             (Ok(transferred_data_len), RequestType::GetDeviceID) => {
                 Status::from_data(self.data_len, transferred_data_len, true)
             }
+            (Ok(_), RequestType::Discard) => {
+                block_metrics.discard_count.inc();
+                Status::Ok {
+                    num_bytes_to_mem: 0,
+                }
+            }
             (_, RequestType::Unsupported(op)) => Status::Unsupported { op },
             (Err(err), _) => Status::IoErr {
                 num_bytes_to_mem: 0,
@@ -231,13 +241,26 @@ impl RequestHeader {
     }
 }
 
+// For this, please see VirtIO v1.2. This struct mimics that of the implementation in Virtio.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[repr(C)]
+pub struct DiscardSegment {
+    sector: u64,
+    num_sectors: u32,
+    flags: u32,
+}
+// SAFETY: Safe because `DiscardSegment` only contains plain data (POD) with no padding-dependent
+// invariants.
+unsafe impl ByteValued for DiscardSegment {}
+
 #[derive(Debug, PartialEq, Eq)]
 pub struct Request {
     pub r#type: RequestType,
     pub data_len: u32,
     pub status_addr: GuestAddress,
     sector: u64,
     data_addr: GuestAddress,
+    discard_segments: Option<Vec<DiscardSegment>>,
 }
 
 impl Request {
@@ -258,10 +281,11 @@ impl Request {
             data_addr: GuestAddress(0),
             data_len: 0,
             status_addr: GuestAddress(0),
+            discard_segments: None,
         };
 
         let data_desc;
-        let status_desc;
+        let mut status_desc;
         let desc = avail_desc
             .next_descriptor()
             .ok_or(VirtioBlockError::DescriptorChainTooShort)?;
@@ -272,6 +296,7 @@ impl Request {
             if req.r#type != RequestType::Flush {
                 return Err(VirtioBlockError::DescriptorChainTooShort);
             }
+            data_desc = desc;
         } else {
             data_desc = desc;
             status_desc = data_desc
@@ -287,6 +312,9 @@ impl Request {
             if !data_desc.is_write_only() && req.r#type == RequestType::GetDeviceID {
                 return Err(VirtioBlockError::UnexpectedReadOnlyDescriptor);
             }
+            if data_desc.is_write_only() && req.r#type == RequestType::Discard {
+                return Err(VirtioBlockError::UnexpectedWriteOnlyDescriptor);
+            }
 
             req.data_addr = data_desc.addr;
             req.data_len = data_desc.len;
@@ -313,6 +341,48 @@ impl Request {
                     return Err(VirtioBlockError::InvalidDataLength);
                 }
             }
+            RequestType::Discard => {
+                // Validate data length
+                let segment_size = std::mem::size_of::<DiscardSegment>();
+                if (data_desc.len as usize) % segment_size != 0 {
+                    return Err(VirtioBlockError::InvalidDataLength);
+                }
+
+                // Calculate number of segments
+                let num_segments = (data_desc.len as usize) / segment_size;
+                if num_segments == 0 {
+                    return Err(VirtioBlockError::InvalidDiscardRequest);
+                }
+                let mut segments = Vec::with_capacity(num_segments);
+
+                // Populate DiscardSegments vector
+                for i in 0..num_segments {
+                    let offset = i * segment_size;
+                    let segment: DiscardSegment = mem
+                        .read_obj(data_desc.addr.unchecked_add(offset as u64))
+                        .map_err(VirtioBlockError::GuestMemory)?;
+                    let end_sector = segment.sector + segment.num_sectors as u64;
+                    if end_sector > num_disk_sectors {
+                        error!(
+                            "Discard request out of bounds: sector={} num_sectors={} \
+                             end_sector={} disk_sectors={}",
+                            segment.sector, segment.num_sectors, end_sector, num_disk_sectors
+                        );
+                        continue;
+                    }
+                    segments.push(segment);
+                }
+
+                // Assign to req.discard_segments
+                req.discard_segments = Some(segments);
+                req.data_len = data_desc.len;
+                status_desc = data_desc
+                    .next_descriptor()
+                    .ok_or(VirtioBlockError::DescriptorChainTooShort)?;
+                if !status_desc.is_write_only() {
+                    return Err(VirtioBlockError::UnexpectedReadOnlyDescriptor);
+                }
+            }
             _ => {}
         }
 
@@ -390,6 +460,11 @@ impl Request {
                     .map_err(IoErr::GetId);
                 return ProcessingResult::Executed(pending.finish(mem, res, block_metrics));
             }
+            RequestType::Discard => {
+                let _metric = block_metrics.write_agg.record_latency_metrics();
+                disk.file_engine
+                    .discard(self.offset(), self.data_len, pending)
+            }
             RequestType::Unsupported(_) => {
                 return ProcessingResult::Executed(pending.finish(mem, Ok(0), block_metrics));
             }
@@ -731,6 +806,7 @@ mod tests {
                 RequestType::Out => VIRTIO_BLK_T_OUT,
                 RequestType::Flush => VIRTIO_BLK_T_FLUSH,
                 RequestType::GetDeviceID => VIRTIO_BLK_T_GET_ID,
+                RequestType::Discard => VIRTIO_BLK_T_DISCARD,
                 RequestType::Unsupported(id) => id,
             }
         }
@@ -743,6 +819,7 @@ mod tests {
             RequestType::Out => VIRTQ_DESC_F_NEXT,
             RequestType::Flush => VIRTQ_DESC_F_NEXT,
             RequestType::GetDeviceID => VIRTQ_DESC_F_NEXT | VIRTQ_DESC_F_WRITE,
+            RequestType::Discard => VIRTQ_DESC_F_NEXT,
             RequestType::Unsupported(_) => VIRTQ_DESC_F_NEXT,
         }
     }
@@ -833,6 +910,7 @@ mod tests {
             data_len: valid_data_len,
             status_addr,
             sector: sector & (NUM_DISK_SECTORS - sectors_len),
+            discard_segments: None,
             data_addr,
         };
         let mut request_header = RequestHeader::new(virtio_request_id, request.sector);