diff --git a/gui/Cargo.toml b/gui/Cargo.toml
index c8672def..cdd4049f 100644
--- a/gui/Cargo.toml
+++ b/gui/Cargo.toml
@@ -9,6 +9,7 @@ crate-type = ["staticlib"]
 [dependencies]
 bitfield-struct = "0.8.0"
 gdbstub = "0.7.2"
+gdbstub_arch = "0.3.1"
 humansize = "2.1.3"
 libc = "0.2.155"
 obconf = { path = "../src/obconf", features = ["serde", "virt"] }
diff --git a/gui/src/vmm/cpu/controller.rs b/gui/src/vmm/cpu/controller.rs
index f1392eab..03c1b72e 100644
--- a/gui/src/vmm/cpu/controller.rs
+++ b/gui/src/vmm/cpu/controller.rs
@@ -1,16 +1,19 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 use std::mem::ManuallyDrop;
+use std::sync::{Arc, Mutex};
 use std::thread::JoinHandle;
 
 /// Contains objects to control a CPU from outside.
 pub struct CpuController {
     thread: ManuallyDrop<JoinHandle<()>>,
+    state: Arc<Mutex<CpuState>>,
 }
 
 impl CpuController {
-    pub fn new(thread: JoinHandle<()>) -> Self {
+    pub fn new(thread: JoinHandle<()>, state: Arc<Mutex<CpuState>>) -> Self {
         Self {
             thread: ManuallyDrop::new(thread),
+            state,
         }
     }
 }
@@ -20,3 +23,8 @@ impl Drop for CpuController {
         unsafe { ManuallyDrop::take(&mut self.thread).join().unwrap() };
     }
 }
+
+/// State of a CPU.
+pub enum CpuState {
+    Running,
+}
diff --git a/gui/src/vmm/cpu/mod.rs b/gui/src/vmm/cpu/mod.rs
index e0531d9e..2f36c158 100644
--- a/gui/src/vmm/cpu/mod.rs
+++ b/gui/src/vmm/cpu/mod.rs
@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
+pub use self::controller::CpuState;
+
 use self::controller::CpuController;
-use super::hv::{Cpu, CpuExit, CpuFeats, CpuIo, Hypervisor};
+use super::hv::{Cpu, CpuExit, CpuIo, Hypervisor};
 use super::hw::{DeviceContext, DeviceTree};
 use super::ram::RamMap;
 use super::screen::Screen;
@@ -9,8 +11,9 @@ use crate::error::RustError;
 use std::collections::BTreeMap;
 use std::error::Error;
 use std::num::NonZero;
+use std::ops::{Deref, DerefMut};
 use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 
 #[cfg_attr(target_arch = "aarch64", path = "aarch64.rs")]
 #[cfg_attr(target_arch = "x86_64", path = "x86_64.rs")]
@@ -21,7 +24,6 @@ mod controller;
 pub struct CpuManager<H: Hypervisor, S: Screen> {
     hv: Arc<H>,
     screen: Arc<S::Buffer>,
-    feats: Arc<CpuFeats>,
     devices: Arc<DeviceTree<H>>,
     event: VmmEventHandler,
     cpus: Vec<CpuController>,
@@ -32,7 +34,6 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
     pub fn new(
         hv: Arc<H>,
         screen: Arc<S::Buffer>,
-        feats: CpuFeats,
         devices: Arc<DeviceTree<H>>,
         event: VmmEventHandler,
         shutdown: Arc<AtomicBool>,
@@ -40,7 +41,6 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
         Self {
             hv,
             screen,
-            feats: Arc::new(feats),
             devices,
             event,
             cpus: Vec::new(),
@@ -53,22 +53,30 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
         let args = Args {
             hv: self.hv.clone(),
             screen: self.screen.clone(),
-            feats: self.feats.clone(),
             devices: self.devices.clone(),
             event: self.event,
             shutdown: self.shutdown.clone(),
         };
 
         // Spawn thread to drive vCPU.
+        let state = Arc::new(Mutex::new(CpuState::Running));
         let t = match map {
-            Some(map) => std::thread::spawn(move || Self::main_cpu(&args, start, map)),
+            Some(map) => std::thread::spawn({
+                let state = state.clone();
+
+                move || Self::main_cpu(args, state, start, map)
+            }),
             None => todo!(),
         };
 
