Auto merge of #104101 - betrusted-io:xous-libstd-initial, r=bjorn3

Add initial libstd support for Xous

This patchset adds some minimal support to the tier-3 target `riscv32imac-unknown-xous-elf`. The following features are supported:

* alloc
* thread creation and joining
* thread sleeping
* thread_local
* panic_abort
* mutex
* condvar
* stdout

Additionally, internal support for the various Xous primitives surrounding IPC have been added as part of the Xous FFI. These may be exposed as part of `std::os::xous::ffi` in the future, however for now they are not public.

This represents the minimum viable product. A future patchset will add support for networking and filesystem support.

12 files changed, 1077 insertions, 0 deletions

diff --git a/library/std/src/sys/mod.rs b/library/std/src/sys/mod.rs
index 63bd783746b..457eb782ccc 100644
--- a/library/std/src/sys/mod.rs
+++ b/library/std/src/sys/mod.rs
@@ -44,6 +44,9 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_family = "wasm")] {
         mod wasm;
         pub use self::wasm::*;
+    } else if #[cfg(target_os = "xous")] {
+        mod xous;
+        pub use self::xous::*;
     } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
         mod sgx;
         pub use self::sgx::*;
diff --git a/library/std/src/sys/xous/alloc.rs b/library/std/src/sys/xous/alloc.rs
new file mode 100644
index 00000000000..b3a3e691e0d
--- /dev/null
+++ b/library/std/src/sys/xous/alloc.rs
@@ -0,0 +1,62 @@
+use crate::alloc::{GlobalAlloc, Layout, System};
+
+static mut DLMALLOC: dlmalloc::Dlmalloc = dlmalloc::Dlmalloc::new();
+
+#[stable(feature = "alloc_system_type", since = "1.28.0")]
+unsafe impl GlobalAlloc for System {
+    #[inline]
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
+        // Calling malloc() is safe because preconditions on this function match the trait method preconditions.
+        let _lock = lock::lock();
+        unsafe { DLMALLOC.malloc(layout.size(), layout.align()) }
+    }
+
+    #[inline]
+    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
+        // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
+        // Calling calloc() is safe because preconditions on this function match the trait method preconditions.
+        let _lock = lock::lock();
+        unsafe { DLMALLOC.calloc(layout.size(), layout.align()) }
+    }
+
+    #[inline]
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
+        // Calling free() is safe because preconditions on this function match the trait method preconditions.
+        let _lock = lock::lock();
+        unsafe { DLMALLOC.free(ptr, layout.size(), layout.align()) }
+    }
+
+    #[inline]
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        // SAFETY: DLMALLOC access is guaranteed to be safe because the lock gives us unique and non-reentrant access.
+        // Calling realloc() is safe because preconditions on this function match the trait method preconditions.
+        let _lock = lock::lock();
+        unsafe { DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size) }
+    }
+}
+
+mod lock {
+    use crate::sync::atomic::{AtomicI32, Ordering::SeqCst};
+
+    static LOCKED: AtomicI32 = AtomicI32::new(0);
+
+    pub struct DropLock;
+
+    pub fn lock() -> DropLock {
+        loop {
+            if LOCKED.swap(1, SeqCst) == 0 {
+                return DropLock;
+            }
+            crate::os::xous::ffi::do_yield();
+        }
+    }
+
+    impl Drop for DropLock {
+        fn drop(&mut self) {
+            let r = LOCKED.swap(0, SeqCst);
+            debug_assert_eq!(r, 1);
+        }
+    }
+}
diff --git a/library/std/src/sys/xous/locks/condvar.rs b/library/std/src/sys/xous/locks/condvar.rs
new file mode 100644
index 00000000000..1bb38dfa341
--- /dev/null
+++ b/library/std/src/sys/xous/locks/condvar.rs
@@ -0,0 +1,111 @@
+use super::mutex::Mutex;
+use crate::os::xous::ffi::{blocking_scalar, scalar};
+use crate::os::xous::services::ticktimer_server;
+use crate::sync::Mutex as StdMutex;
+use crate::time::Duration;
+
+// The implementation is inspired by Andrew D. Birrell's paper
+// "Implementing Condition Variables with Semaphores"
+
+pub struct Condvar {
+    counter: StdMutex<usize>,
+}
+
+unsafe impl Send for Condvar {}
+unsafe impl Sync for Condvar {}
+
+impl Condvar {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> Condvar {
+        Condvar { counter: StdMutex::new(0) }
+    }
+
+    pub fn notify_one(&self) {
+        let mut counter = self.counter.lock().unwrap();
+        if *counter <= 0 {
+            return;
+        } else {
+            *counter -= 1;
+        }
+        let result = blocking_scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::NotifyCondition(self.index(), 1).into(),
+        );
+        drop(counter);
+        result.expect("failure to send NotifyCondition command");
+    }
+
