vm-allocator: free GSIs in GsiAllocator

The old implementation used an ever monotonically increasing u32 counter
to allocate new GSIs. The counter increased every time a new GSI was
allocated, and freeing GSIs was not possible. Thus, Cloud Hypervisor
can run out of GSIs and panics. This currently happened at the 1024th
GSI [0]. Further, this caused the `KVM_SET_GSI_ROUTING` ioctl to carry
much more payload than needed.

This new implementation uses a bitmap for proper tracking of resources
and can gracefully free GSIs - this is abstracted in type
InterruptAllocator.

Please note that this commit only replaces the old mechanism. The next
commit will introduce freeing used GSIs automatically when an
InterruptRoute is dropped.

While being on this, we also propagate the errors that the allocator may
throw where necessary.

Co-authored-by: Sebastian Eydam <sebastian.eydam@cyberus-technology.de>
On-behalf-of: SAP sebastian.eydam@sap.com
Signed-off-by: Sebastian Eydam <sebastian.eydam@cyberus-technology.de>

On-behalf-of: Philipp Schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

vmm/src/device_manager.rs

@@ -91,7 +91,7 @@ use virtio_devices::{
     AccessPlatformMapping, ActivateError, Block, Endpoint, IommuMapping, VdpaDmaMapping,
     VirtioMemMappingSource,
 };
-use vm_allocator::{AddressAllocator, SystemAllocator};
+use vm_allocator::{AddressAllocator, InterruptAllocError, SystemAllocator};
 use vm_device::dma_mapping::ExternalDmaMapping;
 use vm_device::interrupt::{
     InterruptIndex, InterruptManager, LegacyIrqGroupConfig, MsiIrqGroupConfig,
@@ -272,7 +272,7 @@ pub enum DeviceManagerError {
 
     /// Cannot allocate IRQ.
     #[error("Cannot allocate IRQ")]
-    AllocateIrq,
+    AllocateIrq(#[from] InterruptAllocError),
 
     /// Cannot configure the IRQ.
     #[error("Cannot configure the IRQ")]

vmm/src/interrupt.rs

@@ -46,7 +46,7 @@ impl InterruptRoute {
             None => {
                 let new_gsi = allocator
                     .allocate_gsi()
-                    .ok_or_else(|| io::Error::other("Failed allocating new GSI"))?;
+                    .map_err(|e| io::Error::other(format!("Failed allocating new GSI: {e}")))?;
                 self.gsi = Some(new_gsi);
                 Ok(new_gsi)
             }

vmm/src/pci_segment.rs

@@ -209,7 +209,7 @@ impl PciSegment {
                     .lock()
                     .unwrap()
                     .allocate_irq()
-                    .ok_or(DeviceManagerError::AllocateIrq)? as u8,
+                    .map_err(DeviceManagerError::AllocateIrq)? as u8,
             );
         }