Reuse buffer for processing descriptor chains in virtio-rng device

[ShadowCurse]

Changes

Add a buffer to the Entropy struct which will be reused for descriptor chain processing. This will remove runtime overhead of recreating this buffer for each request.

Reason

Less overhead in rng device.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.96%. Comparing base (32332d9) to head (41dfa50).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vmm/src/devices/virtio/rng/device.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4846      +/-   ##
==========================================
- Coverage   83.96%   83.96%   -0.01%     
==========================================
  Files         250      250              
  Lines       27758    27756       -2     
==========================================
- Hits        23306    23304       -2     
  Misses       4452     4452              

Flag	Coverage Δ
5.10-c5n.metal	84.58% <93.33%> (-0.01%)	⬇️
5.10-m5n.metal	84.56% <93.33%> (-0.01%)	⬇️
5.10-m6a.metal	83.85% <93.33%> (-0.01%)	⬇️
5.10-m6g.metal	80.51% <93.33%> (-0.01%)	⬇️
5.10-m6i.metal	84.56% <93.33%> (-0.01%)	⬇️
5.10-m7g.metal	80.51% <93.33%> (-0.01%)	⬇️
6.1-c5n.metal	84.58% <93.33%> (-0.01%)	⬇️
6.1-m5n.metal	84.56% <93.33%> (-0.01%)	⬇️
6.1-m6a.metal	83.85% <93.33%> (-0.01%)	⬇️
6.1-m6g.metal	80.50% <93.33%> (-0.01%)	⬇️
6.1-m6i.metal	84.56% <93.33%> (-0.01%)	⬇️
6.1-m7g.metal	80.51% <93.33%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[roypat]

Could we avoid the lifetime issues like this instead of doing Self:: everywhere? :o

diff --git a/src/vmm/src/devices/virtio/rng/device.rs b/src/vmm/src/devices/virtio/rng/device.rs
index f9bb3c828..622268e54 100644
--- a/src/vmm/src/devices/virtio/rng/device.rs
+++ b/src/vmm/src/devices/virtio/rng/device.rs
@@ -91,13 +91,13 @@ impl Entropy {
             .map_err(DeviceError::FailedSignalingIrq)
     }
 
-    fn rate_limit_request(rate_limiter: &mut RateLimiter, bytes: u64) -> bool {
-        if !rate_limiter.consume(1, TokenType::Ops) {
+    fn rate_limit_request(&mut self, bytes: u64) -> bool {
+        if !self.rate_limiter.consume(1, TokenType::Ops) {
             return false;
         }
 
-        if !rate_limiter.consume(bytes, TokenType::Bytes) {
-            rate_limiter.manual_replenish(1, TokenType::Ops);
+        if !self.rate_limiter.consume(bytes, TokenType::Bytes) {
+            self.rate_limiter.manual_replenish(1, TokenType::Ops);
             return false;
         }
 
@@ -109,29 +109,29 @@ impl Entropy {
         rate_limiter.manual_replenish(bytes, TokenType::Bytes);
     }
 
-    fn handle_one(iovec: &mut IoVecBufferMut) -> Result<u32, EntropyError> {
+    fn handle_one(&mut self) -> Result<u32, EntropyError> {
         // If guest provided us with an empty buffer just return directly
-        if iovec.len() == 0 {
+        if self.buffer.len() == 0 {
             return Ok(0);
         }
 
-        let mut rand_bytes = vec![0; iovec.len() as usize];
+        let mut rand_bytes = vec![0; self.buffer.len() as usize];
         rand::fill(&mut rand_bytes).map_err(|err| {
             METRICS.host_rng_fails.inc();
             err
         })?;
 
         // It is ok to unwrap here. We are writing `iovec.len()` bytes at offset 0.
-        iovec.write_all_volatile_at(&rand_bytes, 0).unwrap();
-        Ok(iovec.len())
+        self.buffer.write_all_volatile_at(&rand_bytes, 0).unwrap();
+        Ok(self.buffer.len())
     }
 
     fn process_entropy_queue(&mut self) {
-        // This is safe since we checked in the event handler that the device is activated.
-        let mem = self.device_state.mem().unwrap();
 
         let mut used_any = false;
         while let Some(desc) = self.queues[RNG_QUEUE].pop() {
+            // This is safe since we checked in the event handler that the device is activated.
+            let mem = self.device_state.mem().unwrap();
             let index = desc.index;
             METRICS.entropy_event_count.inc();
 
@@ -148,8 +148,7 @@ impl Entropy {
                     // Check for available rate limiting budget.
                     // If not enough budget is available, leave the request descriptor in the queue
                     // to handle once we do have budget.
-                    if !Self::rate_limit_request(
-                        &mut self.rate_limiter,
+                    if !self.rate_limit_request(
                         u64::from(self.buffer.len()),
                     ) {
                         debug!("entropy: throttling entropy queue");
@@ -158,7 +157,7 @@ impl Entropy {
                         break;
                     }
 
-                    Self::handle_one(&mut self.buffer).unwrap_or_else(|err| {
+                    self.handle_one().unwrap_or_else(|err| {
                         error!("entropy: {err}");
                         METRICS.entropy_event_fails.inc();
                         0