-        self.cpus.push(CpuController::new(t));
+        self.cpus.push(CpuController::new(t, state));
+    }
+
+    pub fn debug_lock(&mut self) -> DebugLock<H, S> {
+        DebugLock(self)
     }
 
-    fn main_cpu(args: &Args<H, S>, entry: usize, map: RamMap) {
+    fn main_cpu(args: Args<H, S>, state: Arc<Mutex<CpuState>>, entry: usize, map: RamMap) {
         let mut cpu = match args.hv.create_cpu(0) {
             Ok(v) => v,
             Err(e) => {
@@ -78,16 +86,16 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
             }
         };
 
-        if let Err(e) = super::arch::setup_main_cpu(&mut cpu, entry, map, &args.feats) {
+        if let Err(e) = super::arch::setup_main_cpu(&mut cpu, entry, map, args.hv.cpu_features()) {
             let e = RustError::with_source("couldn't setup main CPU", e);
             unsafe { args.event.invoke(VmmEvent::Error { reason: &e }) };
             return;
         }
 
-        Self::run_cpu(cpu, args);
+        Self::run_cpu(&args, &state, cpu);
     }
 
-    fn run_cpu<'a>(mut cpu: H::Cpu<'a>, args: &'a Args<H, S>) {
+    fn run_cpu<'a>(args: &'a Args<H, S>, state: &'a Mutex<CpuState>, mut cpu: H::Cpu<'a>) {
         // Build device contexts for this CPU.
         let mut devices = args
             .devices
@@ -95,7 +103,7 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
             .map(|(addr, dev)| {
                 let end = dev.len().checked_add(addr).unwrap();
 
-                (addr, (dev.create_context(&args.hv), end))
+                (addr, (dev.create_context(&args.hv, state), end))
             })
             .collect::<BTreeMap<usize, (Box<dyn DeviceContext<H::Cpu<'a>>>, NonZero<usize>)>>();
 
@@ -149,11 +157,33 @@ impl<H: Hypervisor, S: Screen> CpuManager<H, S> {
     }
 }
 
+/// RAII struct to unlock all CPUs when dropped.
+pub struct DebugLock<'a, H: Hypervisor, S: Screen>(&'a mut CpuManager<H, S>);
+
+impl<'a, H: Hypervisor, S: Screen> Drop for DebugLock<'a, H, S> {
+    fn drop(&mut self) {
+        todo!()
+    }
+}
+
+impl<'a, H: Hypervisor, S: Screen> Deref for DebugLock<'a, H, S> {
+    type Target = CpuManager<H, S>;
+
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a, H: Hypervisor, S: Screen> DerefMut for DebugLock<'a, H, S> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.0
+    }
+}
+
 /// Encapsulates arguments for a function to run a CPU.
 struct Args<H: Hypervisor, S: Screen> {
     hv: Arc<H>,
     screen: Arc<S::Buffer>,
-    feats: Arc<CpuFeats>,
     devices: Arc<DeviceTree<H>>,
     event: VmmEventHandler,
     shutdown: Arc<AtomicBool>,
diff --git a/gui/src/vmm/cpu/x86_64.rs b/gui/src/vmm/cpu/x86_64.rs
index 8bbbde0c..a21e515e 100644
--- a/gui/src/vmm/cpu/x86_64.rs
+++ b/gui/src/vmm/cpu/x86_64.rs
@@ -10,10 +10,13 @@ use gdbstub::target::ext::breakpoints::{
 };
 use gdbstub::target::ext::thread_extra_info::{ThreadExtraInfo, ThreadExtraInfoOps};
 use gdbstub::target::TargetResult;
+use gdbstub_arch::x86::reg::X86_64CoreRegs;
+use gdbstub_arch::x86::X86_64_SSE;
+use std::num::NonZero;
 use thiserror::Error;
 
 impl<H: Hypervisor, S: Screen> gdbstub::target::Target for CpuManager<H, S> {
-    type Arch = Arch;
+    type Arch = X86_64_SSE;
     type Error = TargetError;
 
     fn base_ops(&mut self) -> BaseOps<'_, Self::Arch, Self::Error> {
@@ -26,11 +29,11 @@ impl<H: Hypervisor, S: Screen> gdbstub::target::Target for CpuManager<H, S> {
 }
 
 impl<H: Hypervisor, S: Screen> MultiThreadBase for CpuManager<H, S> {
-    fn read_registers(&mut self, regs: &mut Registers, tid: Tid) -> TargetResult<(), Self> {
+    fn read_registers(&mut self, regs: &mut X86_64CoreRegs, tid: Tid) -> TargetResult<(), Self> {
         todo!()
     }
 