+    pub fn notify_all(&self) {
+        let mut counter = self.counter.lock().unwrap();
+        if *counter <= 0 {
+            return;
+        }
+        let result = blocking_scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::NotifyCondition(self.index(), *counter)
+                .into(),
+        );
+        *counter = 0;
+        drop(counter);
+
+        result.expect("failure to send NotifyCondition command");
+    }
+
+    fn index(&self) -> usize {
+        self as *const Condvar as usize
+    }
+
+    pub unsafe fn wait(&self, mutex: &Mutex) {
+        let mut counter = self.counter.lock().unwrap();
+        *counter += 1;
+        unsafe { mutex.unlock() };
+        drop(counter);
+
+        let result = blocking_scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::WaitForCondition(self.index(), 0).into(),
+        );
+        unsafe { mutex.lock() };
+
+        result.expect("Ticktimer: failure to send WaitForCondition command");
+    }
+
+    pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
+        let mut counter = self.counter.lock().unwrap();
+        *counter += 1;
+        unsafe { mutex.unlock() };
+        drop(counter);
+
+        let mut millis = dur.as_millis() as usize;
+        if millis == 0 {
+            millis = 1;
+        }
+
+        let result = blocking_scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::WaitForCondition(self.index(), millis)
+                .into(),
+        );
+        unsafe { mutex.lock() };
+
+        let result = result.expect("Ticktimer: failure to send WaitForCondition command")[0] == 0;
+
+        // If we awoke due to a timeout, decrement the wake count, as that would not have
+        // been done in the `notify()` call.
+        if !result {
+            *self.counter.lock().unwrap() -= 1;
+        }
+        result
+    }
+}
+
+impl Drop for Condvar {
+    fn drop(&mut self) {
+        scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::FreeCondition(self.index()).into(),
+        )
+        .ok();
+    }
+}
diff --git a/library/std/src/sys/xous/locks/mod.rs b/library/std/src/sys/xous/locks/mod.rs
new file mode 100644
index 00000000000..f3c5c5d9fb0
--- /dev/null
+++ b/library/std/src/sys/xous/locks/mod.rs
@@ -0,0 +1,7 @@
+mod condvar;
+mod mutex;
+mod rwlock;
+
+pub use condvar::*;
+pub use mutex::*;
+pub use rwlock::*;
diff --git a/library/std/src/sys/xous/locks/mutex.rs b/library/std/src/sys/xous/locks/mutex.rs
new file mode 100644
index 00000000000..ea51776d54e
--- /dev/null
+++ b/library/std/src/sys/xous/locks/mutex.rs
@@ -0,0 +1,116 @@
+use crate::os::xous::ffi::{blocking_scalar, do_yield, scalar};
+use crate::os::xous::services::ticktimer_server;
+use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering::Relaxed, Ordering::SeqCst};
+
+pub struct Mutex {
+    /// The "locked" value indicates how many threads are waiting on this
+    /// Mutex. Possible values are:
+    ///     0: The lock is unlocked
+    ///     1: The lock is locked and uncontended
+    ///   >=2: The lock is locked and contended
+    ///
+    /// A lock is "contended" when there is more than one thread waiting
+    /// for a lock, or it is locked for long periods of time. Rather than
+    /// spinning, these locks send a Message to the ticktimer server
+    /// requesting that they be woken up when a lock is unlocked.
+    locked: AtomicUsize,
+
+    /// Whether this Mutex ever was contended, and therefore made a trip
+    /// to the ticktimer server. If this was never set, then we were never
+    /// on the slow path and can skip deregistering the mutex.
+    contended: AtomicBool,
+}
+
+impl Mutex {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> Mutex {
+        Mutex { locked: AtomicUsize::new(0), contended: AtomicBool::new(false) }
+    }
+
+    fn index(&self) -> usize {
+        self as *const Mutex as usize
+    }
+
+    #[inline]
+    pub unsafe fn lock(&self) {
+        // Try multiple times to acquire the lock without resorting to the ticktimer
+        // server. For locks that are held for a short amount of time, this will
+        // result in the ticktimer server never getting invoked. The `locked` value
+        // will be either 0 or 1.
+        for _attempts in 0..3 {
+            if unsafe { self.try_lock() } {
+                return;
+            }
+            do_yield();
+        }
+
+        // Try one more time to lock. If the lock is released between the previous code and
+        // here, then the inner `locked` value will be 1 at the end of this. If it was not
+        // locked, then the value will be more than 1, for example if there are multiple other
+        // threads waiting on this lock.
+        if unsafe { self.try_lock_or_poison() } {
+            return;
+        }
+
+        // When this mutex is dropped, we will need to deregister it with the server.
