vmm: Add Debuggable trait implementation

This commit adds initial gdb.rs implementation for `Debuggable` trait to
describe a debuggable component. Some part of the trait bound
implementations is based on the crosvm GDB stub code [1].

[1] https://github.com/google/crosvm/blob/main/src/gdb.rs

Signed-off-by: Akira Moroo <retrage01@gmail.com>

vmm/src/gdb.rs

@@ -0,0 +1,53 @@
+// Copyright 2022 Akira Moroo.
+// Portions Copyright 2020 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE-BSD-3-Clause file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+#[cfg(target_arch = "x86_64")]
+use gdbstub_arch::x86::reg::X86_64CoreRegs as CoreRegs;
+use vm_memory::GuestAddress;
+
+#[derive(Debug)]
+pub enum DebuggableError {
+    SetDebug(hypervisor::HypervisorCpuError),
+    Pause(vm_migration::MigratableError),
+    Resume(vm_migration::MigratableError),
+    ReadRegs(crate::cpu::Error),
+    WriteRegs(crate::cpu::Error),
+    ReadMem(hypervisor::HypervisorVmError),
+    WriteMem(hypervisor::HypervisorVmError),
+    TranslateGva(crate::cpu::Error),
+    PoisonedState,
+}
+
+pub trait Debuggable: vm_migration::Pausable {
+    fn set_guest_debug(
+        &self,
+        cpu_id: usize,
+        addrs: &[GuestAddress],
+        singlestep: bool,
+    ) -> Result<(), DebuggableError>;
+    fn debug_pause(&mut self) -> std::result::Result<(), DebuggableError>;
+    fn debug_resume(&mut self) -> std::result::Result<(), DebuggableError>;
+    fn read_regs(&self, cpu_id: usize) -> std::result::Result<CoreRegs, DebuggableError>;
+    fn write_regs(
+        &self,
+        cpu_id: usize,
+        regs: &CoreRegs,
+    ) -> std::result::Result<(), DebuggableError>;
+    fn read_mem(
+        &self,
+        cpu_id: usize,
+        vaddr: GuestAddress,
+        len: usize,
+    ) -> std::result::Result<Vec<u8>, DebuggableError>;
+    fn write_mem(
+        &self,
+        cpu_id: usize,
+        vaddr: &GuestAddress,
+        data: &[u8],
+    ) -> std::result::Result<(), DebuggableError>;
+    fn active_vcpus(&self) -> usize;
+}