mmap: Wrap MmapRegion in Arc<>

The vhost-user-backend crate will need to be able to modify all existing
memory regions to use the VMM user address instead of the guest physical
address once the IOMMU feature is switched on, and vice versa.  To do
so, it needs to be able to modify regions’ base address.

Because `GuestMemoryMmap` stores regions wrapped in an `Arc<_>`, we
cannot mutate them after they have been put into the `GuestMemoryMmap`
object; and `MmapRegion` itself is by its nature not clonable.  So to
modify the regions’ base addresses, we need some way to create a new
`GuestRegionMmap` referencing the same `MmapRegion` as another one, but
with a different base address.

We can do that by having `GuestRegionMmap` wrap its `MmapRegion` in an
`Arc`, and adding a method to return a reference to that `Arc`, and a
method to construct a `GuestRegionMmap` object from such a cloned `Arc.`

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

Author: XanClic

src/mmap/mod.rs

@@ -77,21 +77,29 @@ pub enum Error {
 /// in the virtual address space of the calling process.
 #[derive(Debug)]
 pub struct GuestRegionMmap<B = ()> {
-    mapping: MmapRegion<B>,
+    mapping: Arc<MmapRegion<B>>,
     guest_base: GuestAddress,
 }
 
 impl<B> Deref for GuestRegionMmap<B> {
     type Target = MmapRegion<B>;
 
     fn deref(&self) -> &MmapRegion<B> {
-        &self.mapping
+        self.mapping.as_ref()
     }
 }
 
 impl<B: Bitmap> GuestRegionMmap<B> {
     /// Create a new memory-mapped memory region for the guest's physical memory.
     pub fn new(mapping: MmapRegion<B>, guest_base: GuestAddress) -> result::Result<Self, Error> {
+        Self::with_arc(Arc::new(mapping), guest_base)
+    }
+
+    /// Same as [`Self::new()`], but takes an `Arc`-wrapped `mapping`.
+    pub fn with_arc(
+        mapping: Arc<MmapRegion<B>>,
+        guest_base: GuestAddress,
+    ) -> result::Result<Self, Error> {
         if guest_base.0.checked_add(mapping.size() as u64).is_none() {
             return Err(Error::InvalidGuestRegion);
         }
@@ -101,6 +109,16 @@ impl<B: Bitmap> GuestRegionMmap<B> {
             guest_base,
         })
     }
+
+    /// Return a reference to the inner `Arc<MmapRegion>` (as opposed to
+    /// [`.deref()`](Self::deref()), which does not reference the `Arc`).
+    ///
+    /// The returned reference can be cloned to construct a new `GuestRegionMmap` with a different
+    /// base address (e.g. when switching between memory address spaces based on the guest physical
+    /// address vs.  the VMM userspace virtual address).
+    pub fn get_mapping(&self) -> &Arc<MmapRegion<B>> {
+        &self.mapping
+    }
 }
 
 #[cfg(not(feature = "xen"))]