+        self.contended.store(true, Relaxed);
+
+        // The lock is now "contended". When the lock is released, a Message will get sent to the
+        // ticktimer server to wake it up. Note that this may already have happened, so the actual
+        // value of `lock` may be anything (0, 1, 2, ...).
+        blocking_scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::LockMutex(self.index()).into(),
+        )
+        .expect("failure to send LockMutex command");
+    }
+
+    #[inline]
+    pub unsafe fn unlock(&self) {
+        let prev = self.locked.fetch_sub(1, SeqCst);
+
+        // If the previous value was 1, then this was a "fast path" unlock, so no
+        // need to involve the Ticktimer server
+        if prev == 1 {
+            return;
+        }
+
+        // If it was 0, then something has gone seriously wrong and the counter
+        // has just wrapped around.
+        if prev == 0 {
+            panic!("mutex lock count underflowed");
+        }
+
+        // Unblock one thread that is waiting on this message.
+        scalar(
+            ticktimer_server(),
+            crate::os::xous::services::TicktimerScalar::UnlockMutex(self.index()).into(),
+        )
+        .expect("failure to send UnlockMutex command");
+    }
+
+    #[inline]
+    pub unsafe fn try_lock(&self) -> bool {
+        self.locked.compare_exchange(0, 1, SeqCst, SeqCst).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn try_lock_or_poison(&self) -> bool {
+        self.locked.fetch_add(1, SeqCst) == 0
+    }
+}
+
+impl Drop for Mutex {
+    fn drop(&mut self) {
+        // If there was Mutex contention, then we involved the ticktimer. Free
+        // the resources associated with this Mutex as it is deallocated.
+        if self.contended.load(Relaxed) {
+            scalar(
+                ticktimer_server(),
+                crate::os::xous::services::TicktimerScalar::FreeMutex(self.index()).into(),
+            )
+            .ok();
+        }
+    }
+}
diff --git a/library/std/src/sys/xous/locks/rwlock.rs b/library/std/src/sys/xous/locks/rwlock.rs
new file mode 100644
index 00000000000..618da758adf
--- /dev/null
+++ b/library/std/src/sys/xous/locks/rwlock.rs
@@ -0,0 +1,72 @@
+use crate::os::xous::ffi::do_yield;
+use crate::sync::atomic::{AtomicIsize, Ordering::SeqCst};
+
+pub struct RwLock {
+    /// The "mode" value indicates how many threads are waiting on this
+    /// Mutex. Possible values are:
+    ///    -1: The lock is locked for writing
+    ///     0: The lock is unlocked
+    ///   >=1: The lock is locked for reading
+    ///
+    /// This currently spins waiting for the lock to be freed. An
+    /// optimization would be to involve the ticktimer server to
+    /// coordinate unlocks.
+    mode: AtomicIsize,
+}
+
+unsafe impl Send for RwLock {}
+unsafe impl Sync for RwLock {}
+
+impl RwLock {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> RwLock {
+        RwLock { mode: AtomicIsize::new(0) }
+    }
+
+    #[inline]
+    pub unsafe fn read(&self) {
+        while !unsafe { self.try_read() } {
+            do_yield();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn try_read(&self) -> bool {
+        // Non-atomically determine the current value.
+        let current = self.mode.load(SeqCst);
+
+        // If it's currently locked for writing, then we cannot read.
+        if current < 0 {
+            return false;
+        }
+
+        // Attempt to lock. If the `current` value has changed, then this
+        // operation will fail and we will not obtain the lock even if we
+        // could potentially keep it.
