From 710a860e9b627d122dba0220c7ce7a289ec3c83e Mon Sep 17 00:00:00 2001
From: Bo Chen <chen.bo@intel.com>
Date: Wed, 19 Oct 2022 14:19:06 -0700
Subject: [PATCH] virtio-devices: iommu: Avoid using vector and direct indexing

The the number of queues and associated events is known and fixed. We
can define and use each of them explicitly and avoid using vector (and
hence direct indexing), which is cleaner and slightly more efficient.
Also, this refactoring makes it clearer that we are not handling "event
queue" events (as "_event_queue" is not being used intentionally).

Signed-off-by: Bo Chen <chen.bo@intel.com>
---
 virtio-devices/src/iommu.rs | 35 ++++++++++++++++++-----------------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/virtio-devices/src/iommu.rs b/virtio-devices/src/iommu.rs
index ed03a3f12..227748b23 100644
--- a/virtio-devices/src/iommu.rs
+++ b/virtio-devices/src/iommu.rs
@@ -630,9 +630,11 @@ impl Request {
 
 struct IommuEpollHandler {
     mem: GuestMemoryAtomic<GuestMemoryMmap>,
-    queues: Vec<Queue>,
+    request_queue: Queue,
+    _event_queue: Queue,
     interrupt_cb: Arc<dyn VirtioInterrupt>,
-    queue_evts: Vec<EventFd>,
+    request_queue_evt: EventFd,
+    event_queue_evt: EventFd,
     kill_evt: EventFd,
     pause_evt: EventFd,
     mapping: Arc<IommuMapping>,
@@ -643,7 +645,8 @@ struct IommuEpollHandler {
 impl IommuEpollHandler {
     fn request_queue(&mut self) -> Result<bool, Error> {
         let mut used_descs = false;
-        while let Some(mut desc_chain) = self.queues[0].pop_descriptor_chain(self.mem.memory()) {
+        while let Some(mut desc_chain) = self.request_queue.pop_descriptor_chain(self.mem.memory())
+        {
             let len = Request::parse(
                 &mut desc_chain,
                 &self.mapping,
@@ -651,7 +654,7 @@ impl IommuEpollHandler {
                 self.msi_iova_space,
             )?;
 
-            self.queues[0]
+            self.request_queue
                 .add_used(desc_chain.memory(), desc_chain.head_index(), len as u32)
                 .map_err(Error::QueueAddUsed)?;
 
@@ -680,8 +683,8 @@ impl IommuEpollHandler {
         paused_sync: Arc<Barrier>,
     ) -> result::Result<(), EpollHelperError> {
         let mut helper = EpollHelper::new(&self.kill_evt, &self.pause_evt)?;
-        helper.add_event(self.queue_evts[0].as_raw_fd(), REQUEST_Q_EVENT)?;
-        helper.add_event(self.queue_evts[1].as_raw_fd(), EVENT_Q_EVENT)?;
+        helper.add_event(self.request_queue_evt.as_raw_fd(), REQUEST_Q_EVENT)?;
+        helper.add_event(self.event_queue_evt.as_raw_fd(), EVENT_Q_EVENT)?;
         helper.run(paused, paused_sync, self)?;
 
         Ok(())
@@ -697,7 +700,7 @@ impl EpollHelperHandler for IommuEpollHandler {
         let ev_type = event.data as u16;
         match ev_type {
             REQUEST_Q_EVENT => {
-                self.queue_evts[0].read().map_err(|e| {
+                self.request_queue_evt.read().map_err(|e| {
                     EpollHelperError::HandleEvent(anyhow!("Failed to get queue event: {:?}", e))
                 })?;
 
@@ -717,7 +720,7 @@ impl EpollHelperHandler for IommuEpollHandler {
                 }
             }
             EVENT_Q_EVENT => {
-                self.queue_evts[1].read().map_err(|e| {
+                self.event_queue_evt.read().map_err(|e| {
                     EpollHelperError::HandleEvent(anyhow!("Failed to get queue event: {:?}", e))
                 })?;
 
@@ -1034,23 +1037,21 @@ impl VirtioDevice for Iommu {
         &mut self,
         mem: GuestMemoryAtomic<GuestMemoryMmap>,
         interrupt_cb: Arc<dyn VirtioInterrupt>,
-        queues: Vec<(usize, Queue, EventFd)>,
+        mut queues: Vec<(usize, Queue, EventFd)>,
     ) -> ActivateResult {
         self.common.activate(&queues, &interrupt_cb)?;
         let (kill_evt, pause_evt) = self.common.dup_eventfds();
 
-        let mut virtqueues = Vec::new();
-        let mut queue_evts = Vec::new();
-        for (_, queue, queue_evt) in queues {
-            virtqueues.push(queue);
-            queue_evts.push(queue_evt);
-        }
+        let (_, request_queue, request_queue_evt) = queues.remove(0);
+        let (_, _event_queue, event_queue_evt) = queues.remove(0);
 
         let mut handler = IommuEpollHandler {
             mem,
-            queues: virtqueues,
+            request_queue,
+            _event_queue,
             interrupt_cb,
-            queue_evts,
+            request_queue_evt,
+            event_queue_evt,
             kill_evt,
             pause_evt,
             mapping: self.mapping.clone(),