KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 80 insertions, 0 deletions

diff --git a/Documentation/target/target-export-device b/Documentation/target/target-export-device
new file mode 100755
index 000000000..b803f4f88
--- /dev/null
+++ b/Documentation/target/target-export-device
@@ -0,0 +1,80 @@
+#!/bin/sh
+#
+# This script illustrates the sequence of operations in configfs to
+# create a very simple LIO iSCSI target with a file or block device
+# backstore.
+#
+# (C) Copyright 2014 Christophe Vu-Brugier <cvubrugier@fastmail.fm>
+#
+
+print_usage() {
+    cat <<EOF
+Usage: $(basename $0) [-p PORTAL] DEVICE|FILE
+Export a block device or a file as an iSCSI target with a single LUN
+EOF
+}
+
+die() {
+    echo $1
+    exit 1
+}
+
+while getopts "hp:" arg; do
+    case $arg in
+        h) print_usage; exit 0;;
+        p) PORTAL=${OPTARG};;
+    esac
+done
+shift $(($OPTIND - 1))
+
+DEVICE=$1
+[ -n "$DEVICE" ] || die "Missing device or file argument"
+[ -b $DEVICE -o -f $DEVICE ] || die "Invalid device or file: ${DEVICE}"
+IQN="iqn.2003-01.org.linux-iscsi.$(hostname):$(basename $DEVICE)"
+[ -n "$PORTAL" ] || PORTAL="0.0.0.0:3260"
+
+CONFIGFS=/sys/kernel/config
+CORE_DIR=$CONFIGFS/target/core
+ISCSI_DIR=$CONFIGFS/target/iscsi
+
+# Load the target modules and mount the config file system
+lsmod | grep -q configfs || modprobe configfs
+lsmod | grep -q target_core_mod || modprobe target_core_mod
+mount | grep -q ^configfs || mount -t configfs none $CONFIGFS
+mkdir -p $ISCSI_DIR
+
+# Create a backstore
+if [ -b $DEVICE ]; then
+    BACKSTORE_DIR=$CORE_DIR/iblock_0/data
+    mkdir -p $BACKSTORE_DIR
+    echo "udev_path=${DEVICE}" > $BACKSTORE_DIR/control
+else
+    BACKSTORE_DIR=$CORE_DIR/fileio_0/data
+    mkdir -p $BACKSTORE_DIR
+    DEVICE_SIZE=$(du -b $DEVICE | cut -f1)
+    echo "fd_dev_name=${DEVICE}" > $BACKSTORE_DIR/control
+    echo "fd_dev_size=${DEVICE_SIZE}" > $BACKSTORE_DIR/control
+    echo 1 > $BACKSTORE_DIR/attrib/emulate_write_cache
+fi
+echo 1 > $BACKSTORE_DIR/enable
+
+# Create an iSCSI target and a target portal group (TPG)
+mkdir $ISCSI_DIR/$IQN
+mkdir $ISCSI_DIR/$IQN/tpgt_1/
+
+# Create a LUN
+mkdir $ISCSI_DIR/$IQN/tpgt_1/lun/lun_0
+ln -s $BACKSTORE_DIR $ISCSI_DIR/$IQN/tpgt_1/lun/lun_0/data
+echo 1 > $ISCSI_DIR/$IQN/tpgt_1/enable
+
+# Create a network portal
+mkdir $ISCSI_DIR/$IQN/tpgt_1/np/$PORTAL
+
+# Disable authentication
+echo 0 > $ISCSI_DIR/$IQN/tpgt_1/attrib/authentication
+echo 1 > $ISCSI_DIR/$IQN/tpgt_1/attrib/generate_node_acls
+
+# Allow write access for non authenticated initiators
+echo 0 > $ISCSI_DIR/$IQN/tpgt_1/attrib/demo_mode_write_protect
+
+echo "Target ${IQN}, portal ${PORTAL} has been created"