feat: pmem: cache the msync result

There is only one type of request virto-pmem accepts, but guest still
can issue many of them and Firecracker will need to process it all.
Instead of doing one `msync` per request, it is less resource intensive
to do it once on the first valid descriptor and then duplicate the
result to other descriptors. This is safe since the guest will only
know the result of the execution after Firecracker will signal it,
which will only happen after all descriptors are processed.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

Author: ShadowCurse

src/vmm/src/devices/virtio/pmem/device.rs

@@ -254,8 +254,9 @@ impl Pmem {
         // This is safe since we checked in the event handler that the device is activated.
         let active_state = self.device_state.active_state().unwrap();
 
+        let mut cached_result = None;
         while let Some(head) = self.queues[0].pop()? {
-            let add_result = match self.process_chain(head) {
+            let add_result = match self.process_chain(head, &mut cached_result) {
                 Ok(()) => self.queues[0].add_used(head.index, 4),
                 Err(err) => {
                     error!("pmem: {err}");
@@ -283,7 +284,11 @@ impl Pmem {
         Ok(())
     }
 
-    fn process_chain(&self, head: DescriptorChain) -> Result<(), PmemError> {
+    fn process_chain(
+        &self,
+        head: DescriptorChain,
+        cached_result: &mut Option<i32>,
+    ) -> Result<(), PmemError> {
         // This is safe since we checked in the event handler that the device is activated.
         let active_state = self.device_state.active_state().unwrap();
 
@@ -310,21 +315,29 @@ impl Pmem {
             return Err(PmemError::Non4byteStatusDescriptor(status_descriptor.len));
         }
 
-        let mut result = SUCCESS;
-        // SAFETY: We are calling the system call with valid arguments and checking the returned
-        // value
-        unsafe {
-            let ret = libc::msync(
-                self.mmap_ptr as *mut libc::c_void,
-                u64_to_usize(self.file_len),
-                libc::MS_SYNC,
-            );
-            if ret < 0 {
-                error!("pmem: Unable to msync the file. Error: {}", ret);
-                result = FAILURE;
+        if let Some(result) = cached_result {
+            active_state
+                .mem
+                .write_obj(*result, status_descriptor.addr)?;
+        } else {
+            let mut status = SUCCESS;
+            // SAFETY: We are calling the system call with valid arguments and checking the returned
+            // value
+            unsafe {
+                let ret = libc::msync(
+                    self.mmap_ptr as *mut libc::c_void,
+                    u64_to_usize(self.file_len),
+                    libc::MS_SYNC,
+                );
+                if ret < 0 {
+                    error!("pmem: Unable to msync the file. Error: {}", ret);
+                    status = FAILURE;
+                }
             }
+            *cached_result = Some(status);
+
+            active_state.mem.write_obj(status, status_descriptor.addr)?;
         }
-        active_state.mem.write_obj(result, status_descriptor.addr)?;
         Ok(())
     }
 
@@ -499,8 +512,10 @@ mod tests {
             vq.used.idx.set(0);
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
-            pmem.process_chain(head).unwrap();
+            let mut result = None;
+            pmem.process_chain(head, &mut result).unwrap();
             assert_eq!(mem.read_obj::<u32>(GuestAddress(0x2000)).unwrap(), 0);
+            assert!(result.is_some());
         }
 
         // Invalid request type
@@ -514,7 +529,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::UnknownRequestType(0x69),
             ));
         }
@@ -530,7 +545,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::DescriptorChainTooShort,
             ));
         }
@@ -548,7 +563,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::WriteOnlyDescriptor,
             ));
         }
@@ -566,7 +581,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::ReadOnlyDescriptor,
             ));
         }
@@ -582,7 +597,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::Non4byteHeadDescriptor(0x69),
             ));
         }
@@ -601,7 +616,7 @@ mod tests {
             vq.avail.idx.set(1);
             let head = pmem.queues[0].pop().unwrap().unwrap();
             assert!(matches!(
-                pmem.process_chain(head).unwrap_err(),
+                pmem.process_chain(head, &mut None).unwrap_err(),
                 PmemError::Non4byteStatusDescriptor(0x69),
             ));
         }