-    fn write_registers(&mut self, regs: &Registers, tid: Tid) -> TargetResult<(), Self> {
+    fn write_registers(&mut self, regs: &X86_64CoreRegs, tid: Tid) -> TargetResult<(), Self> {
         todo!()
     }
 
@@ -55,7 +58,11 @@ impl<H: Hypervisor, S: Screen> MultiThreadBase for CpuManager<H, S> {
         &mut self,
         thread_is_active: &mut dyn FnMut(Tid),
     ) -> Result<(), Self::Error> {
-        todo!()
+        for id in (0..self.cpus.len()).map(|v| unsafe { NonZero::new_unchecked(v + 1) }) {
+            thread_is_active(id);
+        }
+
+        Ok(())
     }
 
     #[inline(always)]
@@ -71,15 +78,11 @@ impl<H: Hypervisor, S: Screen> Breakpoints for CpuManager<H, S> {
 }
 
 impl<H: Hypervisor, S: Screen> SwBreakpoint for CpuManager<H, S> {
-    fn add_sw_breakpoint(&mut self, addr: u64, kind: BreakpointKind) -> TargetResult<bool, Self> {
+    fn add_sw_breakpoint(&mut self, addr: u64, kind: usize) -> TargetResult<bool, Self> {
         todo!()
     }
 
-    fn remove_sw_breakpoint(
-        &mut self,
-        addr: u64,
-        kind: BreakpointKind,
-    ) -> TargetResult<bool, Self> {
+    fn remove_sw_breakpoint(&mut self, addr: u64, kind: usize) -> TargetResult<bool, Self> {
         todo!()
     }
 }
@@ -90,56 +93,6 @@ impl<H: Hypervisor, S: Screen> ThreadExtraInfo for CpuManager<H, S> {
     }
 }
 
-/// Implementation of [`gdbstub::arch::Arch`] for x86-64.
-pub enum Arch {}
-
-impl gdbstub::arch::Arch for Arch {
-    type Usize = u64;
-    type Registers = Registers;
-    type BreakpointKind = BreakpointKind;
-    type RegId = RegId;
-}
-
-/// Implementation of [`gdbstub::arch::Registers`] for x86-64.
-#[derive(Default, Debug, Clone, PartialEq)]
-pub struct Registers {}
-
-impl gdbstub::arch::Registers for Registers {
-    type ProgramCounter = u64;
-
-    fn pc(&self) -> Self::ProgramCounter {
-        todo!()
-    }
-
-    fn gdb_serialize(&self, write_byte: impl FnMut(Option<u8>)) {
-        todo!()
-    }
-
-    fn gdb_deserialize(&mut self, bytes: &[u8]) -> Result<(), ()> {
-        todo!()
-    }
-}
-
-/// Implementation of [`gdbstub::arch::BreakpointKind`] for x86-64.
-#[derive(Debug)]
-pub struct BreakpointKind {}
-
-impl gdbstub::arch::BreakpointKind for BreakpointKind {
-    fn from_usize(kind: usize) -> Option<Self> {
-        todo!()
-    }
-}
-
-/// Implementation of [`gdbstub::arch::RegId`] for x86-64.
-#[derive(Debug)]
-pub struct RegId {}
-
-impl gdbstub::arch::RegId for RegId {
-    fn from_raw_id(id: usize) -> Option<(Self, Option<std::num::NonZeroUsize>)> {
-        todo!()
-    }
-}
-
 /// Implementation of [`gdbstub::target::Target::Error`] for x86-64.
 #[derive(Debug, Error)]
 pub enum TargetError {}
diff --git a/gui/src/vmm/hw/console/mod.rs b/gui/src/vmm/hw/console/mod.rs
index 89ade5e6..7cd92281 100644
--- a/gui/src/vmm/hw/console/mod.rs
+++ b/gui/src/vmm/hw/console/mod.rs
@@ -1,10 +1,12 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 use self::context::Context;
 use super::{Device, DeviceContext};
+use crate::vmm::cpu::CpuState;
 use crate::vmm::hv::Hypervisor;
 use crate::vmm::VmmEventHandler;
 use obconf::ConsoleMemory;
 use std::num::NonZero;
+use std::sync::Mutex;
 
 mod context;
 