+        let new = current + 1;
+        self.mode.compare_exchange(current, new, SeqCst, SeqCst).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn write(&self) {
+        while !unsafe { self.try_write() } {
+            do_yield();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn try_write(&self) -> bool {
+        self.mode.compare_exchange(0, -1, SeqCst, SeqCst).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn read_unlock(&self) {
+        self.mode.fetch_sub(1, SeqCst);
+    }
+
+    #[inline]
+    pub unsafe fn write_unlock(&self) {
+        assert_eq!(self.mode.compare_exchange(-1, 0, SeqCst, SeqCst), Ok(-1));
+    }
+}
diff --git a/library/std/src/sys/xous/mod.rs b/library/std/src/sys/xous/mod.rs
new file mode 100644
index 00000000000..6d5c218d195
--- /dev/null
+++ b/library/std/src/sys/xous/mod.rs
@@ -0,0 +1,37 @@
+#![deny(unsafe_op_in_unsafe_fn)]
+
+pub mod alloc;
+#[path = "../unsupported/args.rs"]
+pub mod args;
+#[path = "../unix/cmath.rs"]
+pub mod cmath;
+#[path = "../unsupported/env.rs"]
+pub mod env;
+#[path = "../unsupported/fs.rs"]
+pub mod fs;
+#[path = "../unsupported/io.rs"]
+pub mod io;
+pub mod locks;
+#[path = "../unsupported/net.rs"]
+pub mod net;
+#[path = "../unsupported/once.rs"]
+pub mod once;
+pub mod os;
+#[path = "../unix/os_str.rs"]
+pub mod os_str;
+#[path = "../unix/path.rs"]
+pub mod path;
+#[path = "../unsupported/pipe.rs"]
+pub mod pipe;
+#[path = "../unsupported/process.rs"]
+pub mod process;
+pub mod stdio;
+pub mod thread;
+pub mod thread_local_key;
+#[path = "../unsupported/thread_parking.rs"]
+pub mod thread_parking;
+pub mod time;
+
+#[path = "../unsupported/common.rs"]
+mod common;
+pub use common::*;
diff --git a/library/std/src/sys/xous/os.rs b/library/std/src/sys/xous/os.rs
new file mode 100644
index 00000000000..3d19fa4b31a
--- /dev/null
+++ b/library/std/src/sys/xous/os.rs
@@ -0,0 +1,147 @@
+use super::unsupported;
+use crate::error::Error as StdError;
+use crate::ffi::{OsStr, OsString};
+use crate::fmt;
+use crate::io;
+use crate::marker::PhantomData;
+use crate::os::xous::ffi::Error as XousError;
+use crate::path::{self, PathBuf};
+
+#[cfg(not(test))]
+mod c_compat {
+    use crate::os::xous::ffi::exit;
+    extern "C" {
+        fn main() -> u32;
+    }
+
+    #[no_mangle]
+    pub extern "C" fn abort() {
+        exit(1);
+    }
+
+    #[no_mangle]
+    pub extern "C" fn _start() {
+        exit(unsafe { main() });
+    }
+
+    // This function is needed by the panic runtime. The symbol is named in
+    // pre-link args for the target specification, so keep that in sync.
+    #[no_mangle]
+    // NB. used by both libunwind and libpanic_abort
+    pub extern "C" fn __rust_abort() -> ! {
+        exit(101);
+    }
+}
+
+pub fn errno() -> i32 {
+    0
+}
+
+pub fn error_string(errno: i32) -> String {
+    Into::<XousError>::into(errno).to_string()
+}
+
+pub fn getcwd() -> io::Result<PathBuf> {
+    unsupported()
+}
+
+pub fn chdir(_: &path::Path) -> io::Result<()> {
+    unsupported()
+}
+
+pub struct SplitPaths<'a>(!, PhantomData<&'a ()>);
+
+pub fn split_paths(_unparsed: &OsStr) -> SplitPaths<'_> {
+    panic!("unsupported")
+}
+
+impl<'a> Iterator for SplitPaths<'a> {
+    type Item = PathBuf;
+    fn next(&mut self) -> Option<PathBuf> {
+        self.0
+    }
+}
+
+#[derive(Debug)]
+pub struct JoinPathsError;
+
+pub fn join_paths<I, T>(_paths: I) -> Result<OsString, JoinPathsError>
+where
+    I: Iterator<Item = T>,
+    T: AsRef<OsStr>,
+{
+    Err(JoinPathsError)
+}
+
+impl fmt::Display for JoinPathsError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "not supported on this platform yet".fmt(f)
+    }
+}
+
+impl StdError for JoinPathsError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "not supported on this platform yet"
+    }
+}
+
+pub fn current_exe() -> io::Result<PathBuf> {
+    unsupported()
+}
+
+pub struct Env(!);
+
+impl Env {
+    // FIXME(https://github.com/rust-lang/rust/issues/114583): Remove this when <OsStr as Debug>::fmt matches <str as Debug>::fmt.
