guest_memory: implement error trait

This implements the error trait so we can get more useful error
information back on failures.

BUG=chromium:797868
TEST=./build_test passes on all architectures
TEST=crosvm runs on caroline

Change-Id: Ia131b1eb14d24709c10f2ba752a962e822134a42
Reviewed-on: https://chromium-review.googlesource.com/938312
Commit-Ready: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>

1 files changed, 23 insertions, 1 deletions

diff --git a/sys_util/src/guest_memory.rs b/sys_util/src/guest_memory.rs
index f9dc716..23c8a2f 100644
--- a/sys_util/src/guest_memory.rs
+++ b/sys_util/src/guest_memory.rs
@@ -7,6 +7,8 @@
 use std::io::{Read, Write};
 use std::result;
 use std::sync::Arc;
+use std::error::{self, Error as GuestMemoryError};
+use std::fmt::{self, Display};
 
 use data_model::DataInit;
 use data_model::volatile_memory::*;
@@ -19,10 +21,30 @@ pub enum Error {
     MemoryAccess(GuestAddress, mmap::Error),
     MemoryMappingFailed(mmap::Error),
     MemoryRegionOverlap,
-    RegionOperationFailed,
 }
 pub type Result<T> = result::Result<T, Error>;
 
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        match self {
+            &Error::InvalidGuestAddress(_) =>
+                "Invalid Guest Address",
+            &Error::MemoryAccess(_, _) =>
+                "Invalid Guest Memory Access",
+            &Error::MemoryMappingFailed(_) =>
+                "Failed to map guest memory",
+            &Error::MemoryRegionOverlap =>
+                "Memory regions overlap",
+        }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Guest memory error: {}", Error::description(self))
+    }
+}
+
 struct MemoryRegion {
     mapping: MemoryMapping,
     guest_base: GuestAddress,