rust: chardev: provide basic bindings to character devices

Most of the character device API is pretty simple, with "0 or -errno"
or "number of bytes or -errno" as the convention for return codes.
Add safe wrappers for the API to the CharBackend bindgen-generated
struct.

The API is not complete, but it covers the parts that are used
by the PL011 device, plus qemu_chr_fe_write which is needed to
implement the standard library Write trait.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/chardev.rs

@@ -3,10 +3,28 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 //! Bindings for character devices
+//!
+//! Character devices in QEMU can run under the big QEMU lock or in a separate
+//! `GMainContext`. Here we only support the former, because the bindings
+//! enforce that the BQL is taken whenever the functions in [`CharBackend`] are
+//! called.
 
-use std::ffi::CStr;
+use std::{
+    ffi::CStr,
+    fmt::{self, Debug},
+    io::{self, ErrorKind, Write},
+    marker::PhantomPinned,
+    os::raw::{c_int, c_void},
+    ptr::addr_of_mut,
+    slice,
+};
 
-use crate::{bindings, cell::Opaque, prelude::*};
+use crate::{
+    bindings,
+    callbacks::FnCall,
+    cell::{BqlRefMut, Opaque},
+    prelude::*,
+};
 
 /// A safe wrapper around [`bindings::Chardev`].
 #[repr(transparent)]
@@ -14,6 +32,226 @@ use crate::{bindings, cell::Opaque, prelude::*};
 pub struct Chardev(Opaque<bindings::Chardev>);
 
