diff --git a/xCAT-test/unit/diskless_flat_vmothersetting_machine.t b/xCAT-test/unit/diskless_flat_vmothersetting_machine.t
new file mode 100644
index 000000000..7269980ec
--- /dev/null
+++ b/xCAT-test/unit/diskless_flat_vmothersetting_machine.t
@@ -0,0 +1,74 @@
+#!/usr/bin/perl
+# reg_linux_diskless_installation_flat corrupts the KVM machine type of the compute node, checks
+# that the node fails to boot, and then restores it. Both the restore and the check after it read a
+# machine type chosen by architecture. The ladder that chooses it names ppc64 and x86_64 only, so on
+# any other architecture str3 stays empty: the restore writes an EMPTY vmothersetting, and the check
+# that follows -- "vmothersetting contains machine" -- fails. The compute node has provisioned
+# correctly by then, so the case reports a red cell for a gap in its own arithmetic.
+#
+# The two commands are lifted out of the case file and RUN, with lsdef and chdef shadowed, so the
+# assertions read the value the case would write. BAIL_OUT when an extraction stops matching, so a
+# rewrite fails loudly instead of covering nothing.
+use strict;
+use warnings;
+use Test::More;
+use File::Basename qw(dirname);
+
+my $case = dirname(__FILE__)
+    . '/../autotest/testcase/installation/reg_linux_diskless_installation_flat';
+open my $fh, '<', $case or BAIL_OUT("cannot read $case: $!");
+my @lines = <$fh>;
+close $fh;
+
+# The command that restores the machine type, and the one that removes it again afterwards.
+my ($restore) = grep { /^cmd:.*str2="machine:invalid".*chdef \$\$CN vmothersetting=\$str5/ } @lines;
+my ($remove)  = grep { /^cmd:.*str2=";".*=~ "ppc64".*chdef \$\$CN vmothersetting=/ } @lines;
+BAIL_OUT('cannot find the command that restores the machine type') unless $restore;
+BAIL_OUT('cannot find the command that removes the machine type')  unless $remove;
+
+# Render one command the way xcattest does, then run it with lsdef and chdef shadowed. bash
+# resolves a function ahead of PATH, so the case's own backticks read the stub.
+my $run = sub {
+    my ($cmd, $arch, $lsdef_value) = @_;
+    $cmd =~ s/^cmd://;
+    $cmd =~ s/__GETNODEATTR\(\$\$CN,arch\)__/$arch/g;
+    $cmd =~ s/__GETNODEATTR\(\$\$CN,mgt\)__/kvm/g;
+    $cmd =~ s/\$\$CN/cn1/g;
+    my $prelude = "lsdef() { echo '    vmothersetting=$lsdef_value'; }\n"
+                . "chdef() { echo \"CHDEF:\$*\"; }\n";
+    my $out = qx{bash -c @{[ quotemeta_cmd($prelude . $cmd) ]} 2>&1};
+    my ($written) = $out =~ /^CHDEF:cn1 vmothersetting=(.*)$/m;
+    return { out => $out, written => $written, rc => $? >> 8 };
+};
+
+sub quotemeta_cmd { my ($s) = @_; $s =~ s/'/'\\''/g; return "'$s'" }
+
+# The machine type each architecture must end up with. riscv64 guests run the qemu "virt" machine;
+# kvm.pm sets it in guest_arch_profile.
+my %machine = (
+    ppc64le => 'pseries',
+    x86_64  => 'pc',
+    riscv64 => 'virt',
+);
+
+for my $arch (sort keys %machine) {
+    my $r = $run->($restore, $arch, 'machine:invalid');
+    ok( defined $r->{written} && length $r->{written},
+        "$arch: the restore writes a vmothersetting rather than an empty one" );
+    like( $r->{written} // '', qr/\bmachine:/,
+        "$arch: the restored vmothersetting names a machine type" );
+    like( $r->{written} // '', qr/\Q$machine{$arch}\E/,
+        "$arch: the restored machine type is $machine{$arch}" );
+
+    # The check the case runs straight after the restore.
+    my $keeps_machine = ( $r->{written} // '' ) =~ /machine/ ? 0 : 1;
+    is( $keeps_machine, 0,
+        "$arch: the check after the restore, 'vmothersetting contains machine', passes" );
+
+    # The remove path reads the same ladder; it must not die on an empty str3.
+    my $d = $run->($remove, $arch, "machine:$machine{$arch}");
+    unlike( $d->{out}, qr/unary operator expected/,
+        "$arch: the remove path compares two defined strings" );
+}
+
+done_testing();