From 18d615485679b698fe3b496ad9fe817eb220579e Mon Sep 17 00:00:00 2001
From: Philipp Schuster <philipp.schuster@cyberus-technology.de>
Date: Wed, 1 Jul 2026 15:12:26 +0200
Subject: [PATCH] vmm, vm-migration: streamline error chain message with
 upstream

Streamlines the printing of the error message with upstream [0, 1].

[0] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/8469/changes/fcf2b5bbe6afeebb17dba5bace72a6896531be5e
[1] https://github.com/cloud-hypervisor/cloud-hypervisor/commit/252702049e55cc2903a07fd24e6a165a33c54026

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
---
 vm-migration/src/lib.rs      | 18 +++++++++++++++++
 vm-migration/src/progress.rs |  4 +++-
 vmm/src/lib.rs               | 38 +++++-------------------------------
 3 files changed, 26 insertions(+), 34 deletions(-)

diff --git a/vm-migration/src/lib.rs b/vm-migration/src/lib.rs
index 60b1a47496..1941e6f830 100644
--- a/vm-migration/src/lib.rs
+++ b/vm-migration/src/lib.rs
@@ -3,6 +3,9 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 //
 
+use std::error::Error;
+use std::iter;
+
 use anyhow::anyhow;
 pub use context::{
     CompletedMigrationContext, DowntimeContext, MemoryMigrationContext, MigrationContextError,
@@ -20,6 +23,21 @@ pub mod progress;
 pub mod protocol;
 pub mod tls;
 
+/// Mimics the error chain printing of CH for migration-related errors, where we
+/// do not exit the VMM (which would print the error chain).
+pub fn nested_error_to_flat_chain_as_string(top_error: &dyn Error) -> String {
+    iter::successors(Some(top_error), |sub_error| {
+        // Dereference necessary to mitigate rustc compiler bug.
+        // See <https://github.com/rust-lang/rust/issues/141673>
+        (*sub_error).source()
+    })
+    // Important to use the plain Display impl to not interfere
+    // with anyhow's "smart" printing
+    .map(|e| format!("{e}"))
+    .collect::<Vec<_>>()
+    .join(" => ")
+}
+
 #[derive(Error, Debug)]
 pub enum UffdError {
     #[error("Snapshot ranges are not page-aligned")]
diff --git a/vm-migration/src/progress.rs b/vm-migration/src/progress.rs
index c5babdad92..dc2f358b82 100644
--- a/vm-migration/src/progress.rs
+++ b/vm-migration/src/progress.rs
@@ -22,6 +22,8 @@ use std::fmt::Display;
 use std::num::NonZeroU32;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
+use crate::nested_error_to_flat_chain_as_string;
+
 #[derive(
     Clone, Debug, PartialOrd, Ord, PartialEq, Eq, Hash, serde::Serialize, serde::Deserialize,
 )]
@@ -329,7 +331,7 @@ impl MigrationProgress {
         self.timestamp_snapshot_ms = current_unix_timestamp_ms();
         self.timestamp_snapshot_relative_ms = self.timestamp_snapshot_ms - self.timestamp_begin_ms;
         self.state = MigrationState::Failed {
-            error_msg: format!("{error}",),
+            error_msg: nested_error_to_flat_chain_as_string(error),
             error_msg_debug: format!("{error:?}",),
         };
     }
diff --git a/vmm/src/lib.rs b/vmm/src/lib.rs
index 0bb2f65301..07454a0b18 100644
--- a/vmm/src/lib.rs
+++ b/vmm/src/lib.rs
@@ -56,7 +56,7 @@ use vm_migration::progress::{
 use vm_migration::protocol::*;
 use vm_migration::{
     MemoryMigrationContext, Migratable, MigratableError, OngoingMigrationContext, Pausable,
-    Snapshot, Snapshottable, Transportable,
+    Snapshot, Snapshottable, Transportable, nested_error_to_flat_chain_as_string,
 };
 use vmm_sys_util::eventfd::EventFd;
 use vmm_sys_util::signal::unblock_signal;
@@ -2213,37 +2213,6 @@ impl Vmm {
         self.vm.vm_mut().unwrap().restore()
     }
 
-    /// Prints the error chain to `error!()` level, akin to user-facing errors when Cloud Hypervisor
-    /// or ch-remote fail.
-    // TODO: For upstreaming, we should unify this with the code-paths used by ch-remote and
-    // Cloud Hypervisor on failure.
-    fn log_print_error_chain<'a>(top_error: &'a (dyn std::error::Error + 'static)) {
-        // Print chain of errors
-        if top_error.source().is_none() {
-            error!("Migration failed with the following error:");
-            error!("  {top_error}");
-        } else {
-            // In cli_print_error_chain(), we also print the
-            // <top_err as Debug>::fmt() as oneliner so that we can see all
-            // properties. As we use anyhow errors in the migration path,
-            // Debug::fmt() is not helpful for us as it doesn't print the
-            // underlying properties (like the default Debug::fmt() impl would
-            // do). Instead, it would print a trace itself, which is not what
-            // we want to do here.
-
-            error!("Migration failed with the following chain of errors:");
-            std::iter::successors(Some(top_error), |sub_error| {
-                // Dereference necessary to mitigate rustc compiler bug.
-                // See <https://github.com/rust-lang/rust/issues/141673>
-                (*sub_error).source()
-            })
-            .enumerate()
-            .for_each(|(level, error)| {
-                error!("  {level}: {error}");
-            });
-        }
-    }
-
     /// Checks the migration result.
     ///
     /// This should be called when the migration thread indicated a state
@@ -2345,7 +2314,10 @@ impl Vmm {
                 }
             }
             Err(e) => {
-                Self::log_print_error_chain(&e);
+                error!(
+                    "Migration failed: {}",
+                    nested_error_to_flat_chain_as_string(&e)
+                );
                 try_resume_vm(vm);
 
                 // Update migration progress snapshot