+    pub fn str_debug(&self) -> impl fmt::Debug + '_ {
+        let Self(inner) = self;
+        match *inner {}
+    }
+}
+
+impl fmt::Debug for Env {
+    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self(inner) = self;
+        match *inner {}
+    }
+}
+
+impl Iterator for Env {
+    type Item = (OsString, OsString);
+    fn next(&mut self) -> Option<(OsString, OsString)> {
+        self.0
+    }
+}
+
+pub fn env() -> Env {
+    panic!("not supported on this platform")
+}
+
+pub fn getenv(_: &OsStr) -> Option<OsString> {
+    None
+}
+
+pub fn setenv(_: &OsStr, _: &OsStr) -> io::Result<()> {
+    Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot set env vars on this platform"))
+}
+
+pub fn unsetenv(_: &OsStr) -> io::Result<()> {
+    Err(io::const_io_error!(io::ErrorKind::Unsupported, "cannot unset env vars on this platform"))
+}
+
+pub fn temp_dir() -> PathBuf {
+    panic!("no filesystem on this platform")
+}
+
+pub fn home_dir() -> Option<PathBuf> {
+    None
+}
+
+pub fn exit(code: i32) -> ! {
+    crate::os::xous::ffi::exit(code as u32);
+}
+
+pub fn getpid() -> u32 {
+    panic!("no pids on this platform")
+}
diff --git a/library/std/src/sys/xous/stdio.rs b/library/std/src/sys/xous/stdio.rs
new file mode 100644
index 00000000000..2ac694641ba
--- /dev/null
+++ b/library/std/src/sys/xous/stdio.rs
@@ -0,0 +1,131 @@
+use crate::io;
+
+pub struct Stdin;
+pub struct Stdout {}
+pub struct Stderr;
+
+use crate::os::xous::ffi::{lend, try_lend, try_scalar, Connection};
+use crate::os::xous::services::{log_server, try_connect, LogScalar};
+
+impl Stdin {
+    pub const fn new() -> Stdin {
+        Stdin
+    }
+}
+
+impl io::Read for Stdin {
+    fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
+        Ok(0)
+    }
+}
+
+impl Stdout {
+    pub const fn new() -> Stdout {
+        Stdout {}
+    }
+}
+
+impl io::Write for Stdout {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        #[repr(align(4096))]
+        struct LendBuffer([u8; 4096]);
+        let mut lend_buffer = LendBuffer([0u8; 4096]);
+        let connection = log_server();
+        for chunk in buf.chunks(lend_buffer.0.len()) {
+            for (dest, src) in lend_buffer.0.iter_mut().zip(chunk) {
+                *dest = *src;
+            }
+            lend(connection, 1, &lend_buffer.0, 0, chunk.len()).unwrap();
+        }
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+impl Stderr {
+    pub const fn new() -> Stderr {
+        Stderr
+    }
+}
+
+impl io::Write for Stderr {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        #[repr(align(4096))]
+        struct LendBuffer([u8; 4096]);
+        let mut lend_buffer = LendBuffer([0u8; 4096]);
+        let connection = log_server();
+        for chunk in buf.chunks(lend_buffer.0.len()) {
+            for (dest, src) in lend_buffer.0.iter_mut().zip(chunk) {
+                *dest = *src;
+            }
+            lend(connection, 1, &lend_buffer.0, 0, chunk.len()).unwrap();
+        }
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+pub const STDIN_BUF_SIZE: usize = 0;
+
+pub fn is_ebadf(_err: &io::Error) -> bool {
+    true
+}
+
+#[derive(Copy, Clone)]
+pub struct PanicWriter {
+    log: Connection,
+    gfx: Option<Connection>,
+}
+
+impl io::Write for PanicWriter {
+    fn write(&mut self, s: &[u8]) -> core::result::Result<usize, io::Error> {
+        for c in s.chunks(core::mem::size_of::<usize>() * 4) {
+            // Text is grouped into 4x `usize` words. The id is 1100 plus
+            // the number of characters in this message.
+            // Ignore errors since we're already panicking.
+            try_scalar(self.log, LogScalar::AppendPanicMessage(&c).into()).ok();
+        }
+
+        // Serialize the text to the graphics panic handler, only if we were able
+        // to acquire a connection to it. Text length is encoded in the `valid` field,
+        // the data itself in the buffer. Typically several messages are require to
+        // fully transmit the entire panic message.
+        if let Some(gfx) = self.gfx {
+            #[repr(C, align(4096))]
+            struct Request([u8; 4096]);
+            let mut request = Request([0u8; 4096]);
+            for (&s, d) in s.iter().zip(request.0.iter_mut()) {
+                *d = s;
+            }
+            try_lend(gfx, 0 /* AppendPanicText */, &request.0, 0, s.len()).ok();
+        }
+        Ok(s.len())
+    }
+
+    // Tests show that this does not seem to be reliably called at the end of a panic
+    // print, so, we can't rely on this to e.g. trigger a graphics update.
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+pub fn panic_output() -> Option<impl io::Write> {
+    // Generally this won't fail because every server has already connected, so
+    // this is likely to succeed.
+    let log = log_server();
+
+    // Send the "We're panicking" message (1000).
+    try_scalar(log, LogScalar::BeginPanic.into()).ok();
+
+    // This is will fail in the case that the connection table is full, or if the
+    // graphics server is not running. Most servers do not already have this connection.