 pub type ChardevClass = bindings::ChardevClass;
+pub type Event = bindings::QEMUChrEvent;
+
+/// A safe wrapper around [`bindings::CharBackend`], denoting the character
+/// back-end that is used for example by a device.  Compared to the
+/// underlying C struct it adds BQL protection, and is marked as pinned
+/// because the QOM object ([`bindings::Chardev`]) contains a pointer to
+/// the `CharBackend`.
+pub struct CharBackend {
+    inner: BqlRefCell<bindings::CharBackend>,
+    _pin: PhantomPinned,
+}
+
+impl Write for BqlRefMut<'_, bindings::CharBackend> {
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        let chr: &mut bindings::CharBackend = self;
+
+        let len = buf.len().try_into().unwrap();
+        let r = unsafe { bindings::qemu_chr_fe_write(addr_of_mut!(*chr), buf.as_ptr(), len) };
+        errno::into_io_result(r).map(|cnt| cnt as usize)
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        let chr: &mut bindings::CharBackend = self;
+
+        let len = buf.len().try_into().unwrap();
+        let r = unsafe { bindings::qemu_chr_fe_write_all(addr_of_mut!(*chr), buf.as_ptr(), len) };
+        errno::into_io_result(r).and_then(|cnt| {
+            if cnt as usize == buf.len() {
+                Ok(())
+            } else {
+                Err(ErrorKind::WriteZero.into())
+            }
+        })
+    }
+}
+
+impl Debug for CharBackend {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // SAFETY: accessed just to print the values
+        let chr = self.inner.as_ptr();
+        Debug::fmt(unsafe { &*chr }, f)
+    }
+}
+
+// FIXME: use something like PinnedDrop from the pinned_init crate
+impl Drop for CharBackend {
+    fn drop(&mut self) {
+        self.disable_handlers();
+    }
+}
+
+impl CharBackend {
+    /// Enable the front-end's character device handlers, if there is an
+    /// associated `Chardev`.
+    pub fn enable_handlers<
+        'chardev,
+        'owner: 'chardev,
+        T,
+        CanReceiveFn: for<'a> FnCall<(&'a T,), u32>,
+        ReceiveFn: for<'a, 'b> FnCall<(&'a T, &'b [u8])>,
+        EventFn: for<'a> FnCall<(&'a T, Event)>,
+    >(
+        // When "self" is dropped, the handlers are automatically disabled.
+        // However, this is not necessarily true if the owner is dropped.
+        // So require the owner to outlive the character device.
+        &'chardev self,
+        owner: &'owner T,
+        _can_receive: CanReceiveFn,
+        _receive: ReceiveFn,
+        _event: EventFn,
+    ) {
+        unsafe extern "C" fn rust_can_receive_cb<T, F: for<'a> FnCall<(&'a T,), u32>>(
+            opaque: *mut c_void,
+        ) -> c_int {
+            // SAFETY: the values are safe according to the contract of
+            // enable_handlers() and qemu_chr_fe_set_handlers()
+            let owner: &T = unsafe { &*(opaque.cast::<T>()) };
+            let r = F::call((owner,));
+            r.try_into().unwrap()
+        }
+
+        unsafe extern "C" fn rust_receive_cb<T, F: for<'a, 'b> FnCall<(&'a T, &'b [u8])>>(
+            opaque: *mut c_void,
+            buf: *const u8,
+            size: c_int,
+        ) {
+            // SAFETY: the values are safe according to the contract of
+            // enable_handlers() and qemu_chr_fe_set_handlers()
+            let owner: &T = unsafe { &*(opaque.cast::<T>()) };
+            let buf = unsafe { slice::from_raw_parts(buf, size.try_into().unwrap()) };
+            F::call((owner, buf))
+        }
+
+        unsafe extern "C" fn rust_event_cb<T, F: for<'a> FnCall<(&'a T, Event)>>(
+            opaque: *mut c_void,
+            event: Event,
+        ) {
+            // SAFETY: the values are safe according to the contract of
+            // enable_handlers() and qemu_chr_fe_set_handlers()
+            let owner: &T = unsafe { &*(opaque.cast::<T>()) };
+            F::call((owner, event))
+        }
+
+        let _: () = CanReceiveFn::ASSERT_IS_SOME;
+        let receive_cb: Option<unsafe extern "C" fn(*mut c_void, *const u8, c_int)> =
+            if ReceiveFn::is_some() {
+                Some(rust_receive_cb::<T, ReceiveFn>)
+            } else {
+                None
+            };
+        let event_cb: Option<unsafe extern "C" fn(*mut c_void, Event)> = if EventFn::is_some() {
+            Some(rust_event_cb::<T, EventFn>)
+        } else {
+            None
+        };
+
+        let mut chr = self.inner.borrow_mut();
+        // SAFETY: the borrow promises that the BQL is taken
+        unsafe {
+            bindings::qemu_chr_fe_set_handlers(
+                addr_of_mut!(*chr),
+                Some(rust_can_receive_cb::<T, CanReceiveFn>),
+                receive_cb,
+                event_cb,
+                None,
+                (owner as *const T as *mut T).cast::<c_void>(),
+                core::ptr::null_mut(),
+                true,
+            );
+        }
+    }
+
+    /// Disable the front-end's character device handlers.
+    pub fn disable_handlers(&self) {
+        let mut chr = self.inner.borrow_mut();
+        // SAFETY: the borrow promises that the BQL is taken
+        unsafe {
+            bindings::qemu_chr_fe_set_handlers(
+                addr_of_mut!(*chr),
+                None,
+                None,
+                None,
+                None,
+                core::ptr::null_mut(),
+                core::ptr::null_mut(),
+                true,
+            );
+        }
+    }
+
+    /// Notify that the frontend is ready to receive data.
+    pub fn accept_input(&self) {
+        let mut chr = self.inner.borrow_mut();
+        // SAFETY: the borrow promises that the BQL is taken
+        unsafe { bindings::qemu_chr_fe_accept_input(addr_of_mut!(*chr)) }
+    }
+
+    /// Temporarily borrow the character device, allowing it to be used
+    /// as an implementor of `Write`.  Note that it is not valid to drop
+    /// the big QEMU lock while the character device is borrowed, as
+    /// that might cause C code to write to the character device.
+    pub fn borrow_mut(&self) -> impl Write + '_ {
+        self.inner.borrow_mut()
+    }
+
+    /// Send a continuous stream of zero bits on the line if `enabled` is
+    /// true, or a short stream if `enabled` is false.
+    pub fn send_break(&self, long: bool) -> io::Result<()> {
+        let mut chr = self.inner.borrow_mut();
+        let mut duration: c_int = long.into();
+        // SAFETY: the borrow promises that the BQL is taken
+        let r = unsafe {
+            bindings::qemu_chr_fe_ioctl(
+                addr_of_mut!(*chr),
+                bindings::CHR_IOCTL_SERIAL_SET_BREAK as i32,
+                addr_of_mut!(duration).cast::<c_void>(),
+            )
+        };
+
+        errno::into_io_result(r).map(|_| ())
+    }
+
+    /// Write data to a character backend from the front end.  This function
+    /// will send data from the front end to the back end.  Unlike
+    /// `write`, this function will block if the back end cannot
+    /// consume all of the data attempted to be written.
+    ///
+    /// Returns the number of bytes consumed (0 if no associated Chardev) or an
+    /// error.
+    pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
+        let len = buf.len().try_into().unwrap();
+        // SAFETY: qemu_chr_fe_write is thread-safe
+        let r = unsafe { bindings::qemu_chr_fe_write(self.inner.as_ptr(), buf.as_ptr(), len) };
+        errno::into_io_result(r).map(|cnt| cnt as usize)
+    }
+
+    /// Write data to a character backend from the front end.  This function
+    /// will send data from the front end to the back end.  Unlike
+    /// `write`, this function will block if the back end cannot
+    /// consume all of the data attempted to be written.
+    ///
+    /// Returns the number of bytes consumed (0 if no associated Chardev) or an
+    /// error.
+    pub fn write_all(&self, buf: &[u8]) -> io::Result<()> {
+        let len = buf.len().try_into().unwrap();
+        // SAFETY: qemu_chr_fe_write_all is thread-safe
+        let r = unsafe { bindings::qemu_chr_fe_write_all(self.inner.as_ptr(), buf.as_ptr(), len) };
+        errno::into_io_result(r).and_then(|cnt| {
+            if cnt as usize == buf.len() {
+                Ok(())
+            } else {
+                Err(ErrorKind::WriteZero.into())
+            }
+        })
+    }
+}
 
 unsafe impl ObjectType for Chardev {
     type Class = ChardevClass;

rust/qemu-api/src/zeroable.rs

@@ -106,3 +106,4 @@ impl_zeroable!(crate::bindings::MemoryRegionOps__bindgen_ty_1);
 impl_zeroable!(crate::bindings::MemoryRegionOps__bindgen_ty_2);
 impl_zeroable!(crate::bindings::MemoryRegionOps);
 impl_zeroable!(crate::bindings::MemTxAttrs);
+impl_zeroable!(crate::bindings::CharBackend);