vmm: Reject resizing below the boot size with an error

This was already handled with user memory zones but not with the default
memory. Make a small refactoring to move the boot RAM check into
MemoryManager rather than split across Vm and MemoryManager.

Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/memory_manager.rs

@@ -356,6 +356,11 @@ pub enum Error {
     #[error("Impossible to resize guest memory if it is backed by user defined memory regions")]
     InvalidResizeWithMemoryZones,
 
+    /// Forbidden operation. Impossible to resize guest memory below the boot
+    /// RAM as it is not removable with virtio-mem.
+    #[error("Invalid resize request: desired_size = {0} below boot size = {1}")]
+    InvalidResizeBelowBootSize(u64, u64),
+
     /// It's invalid to try applying a NUMA policy to a memory zone that is
     /// memory mapped with MAP_SHARED.
     #[error(
@@ -2583,14 +2588,19 @@ impl MemoryManager {
         let mut region: Option<Arc<GuestRegionMmap>> = None;
         match self.hotplug_method {
             HotplugMethod::VirtioMem => {
-                if desired_ram >= self.boot_ram {
+                if desired_ram < self.boot_ram {
-                    if !self.dynamic {
+                    return Err(Error::InvalidResizeBelowBootSize(
-                        return Ok(region);
+                        desired_ram,
-                    }
+                        self.boot_ram,
-
+                    ));
-                    self.virtio_mem_resize(DEFAULT_MEMORY_ZONE, desired_ram - self.boot_ram)?;
-                    self.current_ram = desired_ram;
                 }
+
+                if !self.dynamic {
+                    return Ok(region);
+                }
+
+                self.virtio_mem_resize(DEFAULT_MEMORY_ZONE, desired_ram - self.boot_ram)?;
+                self.current_ram = desired_ram;
             }
             HotplugMethod::Acpi => {
                 if desired_ram > self.current_ram {
@@ -2607,7 +2617,7 @@ impl MemoryManager {
         Ok(region)
     }
 
-    pub fn resize_zone(&mut self, id: &str, virtio_mem_size: u64) -> Result<(), Error> {
+    pub fn resize_zone(&mut self, zone: &MemoryZoneConfig, desired_ram: u64) -> Result<(), Error> {
         if !self.user_provided_zones {
             error!(
                 "Not allowed to resize guest memory zone when no zone is \
@@ -2616,7 +2626,11 @@ impl MemoryManager {
             return Err(Error::ResizeZone);
         }
 
-        self.virtio_mem_resize(id, virtio_mem_size)
+        if desired_ram < zone.size {
+            return Err(Error::InvalidResizeBelowBootSize(desired_ram, zone.size));
+        }
+
+        self.virtio_mem_resize(&zone.id, desired_ram - zone.size)
     }
 
     pub fn is_hardlink(f: &File) -> bool {

vmm/src/vm.rs

@@ -2158,27 +2158,18 @@ impl Vm {
         if let Some(zones) = &mut memory_config.zones {
             for zone in zones.iter_mut() {
                 if zone.id == id {
-                    if desired_memory >= zone.size {
+                    self.memory_manager
-                        let hotplugged_size = desired_memory - zone.size;
+                        .lock()
-                        self.memory_manager
+                        .unwrap()
-                            .lock()
+                        .resize_zone(zone, desired_memory)
-                            .unwrap()
+                        .map_err(Error::MemoryManager)?;
-                            .resize_zone(id, desired_memory - zone.size)
+                    // We update the memory zone config regardless of the
-                            .map_err(Error::MemoryManager)?;
+                    // actual 'resize-zone' operation result (happened or
-                        // We update the memory zone config regardless of the
+                    // not), so that if the VM reboots it will be running
-                        // actual 'resize-zone' operation result (happened or
+                    // with the last configured memory zone size.
-                        // not), so that if the VM reboots it will be running
+                    zone.hotplugged_size = Some(desired_memory - zone.size);
-                        // with the last configured memory zone size.
-                        zone.hotplugged_size = Some(hotplugged_size);
 
-                        return Ok(());
+                    return Ok(());
-                    }
-                    error!(
-                        "Invalid to ask less ({}) than boot RAM ({}) for \
-                        this memory zone",
-                        desired_memory, zone.size,
-                    );
-                    return Err(Error::ResizeZone);
                 }
             }
         }