+    let gfx = try_connect("panic-to-screen!");
+
+    Some(PanicWriter { log, gfx })
+}
diff --git a/library/std/src/sys/xous/thread.rs b/library/std/src/sys/xous/thread.rs
new file mode 100644
index 00000000000..78c68de7bf3
--- /dev/null
+++ b/library/std/src/sys/xous/thread.rs
@@ -0,0 +1,144 @@
+use crate::ffi::CStr;
+use crate::io;
+use crate::num::NonZeroUsize;
+use crate::os::xous::ffi::{
+    blocking_scalar, create_thread, do_yield, join_thread, map_memory, update_memory_flags,
+    MemoryFlags, Syscall, ThreadId,
+};
+use crate::os::xous::services::{ticktimer_server, TicktimerScalar};
+use crate::time::Duration;
+use core::arch::asm;
+
+pub struct Thread {
+    tid: ThreadId,
+}
+
+pub const DEFAULT_MIN_STACK_SIZE: usize = 131072;
+const MIN_STACK_SIZE: usize = 4096;
+pub const GUARD_PAGE_SIZE: usize = 4096;
+
+impl Thread {
+    // unsafe: see thread::Builder::spawn_unchecked for safety requirements
+    pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
+        let p = Box::into_raw(Box::new(p));
+        let mut stack_size = crate::cmp::max(stack, MIN_STACK_SIZE);
+
+        if (stack_size & 4095) != 0 {
+            stack_size = (stack_size + 4095) & !4095;
+        }
+
+        // Allocate the whole thing, then divide it up after the fact. This ensures that
+        // even if there's a context switch during this function, the whole stack plus
+        // guard pages will remain contiguous.
+        let stack_plus_guard_pages: &mut [u8] = unsafe {
+            map_memory(
+                None,
+                None,
+                GUARD_PAGE_SIZE + stack_size + GUARD_PAGE_SIZE,
+                MemoryFlags::R | MemoryFlags::W | MemoryFlags::X,
+            )
+        }
+        .map_err(|code| io::Error::from_raw_os_error(code as i32))?;
+
+        // No access to this page. Note: Write-only pages are illegal, and will
+        // cause an access violation.
+        unsafe {
+            update_memory_flags(&mut stack_plus_guard_pages[0..GUARD_PAGE_SIZE], MemoryFlags::W)
+                .map_err(|code| io::Error::from_raw_os_error(code as i32))?
+        };
+
+        // No access to this page. Note: Write-only pages are illegal, and will
+        // cause an access violation.
+        unsafe {
+            update_memory_flags(
+                &mut stack_plus_guard_pages[(GUARD_PAGE_SIZE + stack_size)..],
+                MemoryFlags::W,
+            )
+            .map_err(|code| io::Error::from_raw_os_error(code as i32))?
+        };
+
+        let guard_page_pre = stack_plus_guard_pages.as_ptr() as usize;
+        let tid = create_thread(
+            thread_start as *mut usize,
+            &mut stack_plus_guard_pages[GUARD_PAGE_SIZE..(stack_size + GUARD_PAGE_SIZE)],
+            p as usize,
+            guard_page_pre,
+            stack_size,
+            0,
+        )
+        .map_err(|code| io::Error::from_raw_os_error(code as i32))?;
+
+        extern "C" fn thread_start(main: *mut usize, guard_page_pre: usize, stack_size: usize) {
+            unsafe {
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
+            }
+
+            // Destroy TLS, which will free the TLS page and call the destructor for
+            // any thread local storage.
+            unsafe {
+                crate::sys::thread_local_key::destroy_tls();
+            }
+
+            // Deallocate the stack memory, along with the guard pages. Afterwards,
+            // exit the thread by returning to the magic address 0xff80_3000usize,
+            // which tells the kernel to deallocate this thread.
+            let mapped_memory_base = guard_page_pre;
+            let mapped_memory_length = GUARD_PAGE_SIZE + stack_size + GUARD_PAGE_SIZE;
+            unsafe {
+                asm!(
+                    "ecall",
+                    "ret",
+                                        in("a0") Syscall::UnmapMemory as usize,
+                                        in("a1") mapped_memory_base,
+                                        in("a2") mapped_memory_length,
+                                        in("ra") 0xff80_3000usize,
+                                        options(nomem, nostack, noreturn)
+                );
+            }
+        }
+
+        Ok(Thread { tid })
+    }
+
+    pub fn yield_now() {
+        do_yield();
+    }
+
+    pub fn set_name(_name: &CStr) {
+        // nope
+    }
+
+    pub fn sleep(dur: Duration) {
+        // Because the sleep server works on units of `usized milliseconds`, split
+        // the messages up into these chunks. This means we may run into issues
+        // if you try to sleep a thread for more than 49 days on a 32-bit system.
