From ad3dbcd1307295e0c86f50ccf139372053a041b7 Mon Sep 17 00:00:00 2001
From: Rob Bradford <rbradford@meta.com>
Date: Tue, 7 Jul 2026 09:14:23 +0100
Subject: [PATCH] virtio-devices: Use SmallVec for descriptor chains

Rather than instantiating a vector for parsing the descriptor chain in
advance instead use a SmallVec bounded by the expected length of the
descriptor chain. This removes vector allocations from those paths.

As smallvec was already a block dependency move it to a workspace
dependency and use it from there.

Fixes: #5079

Signed-off-by: Rob Bradford <rbradford@meta.com>
---
 Cargo.lock                    | 1 +
 Cargo.toml                    | 1 +
 block/Cargo.toml              | 2 +-
 virtio-devices/Cargo.toml     | 1 +
 virtio-devices/src/balloon.rs | 5 +++--
 virtio-devices/src/console.rs | 5 +++--
 virtio-devices/src/rng.rs     | 3 ++-
 7 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index ea14aa60d..ea3157441 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2707,6 +2707,7 @@ dependencies = [
  "serde",
  "serde_with",
  "serial_buffer",
+ "smallvec",
  "thiserror",
  "vhost",
  "virtio-bindings",
diff --git a/Cargo.toml b/Cargo.toml
index 4683f618d..38bb98818 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -105,6 +105,7 @@ rustls = { version = "0.23.40", default-features = false, features = [
 sha2 = "0.11.0"
 signal-hook = "0.4.4"
 signal-hook-registry = "1.4.8"
+smallvec = "1.15.1"
 thiserror = "2.0.18"
 uuid = { version = "1.23.2" }
 wait-timeout = "0.2.1"
diff --git a/block/Cargo.toml b/block/Cargo.toml
index 974e1afc8..690d5f79e 100644
--- a/block/Cargo.toml
+++ b/block/Cargo.toml
@@ -20,7 +20,7 @@ libc = { workspace = true }
 log = { workspace = true }
 remain = "0.2.15"
 serde = { workspace = true, features = ["derive"] }
-smallvec = "1.15.1"
+smallvec = { workspace = true }
 thiserror = { workspace = true }
 uuid = { workspace = true, features = ["v4"] }
 virtio-bindings = { workspace = true }
diff --git a/virtio-devices/Cargo.toml b/virtio-devices/Cargo.toml
index d2658eeec..2ef499f8d 100644
--- a/virtio-devices/Cargo.toml
+++ b/virtio-devices/Cargo.toml
@@ -30,6 +30,7 @@ serde_with = { workspace = true, default-features = false, features = [
   "macros",
 ] }
 serial_buffer = { path = "../serial_buffer" }
+smallvec = { workspace = true }
 thiserror = { workspace = true }
 vhost = { workspace = true, features = [
   "vhost-kern",
diff --git a/virtio-devices/src/balloon.rs b/virtio-devices/src/balloon.rs
index 1a1e2198e..eb90cd36d 100644
--- a/virtio-devices/src/balloon.rs
+++ b/virtio-devices/src/balloon.rs
@@ -25,6 +25,7 @@ use event_monitor::event;
 use log::{error, info, warn};
 use seccompiler::SeccompAction;
 use serde::{Deserialize, Serialize};
+use smallvec::SmallVec;
 use thiserror::Error;
 use virtio_queue::{Queue, QueueT};
 use vm_allocator::page_size::{align_page_size_down, get_page_size};
@@ -276,7 +277,7 @@ impl BalloonEpollHandler {
         {
             let data_chunk_size = size_of::<u32>();
 
-            let results: Vec<_> = desc_chain
+            let results: SmallVec<[_; 4]> = desc_chain
                 .checked_iter(self.access_platform.as_deref())
                 .collect();
             for result in results {
@@ -366,7 +367,7 @@ impl BalloonEpollHandler {
             self.queues[queue_index].pop_descriptor_chain(self.mem.memory())
         {
             let mut descs_len = 0;
-            let results: Vec<_> = desc_chain
+            let results: SmallVec<[_; 4]> = desc_chain
                 .checked_iter(self.access_platform.as_deref())
                 .collect();
             for result in results {
diff --git a/virtio-devices/src/console.rs b/virtio-devices/src/console.rs
index e05db9e93..608cdbaf8 100644
--- a/virtio-devices/src/console.rs
+++ b/virtio-devices/src/console.rs
@@ -16,6 +16,7 @@ use log::{error, info, warn};
 use seccompiler::SeccompAction;
 use serde::{Deserialize, Serialize};
 use serial_buffer::SerialBuffer;
+use smallvec::SmallVec;
 use thiserror::Error;
 use virtio_queue::{Queue, QueueT};
 use vm_memory::{ByteValued, Bytes, GuestAddressSpace, GuestMemoryAtomic};
@@ -205,7 +206,7 @@ impl ConsoleEpollHandler {
 
             // Validate all descriptors upfront so we never partially fill
             // buffers for a chain that turns out to be invalid.
-            let descs: Vec<_> = desc_chain
+            let descs: SmallVec<[_; 4]> = desc_chain
                 .checked_iter(self.access_platform.as_deref())
                 .collect::<Result<_, _>>()
                 .unwrap_or_default();
@@ -253,7 +254,7 @@ impl ConsoleEpollHandler {
         let mut used_descs = false;
 
         while let Some(mut desc_chain) = trans_queue.pop_descriptor_chain(self.mem.memory()) {
-            let results: Vec<_> = desc_chain
+            let results: SmallVec<[_; 4]> = desc_chain
                 .checked_iter(self.access_platform.as_deref())
                 .collect();
             for result in results {
diff --git a/virtio-devices/src/rng.rs b/virtio-devices/src/rng.rs
index eae688ef0..b76557ed7 100644
--- a/virtio-devices/src/rng.rs
+++ b/virtio-devices/src/rng.rs
@@ -15,6 +15,7 @@ use event_monitor::event;
 use log::{error, info, warn};
 use seccompiler::SeccompAction;
 use serde::{Deserialize, Serialize};
+use smallvec::SmallVec;
 use thiserror::Error;
 use virtio_queue::{Queue, QueueT};
 use vm_memory::{Bytes, GuestAddressSpace, GuestMemoryAtomic};
@@ -66,7 +67,7 @@ impl RngEpollHandler {
             // Validate the entire descriptor chain upfront so we never
             // partially fill buffers for a chain that turns out to be
             // invalid.
-            let descs: Vec<_> = desc_chain
+            let descs: SmallVec<[_; 4]> = desc_chain
                 .checked_iter(self.access_platform.as_deref())
                 .collect::<Result<_, _>>()
                 .unwrap_or_default();