create: Auto-detect image architecture

When combined with --emulated, this enables running VMs with an
architecture different from the host's.

Closes #62.

Signed-off-by: Alberto Faria <afaria@redhat.com>

docs/2-podman-docker.md

@@ -336,8 +336,9 @@ $ podman run \
 ### System emulation
 
 To use system emulation instead of hardware-assisted virtualization, specify the
-`--emulated` flag. Without this flag, attempting to create a VM on a host tbat
-doesn't support KVM will fail.
+`--emulated` flag. Without this flag, attempting to create a VM from a guest
+with a different architecture from the host's or on a host that doesn't support
+KVM will fail.
 
 It's not currently possible to use this flag when the container image is a bootc
 bootable container.

src/commands/create/domain.rs

@@ -58,11 +58,13 @@ fn generate(
         st(w, "memory", &[("unit", "b")], memory.as_str())?;
 
         s(w, "os", &[("firmware", "efi")], |w| {
-            let attrs = match ["x86", "x86_64"].contains(&env::consts::ARCH) {
-                true => [("machine", "q35")].as_slice(),
-                false => [].as_slice(), // use libvirt's default
-            };
-            st(w, "type", attrs, "hvm")?;
+            let guest_arch = vm_image_info.arch.as_deref().unwrap_or(env::consts::ARCH);
+
+            let mut attrs = vec![("arch", guest_arch)];
+            if ["x86", "x86_64"].contains(&guest_arch) {
+                attrs.push(("machine", "q35"));
+            }
+            st(w, "type", &attrs, "hvm")?;
 
             s(w, "firmware", &[], |w| {
                 se(w, "feature", &[("enabled", "no"), ("name", "secure-boot")])

src/commands/create/mod.rs

@@ -290,6 +290,7 @@ fn set_up_vm_image(
         // the image will be generated later
         return Ok(VmImageInfo {
             path: mirror_vm_image_path_in_container,
+            arch: None,
             size: 0,
             format: "qcow2".to_string(),
         });
@@ -315,7 +316,7 @@ fn set_up_vm_image(
     fs::create_dir_all(&image_dir_path)?;
 
     if !image_dir_path.join("image").try_exists()? {
-        fs::hard_link(vm_image_path_in_host, image_dir_path.join("image"))?;
+        fs::hard_link(&vm_image_path_in_host, image_dir_path.join("image"))?;
     }
 
     if custom_options.persistent {
@@ -338,7 +339,8 @@ fn set_up_vm_image(
 
         bind_mount_file(&mirror_vm_image_path_in_host, &mirror_vm_image_path_in_host)?;
 
-        let mut vm_image_info = VmImageInfo::of(&mirror_vm_image_path_in_host)?;
+        let mut vm_image_info =
+            VmImageInfo::of(&mirror_vm_image_path_in_host, custom_options.emulated)?;
         vm_image_info.path = mirror_vm_image_path_in_container;
 
         Ok(vm_image_info)
@@ -366,7 +368,8 @@ fn set_up_vm_image(
         let overlay_vm_image_path_in_container =
             Utf8Path::new("/").join(overlay_vm_image_path_in_container);
 
-        let mut base_vm_image_info = VmImageInfo::of(&mirror_vm_image_path_in_host)?;
+        let mut base_vm_image_info =
+            VmImageInfo::of(&vm_image_path_in_host, custom_options.emulated)?;
         base_vm_image_info.path = mirror_vm_image_path_in_container;
 
         if is_first_create {
@@ -375,6 +378,7 @@ fn set_up_vm_image(
 
         Ok(VmImageInfo {
             path: Utf8Path::new("/").join(overlay_vm_image_path_in_container),
+            arch: base_vm_image_info.arch,
             size: base_vm_image_info.size,
             format: "qcow2".to_string(),
         })

src/util.rs

@@ -1,8 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+use std::env;
 use std::ffi::{c_char, CString, OsStr};
 use std::fs::{self, OpenOptions, Permissions};
-use std::io::{self, ErrorKind};
+use std::io::{self, ErrorKind, Write};
 use std::os::unix::ffi::OsStrExt;
 use std::os::unix::fs::{MetadataExt, PermissionsExt};
 use std::process::{Command, Stdio};
@@ -336,6 +337,9 @@ pub struct VmImageInfo {
     #[serde(skip)]
     pub path: Utf8PathBuf,
 
+    #[serde(skip)]
+    pub arch: Option<String>,
+
     #[serde(rename = "virtual-size")]
     pub size: u64,
 
@@ -343,7 +347,7 @@ pub struct VmImageInfo {
 }
 
 impl VmImageInfo {
-    pub fn of(vm_image_path: impl AsRef<Utf8Path>) -> Result<VmImageInfo> {
+    pub fn of(vm_image_path: impl AsRef<Utf8Path>, identify_arch: bool) -> Result<VmImageInfo> {
         let vm_image_path = vm_image_path.as_ref().to_path_buf();
 
         let output = Command::new("qemu-img")
@@ -362,6 +366,12 @@ impl VmImageInfo {
         let mut info: VmImageInfo = serde_json::from_slice(&output.stdout)?;
         info.path = vm_image_path;
 
+        if identify_arch {
+            info.arch = identify_image_arch(&info.path)?
+                .ok_or_else(|| anyhow!("Could not identify VM image architecture"))?
+                .into();
+        }
+
         Ok(info)
     }
 }
@@ -393,6 +403,62 @@ pub fn create_overlay_vm_image(
     Ok(())
 }
 
+pub fn identify_image_arch(image_path: impl AsRef<Utf8Path>) -> Result<Option<String>> {
+    let xml = virt_inspector([
+        "--add",
+        image_path.as_ref().as_str(),
+        "--no-applications",
+        "--no-icon",
+    ])?;
+
+    xpath(&xml, "string(//arch)")
+}
+
+fn virt_inspector(args: impl IntoIterator<Item = impl AsRef<OsStr>>) -> Result<String> {
+    let cache_dir = format!("/var/tmp/crun-vm-{}", env::var("_CONTAINERS_ROOTLESS_UID")?);
+    fs::create_dir_all(&cache_dir)?;
+
+    let output = Command::new("virt-inspector")
+        .args(args)
+        .env("LIBGUESTFS_BACKEND", "direct")
+        .env("LIBGUESTFS_CACHEDIR", &cache_dir)
+        .output()?;
+
+    ensure!(
+        output.status.success(),
+        "virt-inspector failed: {}",
+        String::from_utf8_lossy(&output.stderr),
+    );
+
+    Ok(String::from_utf8(output.stdout)?)
+}
+
+fn xpath(xml: &str, path: &str) -> Result<Option<String>> {
+    let mut child = Command::new("virt-inspector")
+        .arg("--xpath")
+        .arg(path)
+        .stdin(Stdio::piped())
+        .spawn()?;
+
+    child.stdin.take().unwrap().write_all(xml.as_bytes())?;
+
+    let output = child.wait_with_output()?;
+
+    ensure!(
+        output.status.success(),
+        "virt-inspector --xpath failed: {}",
+        String::from_utf8_lossy(&output.stderr),
+    );
+
+    let result = String::from_utf8(output.stdout)?;
+
+    if result.is_empty() {
+        Ok(None)
+    } else {
+        Ok(Some(result))
+    }
+}
+
 /// Run `crun`.
 ///
 /// `crun` will inherit this process' standard streams.