hypervisor: emulator: Add initial fetch support

When the x86 instruction decoder tells us about some missing bytes from
the instruction stream, we call into the platform fetch method and
emulate one last instruction.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

hypervisor/src/arch/emulator/mod.rs

@@ -71,6 +71,9 @@ pub enum EmulationError<T: Debug> {
     #[error("Instruction Exception: {0}")]
     InstructionException(Exception<T>),
 
+    #[error("Instruction fetching error: {0}")]
+    InstructionFetchingError(#[source] anyhow::Error),
+
     #[error("Platform emulation error: {0}")]
     PlatformEmulationError(PlatformError),
 }
@@ -126,6 +129,14 @@ pub trait PlatformEmulator: Send + Sync {
     /// * `gva` - Guest virtual address to translate.
     ///
     fn gva_to_gpa(&self, gva: u64) -> Result<u64, PlatformError>;
+
+    /// Fetch instruction bytes from memory.
+    ///
+    /// # Arguments
+    ///
+    /// * `ip` - Instruction pointer virtual address to start fetching instructions from.
+    ///
+    fn fetch(&self, ip: u64, instruction_bytes: &mut [u8]) -> Result<(), PlatformError>;
 }
 
 pub type EmulationResult<S, E> = std::result::Result<S, EmulationError<E>>;