use a 'fam-wrappers' feature to gate the FamStructWrappers

The FamStructWrappers definitions as well as the vmm-sys-util
dependency are now gated by an opt-in `fam-wrappers` feature.

Signed-off-by: Adrian Catangiu <[email protected]>

Author: acatangiu

Cargo.toml

@@ -11,6 +11,7 @@ license = "Apache-2.0"
 [features]
 kvm-v4_14_0 = []
 kvm-v4_20_0 = []
+fam-wrappers = ["vmm-sys-util"]
 
 [dependencies]
-vmm-sys-util = ">=0.2.0"
+vmm-sys-util = { version = ">=0.2.0", optional = true }

README.md

@@ -25,7 +25,20 @@ toml:
 ```toml
 kvm-bindings = { version = "0.1", features = ["kvm_v4_20_0"]}
 ```
-In the `kvm-bindings` crate each feature maps to exactly one Linux version
-as follows:
+Bindings are generated for each specific Linux kernel version based on the enabled
+crate features as follows:
 - `kvm_v4_14_0` contains the bindings for the Linux kernel version 4.14
 - `kvm_v4_20_0` contains the bindings for the Linux kernel version 4.20
+
+This crate also offers safe wrappers over FAM structs - FFI structs that have
+a Flexible Array Member in their definition.
+These safe wrappers can be used if the `fam-wrappers` feature is enabled for
+this crate. Example:
+```toml
+kvm-bindings = { version = "0.1", features = ["kvm_v4_20_0", "fam-wrappers"]}
+```
+
+# Dependencies
+The crate has an `optional` dependency to
+[vmm-sys-util](https://crates.io/crates/vmm-sys-util) when enabling the
+`fam-wrappers` feature.

src/lib.rs

@@ -6,7 +6,10 @@
 #![allow(non_snake_case)]
 
 #[macro_use]
-#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+#[cfg(all(
+    feature = "fam-wrappers",
+    any(target_arch = "x86", target_arch = "x86_64")
+))]
 extern crate vmm_sys_util;
 
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]

src/x86/mod.rs

@@ -1,6 +1,7 @@
 // Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+#[cfg(feature = "fam-wrappers")]
 mod fam_wrappers;
 
 #[cfg(feature = "kvm-v4_14_0")]
@@ -24,5 +25,6 @@ pub mod bindings {
     #[cfg(all(not(feature = "kvm-v4_14_0"), not(feature = "kvm-v4_20_0")))]
     pub use super::bindings_v4_20_0::*;
 
+    #[cfg(feature = "fam-wrappers")]
     pub use super::fam_wrappers::*;
 }