+        let mut millis = dur.as_millis();
+        while millis > 0 {
+            let sleep_duration =
+                if millis > (usize::MAX as _) { usize::MAX } else { millis as usize };
+            blocking_scalar(ticktimer_server(), TicktimerScalar::SleepMs(sleep_duration).into())
+                .expect("failed to send message to ticktimer server");
+            millis -= sleep_duration as u128;
+        }
+    }
+
+    pub fn join(self) {
+        join_thread(self.tid).unwrap();
+    }
+}
+
+pub fn available_parallelism() -> io::Result<NonZeroUsize> {
+    // We're unicore right now.
+    Ok(unsafe { NonZeroUsize::new_unchecked(1) })
+}
+
+pub mod guard {
+    pub type Guard = !;
+    pub unsafe fn current() -> Option<Guard> {
+        None
+    }
+    pub unsafe fn init() -> Option<Guard> {
+        None
+    }
+}
diff --git a/library/std/src/sys/xous/thread_local_key.rs b/library/std/src/sys/xous/thread_local_key.rs
new file mode 100644
index 00000000000..3771ea65700
--- /dev/null
+++ b/library/std/src/sys/xous/thread_local_key.rs
@@ -0,0 +1,190 @@
+use crate::mem::ManuallyDrop;
+use crate::ptr;
+use crate::sync::atomic::AtomicPtr;
+use crate::sync::atomic::AtomicUsize;
+use crate::sync::atomic::Ordering::SeqCst;
+use core::arch::asm;
+
+use crate::os::xous::ffi::{map_memory, unmap_memory, MemoryFlags};
+
+/// Thread Local Storage
+///
+/// Currently, we are limited to 1023 TLS entries. The entries
+/// live in a page of memory that's unique per-process, and is
+/// stored in the `$tp` register. If this register is 0, then
+/// TLS has not been initialized and thread cleanup can be skipped.
+///
+/// The index into this register is the `key`. This key is identical
+/// between all threads, but indexes a different offset within this
+/// pointer.
+pub type Key = usize;
+
+pub type Dtor = unsafe extern "C" fn(*mut u8);
+
+const TLS_MEMORY_SIZE: usize = 4096;
+
+/// TLS keys start at `1` to mimic POSIX.
+static TLS_KEY_INDEX: AtomicUsize = AtomicUsize::new(1);
+
+fn tls_ptr_addr() -> *mut usize {
+    let mut tp: usize;
+    unsafe {
+        asm!(
+            "mv {}, tp",
+            out(reg) tp,
+        );
+    }
+    core::ptr::from_exposed_addr_mut::<usize>(tp)
+}
+
+/// Create an area of memory that's unique per thread. This area will
+/// contain all thread local pointers.
+fn tls_ptr() -> *mut usize {
+    let mut tp = tls_ptr_addr();
+
+    // If the TP register is `0`, then this thread hasn't initialized
+    // its TLS yet. Allocate a new page to store this memory.
+    if tp.is_null() {
+        tp = unsafe {
+            map_memory(
+                None,
+                None,
+                TLS_MEMORY_SIZE / core::mem::size_of::<usize>(),
+                MemoryFlags::R | MemoryFlags::W,
+            )
+        }
+        .expect("Unable to allocate memory for thread local storage")
+        .as_mut_ptr();
+
+        unsafe {
+            // Key #0 is currently unused.
+            (tp).write_volatile(0);
+
+            // Set the thread's `$tp` register
+            asm!(
+                "mv tp, {}",
+                in(reg) tp as usize,
+            );
+        }
+    }
+    tp
+}
+
+/// Allocate a new TLS key. These keys are shared among all threads.
+fn tls_alloc() -> usize {
+    TLS_KEY_INDEX.fetch_add(1, SeqCst)
+}
+
+#[inline]
+pub unsafe fn create(dtor: Option<Dtor>) -> Key {
+    let key = tls_alloc();
+    if let Some(f) = dtor {
+        unsafe { register_dtor(key, f) };
+    }
+    key
+}
+
+#[inline]
+pub unsafe fn set(key: Key, value: *mut u8) {
+    assert!((key < 1022) && (key >= 1));
+    unsafe { tls_ptr().add(key).write_volatile(value as usize) };
+}
+
+#[inline]
+pub unsafe fn get(key: Key) -> *mut u8 {
+    assert!((key < 1022) && (key >= 1));
+    core::ptr::from_exposed_addr_mut::<u8>(unsafe { tls_ptr().add(key).read_volatile() })
+}
+
+#[inline]
+pub unsafe fn destroy(_key: Key) {
+    panic!("can't destroy keys on Xous");
+}
+
+// -------------------------------------------------------------------------
+// Dtor registration (stolen from Windows)
+//
+// Xous has no native support for running destructors so we manage our own
+// list of destructors to keep track of how to destroy keys. We then install a
+// callback later to get invoked whenever a thread exits, running all
+// appropriate destructors.
