vmm: extend last MMIO64 allocator to cover full range

The MMIO64 allocator size is computed with alignment truncation:
  size = (range / alignment) * alignment
This loses up to one alignment unit (4 GiB) at the top of the
address space. When a guest (Windows with virtio-win 0.1.285)
programs a BAR near the top of the physical address space, the
allocation fails because the address falls in the truncated gap.

Give the last PCI segment allocator all remaining space up to
the end of the device area, so no addresses are lost.

Signed-off-by: CMGS <ilskdw@gmail.com>

Author: CMGS

vmm/src/device_manager.rs

@@ -1144,7 +1144,15 @@ 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(),
         ));