refactor: replace read_obj in the descriptor chain creation

`read_obj` takes too much time and we don't need it's checks
as we do them before the actual reading happens.
Replace it with either `get_slice` or `get_host_address`.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

src/vmm/src/devices/virtio/queue.rs

@@ -109,17 +109,16 @@ impl<'a, M: GuestMemory> DescriptorChain<'a, M> {
         let desc_head = mem.checked_offset(desc_table, (index as usize) * 16)?;
         mem.checked_offset(desc_head, 16)?;
 
-        // These reads can't fail unless Guest memory is hopelessly broken.
-        let desc = match mem.read_obj::<Descriptor>(desc_head) {
-            Ok(ret) => ret,
-            Err(err) => {
-                error!(
-                    "Failed to read virtio descriptor from memory at address {:#x}: {}",
-                    desc_head.0, err
-                );
-                return None;
-            }
-        };
+        // SAFETY:
+        // This can't fail as we checked the `desc_head`
+        let ptr = mem.get_host_address(desc_head).unwrap();
+
+        // SAFETY:
+        // Safe as we know that `ptr` is inside guest memory and
+        // following `std::mem::size_of::<Descriptor>` bytes belong
+        // to the descriptor table
+        let desc: &Descriptor = unsafe { &*ptr.cast::<Descriptor>() };
+
         let chain = DescriptorChain {
             mem,
             desc_table,
@@ -423,7 +422,12 @@ impl Queue {
         // SAFETY:
         // `desc_index_address` param is bounded by size of the queue as `avail_idx` is
         // modded by `actual_size()`.
-        let desc_index: u16 = mem.read_obj(desc_index_address).unwrap();
+        let slice = mem
+            .get_slice(desc_index_address, std::mem::size_of::<u16>())
+            .unwrap();
+        // SAFETY:
+        // We transforming valid memory slice
+        let desc_index = unsafe { *slice.ptr_guard().as_ptr().cast::<u16>() };
 
         DescriptorChain::checked_new(mem, self.desc_table, self.actual_size(), desc_index)
     }