From 69cf814b34c68c98c803bbfb36c6b68537061661 Mon Sep 17 00:00:00 2001
From: Sandwich <sandwich@archworks.co>
Date: Sat, 10 Oct 2026 02:29:34 +0200
Subject: [PATCH] fix: cpu-isolation hook restores prior allowedcpus instead of
 all cores

---
 scripts/libvirt-hook-cpu-isolation.sh | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/scripts/libvirt-hook-cpu-isolation.sh b/scripts/libvirt-hook-cpu-isolation.sh
index 02a6083..54c06db 100755
--- a/scripts/libvirt-hook-cpu-isolation.sh
+++ b/scripts/libvirt-hook-cpu-isolation.sh
@@ -5,7 +5,8 @@
 #
 # On start it reads the domain's own <cputune> from the XML libvirt passes on
 # stdin, works out which host CPUs the guest occupies, and confines the systemd
-# slices to the rest. On stop it hands everything back.
+# slices to the rest. On stop it puts back what the slices had before, so a
+# partition someone else set survives a guest's lifetime.
 #
 # Measured on a 16 vCPU guest pinned to one CCD of a 7950X: stalls over 10 us in
 # a 10 second window fell from ~100 to ~52, and CPU throughput rose about 2%.
@@ -19,12 +20,7 @@ OPERATION="$2"
 ALL=$(cat /sys/devices/system/cpu/present)      # not nproc: that honours this hook's own affinity
 NCPU=$(( ${ALL##*-} + 1 ))
 SLICES="system.slice user.slice init.scope"
-
-restore() {
-    for s in $SLICES; do
-        systemctl set-property --runtime -- "$s" AllowedCPUs="$ALL" 2>/dev/null
-    done
-}
+STATE="/run/vfio-native/cpu-isolation/$DOMAIN"   # each slice's AllowedCPUs before we confined it
 
 case "$OPERATION" in
 prepare)
@@ -62,6 +58,10 @@ PY
 )
     [ -n "$HOST" ] || exit 0         # nothing left for the host, or it failed
 
+    mkdir -p "${STATE%/*}"
+    [ -f "$STATE" ] || for s in $SLICES; do
+        printf '%s=%s\n' "$s" "$(systemctl show -p AllowedCPUs --value -- "$s")"
+    done > "$STATE"
     for s in $SLICES; do
         systemctl set-property --runtime -- "$s" AllowedCPUs="$HOST" 2>/dev/null
     done
@@ -69,8 +69,12 @@ PY
     ;;
 
 release|stopped)
-    restore
-    logger -t libvirt-cpu-isolation "$DOMAIN $OPERATION: host slices restored to $ALL"
+    [ -f "$STATE" ] || exit 0        # this hook confined nothing for the domain
+    while IFS='=' read -r s cpus; do
+        systemctl set-property --runtime -- "$s" AllowedCPUs="$cpus" 2>/dev/null
+    done < "$STATE"
+    logger -t libvirt-cpu-isolation "$DOMAIN $OPERATION: host slices restored to $(tr '\n' ' ' < "$STATE")"
+    rm -f "$STATE"
     ;;
 *)
     # Deliberately does nothing. An earlier version called virsh here to check