diff --git a/bin/propolis-server/src/lib/initializer.rs b/bin/propolis-server/src/lib/initializer.rs
index 2b6107b18..c0fd10b49 100644
--- a/bin/propolis-server/src/lib/initializer.rs
+++ b/bin/propolis-server/src/lib/initializer.rs
@@ -1041,26 +1041,38 @@ impl<'a> MachineInitializer<'a> {
                 match &spec.device_spec {
                     StorageDevice::Virtio(disk) => {
                         let bdf = parse_bdf(&disk.pci_path)?;
-                        // TODO: check that bus is 0. only support boot devices
-                        // directly attached to the root bus for now.
+                        if bdf.bus.get() != 0 {
+                            return Err(Error::new(
+                                ErrorKind::InvalidInput,
+                                "Boot device currently must be on PCI bus 0",
+                            ));
+                        }
+
                         order.add_disk(bdf.location);
                     }
                     StorageDevice::Nvme(disk) => {
                         let bdf = parse_bdf(&disk.pci_path)?;
-                        // TODO: check that bus is 0. only support boot devices
-                        // directly attached to the root bus for now.
-                        //
+                        if bdf.bus.get() != 0 {
+                            return Err(Error::new(
+                                ErrorKind::InvalidInput,
+                                "Boot device currently must be on PCI bus 0",
+                            ));
+                        }
+
                         // TODO: separately, propolis-standalone passes an eui64
                         // of 0, so do that here too. is that.. ok?
                         order.add_nvme(bdf.location, 0);
                     }
                 };
             } else {
+                // This should be unreachable - we check that the boot disk is
+                // valid when constructing the spec we're initializing from.
                 let message = format!(
-                    "Instance spec included boot entry which does not refer to an existing disk: `{}`",
+                    "Instance spec included boot entry which does not refer \
+                    to an existing disk: `{}`",
                     boot_entry.name.as_str(),
                 );
-                return Err(Error::new(ErrorKind::Other, message));
+                return Err(Error::new(ErrorKind::InvalidInput, message));
             }
         }
 
diff --git a/lib/propolis-client/src/instance_spec.rs b/lib/propolis-client/src/instance_spec.rs
index cb8bbc449..8c75bee98 100644
--- a/lib/propolis-client/src/instance_spec.rs
+++ b/lib/propolis-client/src/instance_spec.rs
@@ -186,14 +186,16 @@ impl SpecBuilderV0 {
 
     /// Sets a boot order. Names here refer to devices included in this spec.
     ///
-    /// Permissible to not this if the implicit boot order is desired, but the implicit boot order
-    /// may be unstable across device addition and removal.
+    /// Permissible to not set this if the implicit boot order is desired, but
+    /// the implicit boot order may be unstable across device addition and
+    /// removal.
     ///
-    /// If any devices named in this order are not actually present in the constructed spec,
-    /// Propolis will return an error when the spec is provided.
+    /// If any devices named in this order are not actually present in the
+    /// constructed spec, Propolis will return an error when the spec is
+    /// provided.
     ///
-    /// XXX: this should certainly return `&mut Self` - all the builders here should. check if any
-    /// of these are chained..?
+    /// XXX: this should certainly return `&mut Self` - all the builders here
+    /// should. check if any of these are chained..?
     pub fn set_boot_order(
         &mut self,
         boot_order: Vec<String>,
diff --git a/phd-tests/framework/src/test_vm/config.rs b/phd-tests/framework/src/test_vm/config.rs
index 2a4b42921..14d92fb32 100644
--- a/phd-tests/framework/src/test_vm/config.rs
+++ b/phd-tests/framework/src/test_vm/config.rs
@@ -132,10 +132,11 @@ impl<'dr> VmConfig<'dr> {
         self
     }
 
