refactor(resources): add method to create a single region

Split the actual allocation from the calculations to find the right
ranges, and add a new function to create a single region.
This will be used to create the hotpluggable region in the future.

Signed-off-by: Riccardo Mancini <[email protected]>

Author: Manciukic

src/vmm/src/resources.rs

@@ -6,6 +6,7 @@ use std::path::PathBuf;
 use std::sync::{Arc, Mutex, MutexGuard};
 
 use serde::{Deserialize, Serialize};
+use vm_memory::GuestAddress;
 
 use crate::cpu_config::templates::CustomCpuTemplate;
 use crate::device_manager::persist::SharedDeviceType;
@@ -463,11 +464,14 @@ impl VmResources {
         Ok(())
     }
 
-    /// Allocates guest memory in a configuration most appropriate for these [`VmResources`].
+    /// Allocates the given guest memory regions.
     ///
     /// If vhost-user-blk devices are in use, allocates memfd-backed shared memory, otherwise
     /// prefers anonymous memory for performance reasons.
-    pub fn allocate_guest_memory(&self) -> Result<Vec<GuestRegionMmap>, MemoryError> {
+    fn allocate_memory_regions(
+        &self,
+        regions: &[(GuestAddress, usize)],
+    ) -> Result<Vec<GuestRegionMmap>, MemoryError> {
         let vhost_user_device_used = self
             .block
             .devices
@@ -483,22 +487,39 @@ impl VmResources {
         // because that would require running a backend process. If in the future we converge to
         // a single way of backing guest memory for vhost-user and non-vhost-user cases,
         // that would not be worth the effort.
-        let regions =
-            crate::arch::arch_memory_regions(mib_to_bytes(self.machine_config.mem_size_mib));
         if vhost_user_device_used {
             memory::memfd_backed(
-                regions.as_ref(),
+                regions,
                 self.machine_config.track_dirty_pages,
                 self.machine_config.huge_pages,
             )
         } else {
             memory::anonymous(
-                regions.into_iter(),
+                regions.iter().copied(),
                 self.machine_config.track_dirty_pages,
                 self.machine_config.huge_pages,
             )
         }
     }
+
+    /// Allocates guest memory in a configuration most appropriate for these [`VmResources`].
+    pub fn allocate_guest_memory(&self) -> Result<Vec<GuestRegionMmap>, MemoryError> {
+        let regions =
+            crate::arch::arch_memory_regions(mib_to_bytes(self.machine_config.mem_size_mib));
+        self.allocate_memory_regions(&regions)
+    }
+
+    /// Allocates a single guest memory region.
+    pub fn allocate_memory_region(
+        &self,
+        start: GuestAddress,
+        size: usize,
+    ) -> Result<GuestRegionMmap, MemoryError> {
+        Ok(self
+            .allocate_memory_regions(&[(start, size)])?
+            .pop()
+            .unwrap())
+    }
 }
 
 impl From<&VmResources> for VmmConfig {