+//
+// Currently unregistration from this list is not supported. A destructor can be
+// registered but cannot be unregistered. There's various simplifying reasons
+// for doing this, the big ones being:
+//
+// 1. Currently we don't even support deallocating TLS keys, so normal operation
+//    doesn't need to deallocate a destructor.
+// 2. There is no point in time where we know we can unregister a destructor
+//    because it could always be getting run by some remote thread.
+//
+// Typically processes have a statically known set of TLS keys which is pretty
+// small, and we'd want to keep this memory alive for the whole process anyway
+// really.
+//
+// Perhaps one day we can fold the `Box` here into a static allocation,
+// expanding the `StaticKey` structure to contain not only a slot for the TLS
+// key but also a slot for the destructor queue on windows. An optimization for
+// another day!
+
+static DTORS: AtomicPtr<Node> = AtomicPtr::new(ptr::null_mut());
+
+struct Node {
+    dtor: Dtor,
+    key: Key,
+    next: *mut Node,
+}
+
+unsafe fn register_dtor(key: Key, dtor: Dtor) {
+    let mut node = ManuallyDrop::new(Box::new(Node { key, dtor, next: ptr::null_mut() }));
+
+    let mut head = DTORS.load(SeqCst);
+    loop {
+        node.next = head;
+        match DTORS.compare_exchange(head, &mut **node, SeqCst, SeqCst) {
+            Ok(_) => return, // nothing to drop, we successfully added the node to the list
+            Err(cur) => head = cur,
+        }
+    }
+}
+
+pub unsafe fn destroy_tls() {
+    let tp = tls_ptr_addr();
+
+    // If the pointer address is 0, then this thread has no TLS.
+    if tp.is_null() {
+        return;
+    }
+    unsafe { run_dtors() };
+
+    // Finally, free the TLS array
+    unsafe {
+        unmap_memory(core::slice::from_raw_parts_mut(
+            tp,
+            TLS_MEMORY_SIZE / core::mem::size_of::<usize>(),
+        ))
+        .unwrap()
+    };
+}
+
+unsafe fn run_dtors() {
+    let mut any_run = true;
+    for _ in 0..5 {
+        if !any_run {
+            break;
+        }
+        any_run = false;
+        let mut cur = DTORS.load(SeqCst);
+        while !cur.is_null() {
+            let ptr = unsafe { get((*cur).key) };
+
+            if !ptr.is_null() {
+                unsafe { set((*cur).key, ptr::null_mut()) };
+                unsafe { ((*cur).dtor)(ptr as *mut _) };
+                any_run = true;
+            }
+
+            unsafe { cur = (*cur).next };
+        }
+    }
+}
diff --git a/library/std/src/sys/xous/time.rs b/library/std/src/sys/xous/time.rs
new file mode 100644
index 00000000000..4e4ae67efff
--- /dev/null
+++ b/library/std/src/sys/xous/time.rs
@@ -0,0 +1,57 @@
+use crate::os::xous::ffi::blocking_scalar;
+use crate::os::xous::services::{
+    systime_server, ticktimer_server, SystimeScalar::GetUtcTimeMs, TicktimerScalar::ElapsedMs,
+};
+use crate::time::Duration;
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub struct Instant(Duration);
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+pub struct SystemTime(Duration);
+
+pub const UNIX_EPOCH: SystemTime = SystemTime(Duration::from_secs(0));
+
+impl Instant {
+    pub fn now() -> Instant {
+        let result = blocking_scalar(ticktimer_server(), ElapsedMs.into())
+            .expect("failed to request elapsed_ms");
+        let lower = result[0];
+        let upper = result[1];
+        Instant { 0: Duration::from_millis(lower as u64 | (upper as u64) << 32) }
+    }
+
+    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
+        self.0.checked_sub(other.0)
+    }
+
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {
+        self.0.checked_add(*other).map(Instant)
+    }
+
+    pub fn checked_sub_duration(&self, other: &Duration) -> Option<Instant> {
+        self.0.checked_sub(*other).map(Instant)
+    }
+}
+
+impl SystemTime {
+    pub fn now() -> SystemTime {
+        let result = blocking_scalar(systime_server(), GetUtcTimeMs.into())
+            .expect("failed to request utc time in ms");
+        let lower = result[0];
+        let upper = result[1];
+        SystemTime { 0: Duration::from_millis((upper as u64) << 32 | lower as u64) }
+    }
+
+    pub fn sub_time(&self, other: &SystemTime) -> Result<Duration, Duration> {
+        self.0.checked_sub(other.0).ok_or_else(|| other.0 - self.0)
+    }
+
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+        Some(SystemTime(self.0.checked_add(*other)?))
+    }
+
+    pub fn checked_sub_duration(&self, other: &Duration) -> Option<SystemTime> {
+        Some(SystemTime(self.0.checked_sub(*other)?))
+    }
+}