fix IovDeque for non 4K pages

CHANGELOG.md

@@ -29,6 +29,10 @@ and this project adheres to
 
 ### Fixed
 
+- [#5222](https://github.com/firecracker-microvm/firecracker/pull/5222): Fixed
+  `IovDeque` wraparound logic which prevented it from working properly on
+  systems with non 4K pages.
+
 ## [1.12.0]
 
 ### Added

src/vmm/src/devices/virtio/iov_deque.rs

@@ -69,7 +69,9 @@
 // Like that, the elements stored in the buffer are always laid out in contiguous virtual memory,
 // so making a slice out of them does not require any copies.
 //
-// The `L` const generic determines the maximum number of `iovec` elements the queue should hold.
+// The `L` const generic determines the maximum number of `iovec` elements the queue should hold
+// at any point in time. The actual capacity of the queue may differ and will depend on the host
+// page size.
 //
 // ```Rust
 // pub struct iovec {
@@ -83,6 +85,7 @@
     pub iov: *mut libc::iovec,
     pub start: u16,
     pub len: u16,
+    pub capacity: u16,
 }
 
 // SAFETY: This is `Send`. We hold sole ownership of the underlying buffer.
@@ -158,6 +161,14 @@
     /// Create a new [`IovDeque`] that can hold memory described by a single VirtIO queue.
     pub fn new() -> Result<Self, IovDequeError> {
         let pages_bytes = Self::pages_bytes();
+        let capacity = pages_bytes / std::mem::size_of::<iovec>();
+        let capacity: u16 = capacity.try_into().unwrap();
+        assert!(
+            L <= capacity,
+            "Actual capacity {} is smaller than requested capacity {}",
+            capacity,
+            L
+        );
 
         let memfd = Self::create_memfd(pages_bytes)?;
         let raw_memfd = memfd.as_file().as_raw_fd();
@@ -201,6 +212,7 @@
             iov: buffer.cast(),
             start: 0,
             len: 0,
+            capacity,
         })
     }
 
@@ -258,8 +270,8 @@
 
         self.start += nr_iovecs;
         self.len -= nr_iovecs;
-        if self.start >= L {
-            self.start -= L;
+        if self.capacity <= self.start {
+            self.start -= self.capacity;
         }
     }
 

src/vmm/src/devices/virtio/iovec.rs

@@ -927,6 +927,7 @@ mod verification {
             iov: mem.cast(),
             start: kani::any_where(|&start| start < FIRECRACKER_MAX_QUEUE_SIZE),
             len: 0,
+            capacity: FIRECRACKER_MAX_QUEUE_SIZE,
         }
     }