pci: rollback BAR address on failed move_bar

When BAR reprogramming is detected, detect_bar_reprogramming()
eagerly updates the BAR address in config space before the actual
MMIO remapping occurs. If the subsequent move_bar() fails (e.g.
the new address falls outside the allocator range), the config
register retains the new address while the MMIO bus still uses
the old one, leaving the device broken.

Add restore_bar_addr() to undo the config space update when
move_bar() fails, so the device remains functional at its
original address.

For 64-bit BARs, restore both the low and high BAR slots as well
as the corresponding config registers, mirroring the two-slot
update logic in detect_bar_reprogramming().

The VirtioPciDevice::restore_bar_addr() delegates directly to
PciConfiguration since DeviceRelocation::move_bar() calls
pci_dev.move_bar() last — if the relocation fails earlier,
bar_regions was never updated and needs no rollback.

Signed-off-by: CMGS <ilskdw@gmail.com>

Author: CMGS

pci/src/bus.rs

@@ -10,7 +10,7 @@ use std::ops::DerefMut;
 use std::sync::{Arc, Barrier, Mutex};
 
 use byteorder::{ByteOrder, LittleEndian};
-use log::error;
+use log::warn;
 use thiserror::Error;
 use vm_device::{Bus, BusDevice, BusDeviceSync};
 
@@ -280,10 +280,15 @@ impl PciConfigIo {
                     device.deref_mut(),
                     params.region_type,
                 ) {
-                    error!(
-                        "Failed moving device BAR: {}: 0x{:x}->0x{:x}(0x{:x})",
+                    warn!(
+                        "Failed moving device BAR: {}: 0x{:x}->0x{:x}(0x{:x}), keeping old BAR",
                         e, params.old_base, params.new_base, params.len
                     );
+                    // Rollback: the config register was already updated to
+                    // new_base by detect_bar_reprogramming(). Restore it by
+                    // writing back the old address so device state stays
+                    // consistent with the MMIO bus mapping.
+                    device.restore_bar_addr(params);
                 }
             }
 
@@ -405,10 +410,11 @@ impl PciConfigMmio {
                     device.deref_mut(),
                     params.region_type,
                 ) {
-                    error!(
-                        "Failed moving device BAR: {}: 0x{:x}->0x{:x}(0x{:x})",
+                    warn!(
+                        "Failed moving device BAR: {}: 0x{:x}->0x{:x}(0x{:x}), keeping old BAR",
                         e, params.old_base, params.new_base, params.len
                     );
+                    device.restore_bar_addr(params);
                 }
             }
         }

pci/src/configuration.rs

@@ -1093,6 +1093,55 @@ impl PciConfiguration {
     pub(crate) fn clear_pending_bar_reprogram(&mut self) {
         self.pending_bar_reprogram = Vec::new();
     }
+
+    /// Restore BAR address after a failed move. This undoes the premature
+    /// address update in detect_bar_reprogramming() so that config space
+    /// stays consistent with the actual MMIO mapping.
+    pub fn restore_bar_addr(&mut self, params: &BarReprogrammingParams) {
+        match params.region_type {
+            PciBarRegionType::Memory64BitRegion => {
+                // 64-bit BAR spans two slots: bars[i] (low, type Memory64BitRegion)
+                // and bars[i+1] (high, type None). Mirror detect_bar_reprogramming
+                // by matching the combined address and restoring both halves.
+                for i in 0..NUM_BAR_REGS - 1 {
+                    if self.bars[i].r#type != Some(PciBarRegionType::Memory64BitRegion) {
+                        continue;
+                    }
+                    let low_mask = self.writable_bits[BAR0_REG + i];
+                    let high_mask = self.writable_bits[BAR0_REG + i + 1];
+                    let current = (u64::from(self.bars[i + 1].addr & high_mask) << 32)
+                        | u64::from(self.bars[i].addr & low_mask);
+                    if current == params.new_base {
+                        let old_low = params.old_base as u32;
+                        let old_high = (params.old_base >> 32) as u32;
+                        self.bars[i].addr = old_low;
+                        self.bars[i + 1].addr = old_high;
+                        self.registers[BAR0_REG + i] =
+                            (self.registers[BAR0_REG + i] & !low_mask) | (old_low & low_mask);
+                        self.registers[BAR0_REG + i + 1] = (self.registers[BAR0_REG + i + 1]
+                            & !high_mask)
+                            | (old_high & high_mask);
+                        return;
+                    }
+                }
+            }
+            _ => {
+                // 32-bit Memory or IO BAR
+                for i in 0..NUM_BAR_REGS {
+                    let mask = self.writable_bits[BAR0_REG + i];
+                    if self.bars[i].r#type == Some(params.region_type)
+                        && u64::from(self.bars[i].addr & mask) == params.new_base
+                    {
+                        let old = params.old_base as u32;
+                        self.bars[i].addr = old;
+                        self.registers[BAR0_REG + i] =
+                            (self.registers[BAR0_REG + i] & !mask) | (old & mask);
+                        return;
+                    }
+                }
+            }
+        }
+    }
 }
 
 impl Pausable for PciConfiguration {}

pci/src/device.rs

@@ -93,6 +93,10 @@ pub trait PciDevice: Send {
     fn move_bar(&mut self, _old_base: u64, _new_base: u64) -> result::Result<(), io::Error> {
         Ok(())
     }
+    /// Restore BAR address in config space after a failed move_bar.
+    /// This rolls back the address update made by detect_bar_reprogramming()
+    /// so that the config register stays consistent with the MMIO bus mapping.
+    fn restore_bar_addr(&mut self, _params: &BarReprogrammingParams) {}
     /// Provides a mutable reference to the Any trait. This is useful to let
     /// the caller have access to the underlying type behind the trait.
     fn as_any_mut(&mut self) -> &mut dyn Any;

virtio-devices/src/transport/pci_device.rs

@@ -1133,6 +1133,10 @@ impl PciDevice for VirtioPciDevice {
         Ok(())
     }
 
+    fn restore_bar_addr(&mut self, params: &BarReprogrammingParams) {
+        self.configuration.restore_bar_addr(params);
+    }
+
     fn read_bar(&mut self, _base: u64, offset: u64, data: &mut [u8]) {
         match offset {
             o if o < COMMON_CONFIG_BAR_OFFSET + COMMON_CONFIG_SIZE => self.common_config.read(