fix(virtio): prevent MSI-X change if irqfd linked

If the MSI-X table entry 0 is mapped to queue 0, and queue 0 is
offloaded to a vhost backend, and later the guest maps table entry
1 to queue 0, we will need to find out the iqrfd of table entry 1,
and send a message to the vhost backend to update the irqfd. This is
much more complicated than just updating table entry 0, which only
involves updating the GSI table at the VMM side.

For now we simply prevent such changes if a queue is already mapped
to an MSI-X entry that is linked to an irqfd.

Signed-off-by: Changyuan Lyu <[email protected]>

alioth/src/virtio/pci.rs

@@ -215,6 +215,23 @@ where
             q.enabled.store(false, Ordering::Release);
         }
     }
+
+    fn msix_change_allowed(&self, old: u16) -> bool {
+        let Some(entry) = self.irq_sender.msix_entries.get(old as usize) else {
+            return true;
+        };
+        let entry = entry.read();
+        if let MsixTableMmioEntry::IrqFd(fd) = &*entry {
+            log::error!(
+                "{}: MSI-X vector {old:#x} was assigned to irqfd {:#x}",
+                self.name,
+                fd.as_fd().as_raw_fd(),
+            );
+            false
+        } else {
+            true
+        }
+    }
 }
 
 impl<M> Mmio for VirtioPciRegisterMmio<M>
@@ -367,11 +384,19 @@ where
             }
             VirtioCommonCfg::LAYOUT_CONFIG_MSIX_VECTOR => {
                 let config_msix = &self.irq_sender.msix_vector.config;
-                let old = config_msix.swap(val as u16, Ordering::AcqRel);
-                log::trace!(
-                    "{}: config MSI-X vector update: {old:#x} -> {val:#x}",
-                    self.name
-                );
+                let old = config_msix.load(Ordering::Acquire);
+                if self.msix_change_allowed(old) {
+                    config_msix.store(val as u16, Ordering::Release);
+                    log::trace!(
+                        "{}: config MSI-X vector update: {old:#x} -> {val:#x}",
+                        self.name
+                    );
+                } else {
+                    log::error!(
+                        "{}: cannot change config MSI-X vector from {old:#x} to {val:#x}",
+                        self.name
+                    )
+                }
             }
             VirtioCommonCfg::LAYOUT_DEVICE_STATUS => {
                 let status = DevStatus::from_bits_truncate(val as u8);
@@ -406,11 +431,19 @@ where
             VirtioCommonCfg::LAYOUT_QUEUE_MSIX_VECTOR => {
                 let q_sel = reg.queue_sel.load(Ordering::Relaxed) as usize;
                 if let Some(msix_vector) = self.irq_sender.msix_vector.queues.get(q_sel) {
-                    let old = msix_vector.swap(val as u16, Ordering::AcqRel);
-                    log::trace!(
-                        "{}: queue {q_sel} MSI-X vector update: {old:#x} -> {val:#x}",
-                        self.name
-                    );
+                    let old = msix_vector.load(Ordering::Acquire);
+                    if self.msix_change_allowed(old) {
+                        msix_vector.store(val as u16, Ordering::Release);
+                        log::trace!(
+                            "{}: queue {q_sel} MSI-X vector update: {old:#x} -> {val:#x}",
+                            self.name
+                        );
+                    } else {
+                        log::error!(
+                            "{}: cannot change queue {q_sel} MSI-X vector from {old:#x} to {val:#x}",
+                            self.name
+                        )
+                    }
                 }
             }
             VirtioCommonCfg::LAYOUT_QUEUE_ENABLE => {