OVMF: enable "cargo xtest run" under NixOS

Author: phip1611

.editorconfig

@@ -12,6 +12,6 @@ trim_trailing_whitespace = true
 indent_style = space
 indent_size = 4
 
-[*.{md,json}]
+[*.{json,md,nix,yml}]
 indent_style = space
 indent_size = 2

nix/nixpkgs.nix

@@ -0,0 +1,13 @@
+# Pinned nixpkgs version.
+
+let
+  # Picked a recent commit from the nixos-22.11-small branch.
+  # https://github.com/NixOS/nixpkgs/tree/nixos-22.11-small
+  #
+  # When you change this, also change the sha256 hash!
+  rev = "a45745ac9e4e1eb86397ab22e2a8823120ab9a4c";
+in
+builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+  sha256 = "sha256:1acllp8yxp1rwncxsxnxl9cwkm97wxfnd6ryclmvll3sa39j9b1z";
+}

shell.nix

@@ -0,0 +1,16 @@
+let
+  pkgsSrc = import ./nix/nixpkgs.nix;
+  pkgs = import pkgsSrc {};
+in
+pkgs.mkShell rec {
+  # CLI Utilities
+  nativeBuildInputs = with pkgs; [
+  ];
+
+  # Header Files, Runtime Dependencies
+  buildInputs = with pkgs; [
+  ];
+
+  # Set ENV var "OVMF"
+  OVMF="${pkgs.OVMF.fd}";
+}

xtask/Cargo.toml

@@ -13,6 +13,7 @@ heck = "0.4.0"
 itertools = "0.10.5"
 mbrman = "0.5.1"
 nix = "0.26.1"
+os_info = { version = "3.6.0", default-features = false }
 proc-macro2 = "1.0.46"
 quote = "1.0.21"
 regex = "1.5.4"

xtask/src/qemu.rs

@@ -14,6 +14,8 @@ use std::io::{BufRead, BufReader, Read, Write};
 use std::path::{Path, PathBuf};
 use std::process::{Child, Command, Stdio};
 use tempfile::TempDir;
+#[cfg(target_os = "linux")]
+use {std::fs::Permissions, std::os::unix::fs::PermissionsExt};
 
 #[derive(Clone, Copy, Debug)]
 enum OvmfFileType {
@@ -127,6 +129,34 @@ impl OvmfPaths {
         }
     }
 
+    /// Get the NixOS OVMF paths for the given guest arch.
+    fn nixos(arch: UefiArch) -> Option<Self> {
+        let os_info = os_info::get();
+        if os_info.os_type() != os_info::Type::NixOS {
+            return None;
+        }
+        let path = std::env::var_os("OVMF").expect("Must have OVMF env var. Run '$ nix-shell'");
+        let path = path.to_str().unwrap();
+        let paths = match arch {
+            // Package "edk2-aarch64".
+            UefiArch::AArch64 => Self {
+                code: format!("{path}/FV/AAVMF_CODE.fd").into(),
+                vars: format!("{path}/FV/AAVMF_VARS.fd").into(),
+            },
+            // Package "edk2-ovmf-ia32".
+            UefiArch::IA32 => Self {
+                code: format!("{path}/FV/OVMF_CODE.fd").into(),
+                vars: format!("{path}/FV/OVMF_VARS.fd").into(),
+            },
+            // Package "edk2-ovmf".
+            UefiArch::X86_64 => Self {
+                code: format!("{path}/FV/OVMF_CODE.fd").into(),
+                vars: format!("{path}/FV/OVMF_VARS.fd").into(),
+            },
+        };
+        Some(paths)
+    }
+
     /// Get the Windows OVMF paths for the given guest arch.
     fn windows(arch: UefiArch) -> Self {
         match arch {
@@ -157,6 +187,9 @@ impl OvmfPaths {
             }
             candidates.push(Self::debian_linux(arch));
             candidates.push(Self::fedora_linux(arch));
+            if let Some(paths) = Self::nixos(arch) {
+                candidates.push(paths);
+            }
         }
         if platform::is_windows() {
             candidates.push(Self::windows(arch));
@@ -497,6 +530,10 @@ pub fn run_qemu(arch: UefiArch, opt: &QemuOpt) -> Result<()> {
     // versions of OVMF won't boot if the vars file isn't writeable.
     let ovmf_vars = tmp_dir.join("ovmf_vars");
     fs_err::copy(&ovmf_paths.vars, &ovmf_vars)?;
+    // Necessary, as for example on NixOS, the files are read-only inside
+    // the Nix store.
+    #[cfg(target_os = "linux")]
+    fs_err::set_permissions(&ovmf_vars, Permissions::from_mode(0o666))?;
 
     add_pflash_args(&mut cmd, &ovmf_paths.code, PflashMode::ReadOnly);
     add_pflash_args(&mut cmd, &ovmf_vars, PflashMode::ReadWrite);