@@ -35,7 +37,11 @@ impl<H: Hypervisor> Device<H> for Console {
         self.len
     }
 
-    fn create_context<'a>(&'a self, hv: &'a H) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
+    fn create_context<'a>(
+        &'a self,
+        hv: &'a H,
+        _: &'a Mutex<CpuState>,
+    ) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
         Box::new(Context::new(self, hv))
     }
 }
diff --git a/gui/src/vmm/hw/debugger/context.rs b/gui/src/vmm/hw/debugger/context.rs
index 1305f7a8..ef4c66ca 100644
--- a/gui/src/vmm/hw/debugger/context.rs
+++ b/gui/src/vmm/hw/debugger/context.rs
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 use super::Debugger;
+use crate::vmm::cpu::CpuState;
 use crate::vmm::hv::{Cpu, CpuExit, CpuIo};
 use crate::vmm::hw::{read_u8, DeviceContext, MmioError};
 use crate::vmm::VmmEvent;
@@ -7,16 +8,18 @@ use obconf::{DebuggerMemory, StopReason};
 use std::error::Error;
 use std::mem::offset_of;
 use std::ptr::null_mut;
+use std::sync::Mutex;
 use thiserror::Error;
 
 /// Implementation of [`DeviceContext`].
 pub struct Context<'a> {
     dev: &'a Debugger,
+    state: &'a Mutex<CpuState>,
 }
 
 impl<'a> Context<'a> {
-    pub fn new(dev: &'a Debugger) -> Self {
-        Self { dev }
+    pub fn new(dev: &'a Debugger, state: &'a Mutex<CpuState>) -> Self {
+        Self { dev, state }
     }
 }
 
@@ -26,10 +29,13 @@ impl<'a, C: Cpu> DeviceContext<C> for Context<'a> {
         let off = exit.addr() - self.dev.addr;
 
         if off == offset_of!(DebuggerMemory, stop) {
+            // Read stop reason.
             let stop = read_u8(exit).map_err(|e| ExecError::ReadFailed(off, e))?;
             let stop: StopReason = stop
                 .try_into()
                 .map_err(|_| Box::new(ExecError::InvalidStop(stop)))?;
+
+            // Notify GUI.
             let stop = match stop {
                 StopReason::WaitForDebugger => null_mut(),
             };
diff --git a/gui/src/vmm/hw/debugger/mod.rs b/gui/src/vmm/hw/debugger/mod.rs
index 16804c0e..bf7c07d2 100644
--- a/gui/src/vmm/hw/debugger/mod.rs
+++ b/gui/src/vmm/hw/debugger/mod.rs
@@ -1,10 +1,12 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 use self::context::Context;
 use super::{Device, DeviceContext};
+use crate::vmm::cpu::CpuState;
 use crate::vmm::hv::Hypervisor;
 use crate::vmm::VmmEventHandler;
 use obconf::DebuggerMemory;
 use std::num::NonZero;
+use std::sync::Mutex;
 
 mod context;
 
@@ -35,7 +37,11 @@ impl<H: Hypervisor> Device<H> for Debugger {
         self.len
     }
 
-    fn create_context<'a>(&'a self, _: &'a H) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
-        Box::new(Context::new(self))
+    fn create_context<'a>(
+        &'a self,
+        _: &'a H,
+        state: &'a Mutex<CpuState>,
+    ) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
+        Box::new(Context::new(self, state))
     }
 }
diff --git a/gui/src/vmm/hw/mod.rs b/gui/src/vmm/hw/mod.rs
index 1137c2d0..9f508eef 100644
--- a/gui/src/vmm/hw/mod.rs
+++ b/gui/src/vmm/hw/mod.rs
@@ -1,16 +1,17 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
+pub use self::console::*;
+pub use self::debugger::*;
+pub use self::vmm::*;
+
+use super::cpu::CpuState;
 use super::hv::{Cpu, CpuExit, CpuIo, Hypervisor, IoBuf};
 use super::VmmEventHandler;
 use std::collections::BTreeMap;
 use std::error::Error;
 use std::num::NonZero;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use thiserror::Error;
 
