From a69d8c63b34c87e1227e1bab6601fbd209152c50 Mon Sep 17 00:00:00 2001
From: Yi Wang <foxywang@tencent.com>
Date: Tue, 14 Nov 2023 15:41:18 +0800
Subject: [PATCH] vmm: speed up JSON load when reading snap files

We found that it's slow to load JSON when reading snap files. As
described in [1], using from_slice instead of from_reader can fix
this.

Also, fix the error type being returned.

1. https://github.com/serde-rs/json/issues/160

Signed-off-by: Yi Wang <foxywang@tencent.com>
---
 vmm/src/migration.rs | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/vmm/src/migration.rs b/vmm/src/migration.rs
index 1906d69fd..7e7b53770 100644
--- a/vmm/src/migration.rs
+++ b/vmm/src/migration.rs
@@ -7,7 +7,7 @@ use crate::coredump::GuestDebuggableError;
 use crate::{config::VmConfig, vm::VmSnapshot};
 use anyhow::anyhow;
 use std::fs::File;
-use std::io::BufReader;
+use std::io::Read;
 use std::path::PathBuf;
 use vm_migration::{MigratableError, Snapshot};
 
@@ -49,10 +49,14 @@ pub fn recv_vm_config(source_url: &str) -> std::result::Result<VmConfig, Migrata
     vm_config_path.push(SNAPSHOT_CONFIG_FILE);
 
     // Try opening the snapshot file
-    let vm_config_file =
-        File::open(vm_config_path).map_err(|e| MigratableError::MigrateSend(e.into()))?;
-    let vm_config_reader = BufReader::new(vm_config_file);
-    serde_json::from_reader(vm_config_reader).map_err(|e| MigratableError::MigrateReceive(e.into()))
+    let mut vm_config_file =
+        File::open(vm_config_path).map_err(|e| MigratableError::MigrateReceive(e.into()))?;
+    let mut bytes = Vec::new();
+    vm_config_file
+        .read_to_end(&mut bytes)
+        .map_err(|e| MigratableError::MigrateReceive(e.into()))?;
+
+    serde_json::from_slice(&bytes).map_err(|e| MigratableError::MigrateReceive(e.into()))
 }
 
 pub fn recv_vm_state(source_url: &str) -> std::result::Result<Snapshot, MigratableError> {
@@ -61,10 +65,14 @@ pub fn recv_vm_state(source_url: &str) -> std::result::Result<Snapshot, Migratab
     vm_state_path.push(SNAPSHOT_STATE_FILE);
 
     // Try opening the snapshot file
-    let vm_state_file =
-        File::open(vm_state_path).map_err(|e| MigratableError::MigrateSend(e.into()))?;
-    let vm_state_reader = BufReader::new(vm_state_file);
-    serde_json::from_reader(vm_state_reader).map_err(|e| MigratableError::MigrateReceive(e.into()))
+    let mut vm_state_file =
+        File::open(vm_state_path).map_err(|e| MigratableError::MigrateReceive(e.into()))?;
+    let mut bytes = Vec::new();
+    vm_state_file
+        .read_to_end(&mut bytes)
+        .map_err(|e| MigratableError::MigrateReceive(e.into()))?;
+
+    serde_json::from_slice(&bytes).map_err(|e| MigratableError::MigrateReceive(e.into()))
 }
 
 pub fn get_vm_snapshot(snapshot: &Snapshot) -> std::result::Result<VmSnapshot, MigratableError> {