pvh: Control PVH capabilities using optional feature

Enable the optional "pvh" feature to build a Firecracker binary
that is able to boot Linux guests using the PVH boot protocol.
Build a firecracker binary with the PVH capability by running:

tools/devtool build [--release] -- --features pvh

and the resulting Firecracker binary will use the PVH boot
protocol as the preferred choice for booting a guest, if the
guest kernel supports it.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Acked-by: Liam Merwick <liam.merwick@oracle.com>

src/firecracker/Cargo.toml

@@ -15,3 +15,6 @@ polly = { path = "../polly" }
 seccomp = { path = "../seccomp" }
 utils = { path = "../utils" }
 vmm = { path = "../vmm" }
+
+[features]
+pvh = ["vmm/pvh"]

src/vmm/Cargo.toml

@@ -29,3 +29,6 @@ cpuid = { path = "../cpuid" }
 
 [dev-dependencies]
 vmm-sys-util = ">=0.4.0"
+
+[features]
+pvh = []

tools/devtool

@@ -475,6 +475,28 @@ cmd_build() {
 
     [ $profile = "release" ] && cargo_args+=("--release")
 
+    # Parse any optional features passed to cargo.
+    # Currently, cargo does not support specifying features in a workspace, so passing
+    # the "--features opt-feature" option to cargo does not enable the optional feature
+    # in the crates that are part of the workspace, as one would expect. In order to
+    # build the individual crates with non-default features, we can use the cargo build
+    # option "--manifest-path", to direct cargo to use the Cargo.toml file that defines
+    # the desired feature. The downside is that other independent crates in the workspace
+    # (i.e. jailer) will not be built.
+    while [ $# -gt 0 ]; do
+        case "$1" in
+            "--features")
+                shift
+                [[ "$1" =~ ^--* ]] && \
+                    die "Must specify a feature name: \"--features <opt_feature>\"."
+                say_warn "WARNING: Building Firecracker with experimental feature." \
+                    "The jailer binary will not be built."
+                cargo_args+=("--manifest-path" "src/firecracker/Cargo.toml")
+                ;;
+        esac
+        shift
+    done
+
     # Run the cargo build process inside the container.
     # We don't need any special privileges for the build phase, so we run the
     # container as the current user/group.