-pub use self::console::*;
-pub use self::debugger::*;
-pub use self::vmm::*;
-
 mod console;
 mod debugger;
 mod vmm;
@@ -126,7 +127,11 @@ pub trait Device<H: Hypervisor>: Send + Sync {
     /// Total size of device memory, in bytes.
     fn len(&self) -> NonZero<usize>;
 
-    fn create_context<'a>(&'a self, hv: &'a H) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a>;
+    fn create_context<'a>(
+        &'a self,
+        hv: &'a H,
+        state: &'a Mutex<CpuState>,
+    ) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a>;
 }
 
 /// Context to execute memory-mapped I/O operations on a virtual device.
diff --git a/gui/src/vmm/hw/vmm/mod.rs b/gui/src/vmm/hw/vmm/mod.rs
index fc5dda11..5ec2efcf 100644
--- a/gui/src/vmm/hw/vmm/mod.rs
+++ b/gui/src/vmm/hw/vmm/mod.rs
@@ -1,10 +1,12 @@
 // SPDX-License-Identifier: MIT OR Apache-2.0
 use self::context::Context;
 use super::{Device, DeviceContext};
+use crate::vmm::cpu::CpuState;
 use crate::vmm::hv::Hypervisor;
 use crate::vmm::VmmEventHandler;
 use obconf::VmmMemory;
 use std::num::NonZero;
+use std::sync::Mutex;
 
 mod context;
 
@@ -35,7 +37,11 @@ impl<H: Hypervisor> Device<H> for Vmm {
         self.len
     }
 
-    fn create_context<'a>(&'a self, _: &'a H) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
+    fn create_context<'a>(
+        &'a self,
+        _: &'a H,
+        _: &'a Mutex<CpuState>,
+    ) -> Box<dyn DeviceContext<H::Cpu<'a>> + 'a> {
         Box::new(Context::new(self))
     }
 }
diff --git a/gui/src/vmm/mod.rs b/gui/src/vmm/mod.rs
index c0ddd7b4..0e585689 100644
--- a/gui/src/vmm/mod.rs
+++ b/gui/src/vmm/mod.rs
@@ -442,7 +442,6 @@ pub unsafe extern "C" fn vmm_start(
     let mut cpu = CpuManager::new(
         Arc::new(hv),
         screen.buffer().clone(),
-        feats,
         devices,
         event,
         shutdown.clone(),
@@ -501,14 +500,14 @@ pub unsafe extern "C" fn vmm_dispatch_debug(vmm: *mut Vmm, stop: *mut KernelStop
 
     // Setup target object.
     let vmm = &mut *vmm;
-    let target = &mut vmm.cpu;
+    let mut target = vmm.cpu.debug_lock();
 
     loop {
         // Check current state.
         let r = match vmm.gdb.take().unwrap() {
-            GdbStubStateMachine::Idle(s) => self::debug::dispatch_idle(target, s),
+            GdbStubStateMachine::Idle(s) => self::debug::dispatch_idle(&mut target, s),
             GdbStubStateMachine::Running(s) => {
-                match self::debug::dispatch_running(target, s, stop.take()) {
+                match self::debug::dispatch_running(&mut target, s, stop.take()) {
                     Ok(Ok(v)) => Ok(v),
                     Ok(Err(v)) => {
                         // No pending data from the debugger.
@@ -519,7 +518,7 @@ pub unsafe extern "C" fn vmm_dispatch_debug(vmm: *mut Vmm, stop: *mut KernelStop
                 }
             }
             GdbStubStateMachine::CtrlCInterrupt(s) => {
-                match s.interrupt_handled(target, None::<MultiThreadStopReason<u64>>) {
+                match s.interrupt_handled(&mut target, None::<MultiThreadStopReason<u64>>) {
                     Ok(v) => vmm.gdb = Some(v),
                     Err(e) => {
                         let r = RustError::with_source("couldn't handle CTRL+C from a debugger", e);