diff --git a/vmm/src/device_manager.rs b/vmm/src/device_manager.rs
index 0ae852644..77ef0cc42 100644
--- a/vmm/src/device_manager.rs
+++ b/vmm/src/device_manager.rs
@@ -1140,6 +1140,8 @@ pub struct DeviceManager {
     ivshmem_device: Option<Arc<Mutex<devices::IvshmemDevice>>>,
 }
 
+/// Create per-PCI-segment MMIO allocators over the range `[start, end]`.
+/// Both `start` and `end` are inclusive addresses.
 fn create_mmio_allocators(
     start: u64,
     end: u64,
@@ -1157,7 +1159,15 @@ fn create_mmio_allocators(
     for segment_id in 0..num_pci_segments as u64 {
         let weight = weights[segment_id as usize] as u64;
         let mmio_start = start + i * pci_segment_mmio_size;
-        let mmio_size = pci_segment_mmio_size * weight;
+        let is_last = segment_id == num_pci_segments as u64 - 1;
+        // Give the last segment all remaining space so no addresses
+        // near the top of the physical address space are lost to
+        // alignment truncation.
+        let mmio_size = if is_last {
+            end - mmio_start + 1
+        } else {
+            pci_segment_mmio_size * weight
+        };
         let allocator = Arc::new(Mutex::new(
             AddressAllocator::new(GuestAddress(mmio_start), mmio_size).unwrap(),
         ));
@@ -1218,7 +1228,8 @@ impl DeviceManager {
         }
 
         let start_of_mmio32_area = layout::MEM_32BIT_DEVICES_START.0;
-        let end_of_mmio32_area = layout::MEM_32BIT_DEVICES_START.0 + layout::MEM_32BIT_DEVICES_SIZE;
+        let end_of_mmio32_area =
+            layout::MEM_32BIT_DEVICES_START.0 + layout::MEM_32BIT_DEVICES_SIZE - 1;
         let pci_mmio32_allocators = create_mmio_allocators(
             start_of_mmio32_area,
             end_of_mmio32_area,
@@ -5739,7 +5750,7 @@ mod unit_tests {
 
     #[test]
     fn test_create_mmio_allocators() {
-        let res = create_mmio_allocators(0x100000, 0x400000, 1, &[1], 4 << 10);
+        let res = create_mmio_allocators(0x100000, 0x3fffff, 1, &[1], 4 << 10);
         assert_eq!(res.len(), 1);
         assert_eq!(
             res[0].lock().unwrap().base(),
@@ -5750,7 +5761,7 @@ mod unit_tests {
             vm_memory::GuestAddress(0x3fffff)
         );
 
-        let res = create_mmio_allocators(0x100000, 0x400000, 2, &[1, 1], 4 << 10);
+        let res = create_mmio_allocators(0x100000, 0x3fffff, 2, &[1, 1], 4 << 10);
         assert_eq!(res.len(), 2);
         assert_eq!(
             res[0].lock().unwrap().base(),
@@ -5769,7 +5780,7 @@ mod unit_tests {
             vm_memory::GuestAddress(0x3fffff)
         );
 
-        let res = create_mmio_allocators(0x100000, 0x400000, 2, &[2, 1], 4 << 10);
+        let res = create_mmio_allocators(0x100000, 0x3fffff, 2, &[2, 1], 4 << 10);
         assert_eq!(res.len(), 2);
         assert_eq!(
             res[0].lock().unwrap().base(),