-    /// Add a new disk to the VM config, and add it to the front of the VM's boot order.
+    /// Add a new disk to the VM config, and add it to the front of the VM's
+    /// boot order.
     ///
-    /// The added disk will have the name `boot-disk`, and replace the previous existing
-    /// `boot-disk`.
+    /// The added disk will have the name `boot-disk`, and replace the previous
+    /// existing `boot-disk`.
     pub fn boot_disk(
         &mut self,
         artifact: &'dr str,
@@ -207,28 +208,37 @@ impl<'dr> VmConfig<'dr> {
             })
             .context("serializing Propolis server config")?;
 
-        // The first disk in the boot list might not be the disk a test *actually* expects to boot.
+        // The first disk in the boot list might not be the disk a test
+        // *actually* expects to boot.
         //
-        // If there are multiple bootable disks in the boot order, we'll assume they're all
-        // the same guest OS kind. So look for `boot-disk` - if there isn't a disk named
-        // `boot-disk` then fall back to hoping the first disk in the boot order is a bootable
-        // disk, and if *that* isn't a bootable disk, maybe the first disk is.
+        // If there are multiple bootable disks in the boot order, we'll assume
+        // they're all the same guest OS kind. So look for `boot-disk` - if
+        // there isn't a disk named `boot-disk` then fall back to hoping the
+        // first disk in the boot order is a bootable disk, and if *that* isn't
+        // a bootable disk, maybe the first disk is.
         //
-        // TODO: theoretically we might want to accept configuration of a specific guest OS adapter
-        // and avoid the guessing games. So far the above supports existing tests and makes them
-        // "Just Work", but a more complicated test may want more control here.
-        let boot_disk =
-            self.disks.iter().find(|d| d.name == "boot-disk")
-                .or_else(|| if let Some(boot_order) = self.boot_order.as_ref() {
-                    boot_order.first().and_then(|name| self.disks.iter().find(|d| &d.name == name))
+        // TODO: theoretically we might want to accept configuration of a
+        // specific guest OS adapter and avoid the guessing games. So far the
+        // above supports existing tests and makes them "Just Work", but a more
+        // complicated test may want more control here.
+        let boot_disk = self
+            .disks
+            .iter()
+            .find(|d| d.name == "boot-disk")
+            .or_else(|| {
+                if let Some(boot_order) = self.boot_order.as_ref() {
+                    boot_order.first().and_then(|name| {
+                        self.disks.iter().find(|d| &d.name == name)
+                    })
                 } else {
                     None
-                })
-                .or_else(|| self.disks.first())
-                .expect("VM config includes at least one disk (and maybe a boot order)?");
+                }
+            })
+            .or_else(|| self.disks.first())
+            .expect("VM config includes at least one disk");
 
-        // XXX: assuming all bootable images are equivalent to the first, or at least the same
-        // guest OS kind.
+        // XXX: assuming all bootable images are equivalent to the first, or at
+        // least the same guest OS kind.
         let DiskSource::Artifact(boot_artifact) = boot_disk.source else {
             unreachable!("boot disks always use artifacts as sources");
         };
@@ -242,7 +252,6 @@ impl<'dr> VmConfig<'dr> {
         let mut disk_handles = Vec::new();
         for disk in self.disks.iter() {
             disk_handles.push(
-                // format!("data-disk-{}", idx)
                 make_disk(disk.name.to_owned(), framework, disk)
                     .await
                     .context("creating disk")?,
diff --git a/phd-tests/tests/src/boot_order.rs b/phd-tests/tests/src/boot_order.rs
index d906abd7d..569a72458 100644
--- a/phd-tests/tests/src/boot_order.rs
+++ b/phd-tests/tests/src/boot_order.rs
@@ -353,11 +353,7 @@ async fn guest_can_adjust_boot_order(ctx: &Framework) {
     write_efivar(&vm, &bootvar(0xfff0), &boot_option_bytes).await?;
     let reread = read_efivar(&vm, &bootvar(0xfff0)).await?;
     if reread.is_empty() {
-        warn!(
-            "guest environment drops EFI variable writes! \
-            exiting test WITHOUT VALIDATING ANYTHING"
-        );
-        return Ok(());
+        phd_skip!("Guest environment drops EFI variable writes");
     } else {
         assert_eq!(
             boot_option_bytes,