vmm, pci, openapi: Add VFIO mmap BAR exclusion

Allow VFIO devices to list BAR indices that should not be
mmapped into the guest. This lets operators skip large BARs that
are known not to be used by their workload.

When a BAR is skipped, the log also calls out that P2P DMA
mapping is skipped because the VFIO DMA map path uses the same
mmap backing.

Signed-off-by: Damian Barabonkov <dbctl@pm.me>
Assisted-by: OpenCode:gpt-5.5

pci/src/vfio.rs

@@ -501,6 +501,13 @@ pub(crate) struct VfioCommon {
     pub(crate) vfio_wrapper: Arc<dyn Vfio>,
     pub(crate) patches: HashMap<usize, ConfigPatch>,
     x_nv_gpudirect_clique: Option<u8>,
+    x_exclude_mmap_bars: Vec<u8>,
+}
+
+#[derive(Default)]
+pub(crate) struct VfioCommonConfig {
+    pub(crate) x_nv_gpudirect_clique: Option<u8>,
+    pub(crate) x_exclude_mmap_bars: Vec<u8>,
 }
 
 impl VfioCommon {
@@ -511,7 +518,7 @@ impl VfioCommon {
         subclass: &dyn PciSubclass,
         bdf: PciBdf,
         snapshot: Option<&Snapshot>,
-        x_nv_gpudirect_clique: Option<u8>,
+        config: VfioCommonConfig,
     ) -> Result<Self, VfioPciError> {
         let pci_configuration_state = vm_migration::state_from_id(snapshot, PCI_CONFIGURATION_ID)
             .map_err(|e| {
@@ -546,7 +553,8 @@ impl VfioCommon {
             legacy_interrupt_group,
             vfio_wrapper,
             patches: HashMap::new(),
-            x_nv_gpudirect_clique,
+            x_nv_gpudirect_clique: config.x_nv_gpudirect_clique,
+            x_exclude_mmap_bars: config.x_exclude_mmap_bars,
         };
 
         let state: Option<VfioCommonState> = snapshot
@@ -1499,6 +1507,7 @@ impl VfioPciDevice {
         memory_slot_allocator: MemorySlotAllocator,
         snapshot: Option<&Snapshot>,
         x_nv_gpudirect_clique: Option<u8>,
+        x_exclude_mmap_bars: Vec<u8>,
         device_path: PathBuf,
     ) -> Result<Self, VfioPciError> {
         let device = Arc::new(device);
@@ -1513,7 +1522,10 @@ impl VfioPciDevice {
             &PciVfioSubclass::VfioSubclass,
             bdf,
             vm_migration::snapshot_from_id(snapshot, VFIO_COMMON_ID),
-            x_nv_gpudirect_clique,
+            VfioCommonConfig {
+                x_nv_gpudirect_clique,
+                x_exclude_mmap_bars,
+            },
         )?;
 
         let vfio_pci_device = VfioPciDevice {
@@ -1649,6 +1661,21 @@ impl VfioPciDevice {
         // SAFETY: fd is guaranteed valid
         let fd = unsafe { BorrowedFd::borrow_raw(fd) };
         for region in self.common.mmio_regions.iter_mut() {
+            if self
+                .common
+                .x_exclude_mmap_bars
+                .contains(&(region.index as u8))
+            {
+                info!(
+                    "Skipping VFIO BAR mmap and P2P DMA mapping for device {} at {} BAR {} (size = 0x{:x})",
+                    self.bdf,
+                    self.device_path.display(),
+                    region.index,
+                    region.length
+                );
+                continue;
+            }
+
             let region_flags = self.device.get_region_flags(region.index);
             if region_flags & VFIO_REGION_INFO_FLAG_MMAP != 0 {
                 let mut prot = 0;

pci/src/vfio_user.rs

@@ -26,7 +26,9 @@ use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottabl
 use vmm_sys_util::eventfd::EventFd;
 
 use crate::mmap::MmapRegion;
-use crate::vfio::{UserMemoryRegion, VFIO_COMMON_ID, Vfio, VfioCommon, VfioError};
+use crate::vfio::{
+    UserMemoryRegion, VFIO_COMMON_ID, Vfio, VfioCommon, VfioCommonConfig, VfioError,
+};
 use crate::{
     BarReprogrammingParams, PciBarConfiguration, PciBdf, PciDevice, PciDeviceError, PciSubclass,
     VfioPciError,
@@ -101,7 +103,7 @@ impl VfioUserPciDevice {
             &PciVfioUserSubclass::VfioUserSubclass,
             bdf,
             vm_migration::snapshot_from_id(snapshot, VFIO_COMMON_ID),
-            None,
+            VfioCommonConfig::default(),
         )
         .map_err(VfioUserPciDeviceError::CreateVfioCommon)?;