Move vmlinuz out of registry dir.

src/qemu.rs

@@ -1,13 +1,4 @@
-//! QEMU: boot a registry entry with qemu-system-x86_64 using direct kernel
-//! boot (-kernel/-initrd or -drive). The kernel is shared across all VMs
-//! at $XDG_DATA_HOME/boco/registry/vmlinuz. A universal /boco/init script (injected
-//! at build time) handles VM bootstrap; /boco/exec runs the container's
-//! CMD/ENTRYPOINT. Runtime overrides are delivered as `boco.cmd=<base64>`
-//! on the kernel cmdline. Host directories are shared into the guest via
-//! virtio-fs: one virtiofsd daemon (under `podman unshare` for rootless
-//! UID mapping) per `--mount`, plus a `-chardev socket` + `-device
-//! vhost-user-fs-pci` pair on the QEMU side. The guest mounts each share
-//! with `mount -t virtiofs <tag> <dest>` (see `boco.mount=` cmdline).
+//! See [`boot`]
 
 use anyhow::{Context, Result, bail};
 use base64::Engine;
@@ -16,7 +7,7 @@ use std::{fmt::Write as _, process::Command};
 
 use crate::{
     forward::PortForward,
-    registry::{registry_base_dir, registry_dir},
+    registry::{registry_dir, vmlinuz_path},
     virtiofs::{self, VirtiofsShare},
 };
 
@@ -46,6 +37,10 @@ pub struct BootCmd {
     mount: Vec<String>,
 }
 
+/// Invoke `qemu` to boot a boco image. See [`BootCmd`] for options
+///
+/// This uses qemu's direct-kernel-boot with `-kernel`, so it does not require a bootloader.
+/// The boco image is just an ext4 rootfs.
 pub(crate) fn boot(
     BootCmd {
         name,
@@ -56,13 +51,13 @@ pub(crate) fn boot(
     }: BootCmd,
 ) -> Result<()> {
     let reg_dir = registry_dir(&name)?;
-    let vmlinuz_path = registry_base_dir()?.join("vmlinuz");
+    let vmlinuz_path = vmlinuz_path()?;
     let initrd_path = reg_dir.join("initrd");
     let qcow2_path = reg_dir.join("image.qcow2");
     if !vmlinuz_path.exists() {
         // TODO: guide user in how to set up a kernel
         bail!(
-            "No kernel available. Place a vmlinuz at {}",
+            "No kernel available. Expected vmlinuz at {}",
             vmlinuz_path.display()
         );
     }

src/registry.rs

@@ -26,6 +26,12 @@ pub(crate) fn validate_image_name(name: &str) -> Result<()> {
     Ok(())
 }
 
+pub(crate) fn vmlinuz_path() -> Result<PathBuf> {
+    let base = xdg::BaseDirectories::with_prefix("boco");
+    base.place_data_file("vmlinuz")
+        .context("Failed to create XDG_DATA_HOME subdirectory")
+}
+
 pub(crate) fn registry_base_dir() -> Result<PathBuf> {
     let base = xdg::BaseDirectories::with_prefix("boco");
     base.create_data_directory("registry")