pci: Allow specific PCI b/d/f to be reserved

In order to let the PciBus user choose where a device should be placed
on the bus, a new function get_device_id() is introduced. This will be
helpful in the context of snapshot/restore as the caller will be able to
place the PCI devices on the same slot they were placed before the
snapshot was taken.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

pci/src/bus.rs

@@ -34,6 +34,8 @@ pub enum PciRootError {
     NoPciDeviceSlotAvailable,
     /// Invalid PCI device identifier provided.
     InvalidPciDeviceSlot(usize),
+    /// Valid PCI device identifier but already used.
+    AlreadyInUsePciDeviceSlot(usize),
 }
 pub type Result<T> = std::result::Result<T, PciRootError>;
 
@@ -155,6 +157,19 @@ impl PciBus {
         Err(PciRootError::NoPciDeviceSlotAvailable)
     }
 
+    pub fn get_device_id(&mut self, id: usize) -> Result<()> {
+        if id < NUM_DEVICE_IDS {
+            if !self.device_ids[id] {
+                self.device_ids[id] = true;
+                Ok(())
+            } else {
+                Err(PciRootError::AlreadyInUsePciDeviceSlot(id))
+            }
+        } else {
+            Err(PciRootError::InvalidPciDeviceSlot(id))
+        }
+    }
+
     pub fn put_device_id(&mut self, id: usize) -> Result<()> {
         if id < NUM_DEVICE_IDS {
             self.device_ids[id] = false;