msr: Adding check for kernel module

Previously, if the kernel module was not present, this was silently
returning an empty file-handle. When the MSR kernel module is not
present, we want to make sure a specific error is returned letting the
user know that it is not present. For now, we are checking this by
looking if the device-file is present.

Signed-off-by: Larry Dewey <larry.dewey@amd.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "msru"
-version = "0.1.0"
+version = "0.2.0"
 authors = [ "Larry Dewey <larry.dewey@amd.com>"]
 edition = "2021"
 rust-version = "1.59"

README.md

@@ -10,7 +10,7 @@ Add the following line to your `Cargo.toml` file:
 
 ```toml
 [dependencies]
-msru = "0.1.0"
+msru = "0.2.0"
 ```
 
 ## Usage
@@ -19,7 +19,9 @@ msru = "0.1.0"
 use msru::Msr;
 
 // X86_64 SYSCFG MSR
-let msr: Msr = Msr::new(0xC0010010, 0);
+let msr: Msr = Msr::new(0xC0010010, 0)?;
+
+let raw_value: u64 = msr.read()?;
 
 // ...
 ```

src/lib.rs

@@ -7,11 +7,40 @@
 //! Currently this crate only supports Linux.
 
 use std::{
+    convert::From,
     fs::{File, OpenOptions},
-    io::{Read, Result, Seek, SeekFrom},
+    io::{Read, Seek, SeekFrom},
     os::unix::fs::FileExt,
+    path::Path,
 };
 
+#[derive(Debug)]
+pub enum MsrError {
+    IoError(std::io::Error),
+    MissingKernelModule,
+    UnknownError,
+}
+
+impl std::error::Error for MsrError {}
+
+impl std::fmt::Display for MsrError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            MsrError::IoError(io_error) => write!(f, "IoError Encountered: {io_error}"),
+            MsrError::MissingKernelModule => write!(f, "MSR Kernel Module not loaded!"),
+            MsrError::UnknownError => write!(f, "An unknown error was encountered!"),
+        }
+    }
+}
+
+impl From<std::io::Error> for MsrError {
+    fn from(value: std::io::Error) -> Self {
+        Self::IoError(value)
+    }
+}
+
+type Result<T> = std::result::Result<T, MsrError>;
+
 /// A Rust-friendly MSR structure.
 pub struct Msr {
     /// A model specific register address we would like to read.
@@ -24,6 +53,9 @@ impl Msr {
     /// Construct an Msr for a specified register and CPU.
     pub fn new(reg: u32, cpu: u16) -> Result<Self> {
         let cpu_msr_path: String = format!("/dev/cpu/{cpu}/msr");
+        if !Path::new(&cpu_msr_path).exists() {
+            return Err(MsrError::MissingKernelModule);
+        }
         Ok(Self {
             reg,
             fh: OpenOptions::new()