On Tue, Feb 18, 2025 at 07:20:12PM +0100, Kevin Wolf wrote: > This adds helper functions that allow running Rust futures to completion > using QEMU's event loops.
This commit is a cliff-hanger. I'm intrigued to find out how timer, fd,
etc event loop integration will work :).
>
> Signed-off-by: Kevin Wolf <kw...@redhat.com>
> ---
> include/qemu/coroutine-rust.h | 24 +++++++++++
> rust/wrapper.h | 1 +
> util/qemu-co-rust-async.c | 55 +++++++++++++++++++++++++
> rust/qemu-api/meson.build | 1 +
> rust/qemu-api/src/futures.rs | 77 +++++++++++++++++++++++++++++++++++
> rust/qemu-api/src/lib.rs | 1 +
> util/meson.build | 3 ++
> 7 files changed, 162 insertions(+)
> create mode 100644 include/qemu/coroutine-rust.h
> create mode 100644 util/qemu-co-rust-async.c
> create mode 100644 rust/qemu-api/src/futures.rs
>
> diff --git a/include/qemu/coroutine-rust.h b/include/qemu/coroutine-rust.h
> new file mode 100644
> index 0000000000..0c5cf42a6b
> --- /dev/null
> +++ b/include/qemu/coroutine-rust.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Helpers to run Rust futures using QEMU coroutines
> + *
> + * Copyright Red Hat
> + *
> + * Author:
> + * Kevin Wolf <kw...@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or
> later.
> + * See the COPYING.LIB file in the top-level directory.
> + */
> +
> +#ifndef QEMU_COROUTINE_RUST_H
> +#define QEMU_COROUTINE_RUST_H
> +
> +typedef struct RustBoxedFuture RustBoxedFuture;
> +typedef void coroutine_fn RunFuture(RustBoxedFuture *future, void *opaque);
> +
> +void no_coroutine_fn rust_run_future(RustBoxedFuture *future,
> + RunFuture *entry,
> + void *opaque);
This adds a blocking (aio_poll()-style) API. The more blocking APIs we
add, the more points are created where QEMU hangs when the async
operation doesn't complete in a reasonable amount of time. It would be
best to avoid introducing new blocking APIs, but sometimes it is
unavoidable.
rust_run_future() is very generic and I think the downsides should be
pointed out to discourage people from using it when not absolutely
necessary. Can you document when it's appropriate to use this API?
> +
> +#endif
> diff --git a/rust/wrapper.h b/rust/wrapper.h
> index 303d7bba7f..3dc385e256 100644
> --- a/rust/wrapper.h
> +++ b/rust/wrapper.h
> @@ -58,3 +58,4 @@ typedef enum memory_order {
> #include "block/block_int.h"
> #include "block/qdict.h"
> #include "qapi/qapi-visit-block-core.h"
> +#include "qemu/coroutine-rust.h"
> diff --git a/util/qemu-co-rust-async.c b/util/qemu-co-rust-async.c
> new file mode 100644
> index 0000000000..d893dfb7bd
> --- /dev/null
> +++ b/util/qemu-co-rust-async.c
> @@ -0,0 +1,55 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Helpers to run Rust futures using QEMU coroutines
> + *
> + * Copyright Red Hat
> + *
> + * Author:
> + * Kevin Wolf <kw...@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or
> later.
> + * See the COPYING.LIB file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +
> +#include "block/aio-wait.h"
> +#include "qemu/coroutine.h"
> +#include "qemu/coroutine-rust.h"
> +#include "qemu/main-loop.h"
> +
> +typedef struct FutureCo {
> + RustBoxedFuture *future;
> + RunFuture *entry;
> + void *opaque;
> + bool done;
> +} FutureCo;
> +
> +static void coroutine_fn rust_co_run_future_entry(void *opaque)
> +{
> + FutureCo *data = opaque;
> +
> + data->entry(data->future, data->opaque);
> + data->done = true;
> + aio_wait_kick();
> +}
> +
> +void no_coroutine_fn rust_run_future(RustBoxedFuture *future,
> + RunFuture *entry,
> + void *opaque)
> +{
> + AioContext *ctx = qemu_get_current_aio_context();
> + Coroutine *co;
> + FutureCo data = {
> + .future = future,
> + .entry = entry,
> + .opaque = opaque,
> + .done = false,
> + };
> +
> + GLOBAL_STATE_CODE();
> +
> + co = qemu_coroutine_create(rust_co_run_future_entry, &data);
> + aio_co_enter(ctx, co);
> + AIO_WAIT_WHILE(ctx, !data.done);
> +}
> diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
> index e0a3052c79..44fd34e193 100644
> --- a/rust/qemu-api/meson.build
> +++ b/rust/qemu-api/meson.build
> @@ -22,6 +22,7 @@ sources_core = [
> 'src/chardev.rs',
> 'src/c_str.rs',
> 'src/errno.rs',
> + 'src/futures.rs',
> 'src/module.rs',
> 'src/offset_of.rs',
> 'src/prelude.rs',
> diff --git a/rust/qemu-api/src/futures.rs b/rust/qemu-api/src/futures.rs
> new file mode 100644
> index 0000000000..cd307a1d62
> --- /dev/null
> +++ b/rust/qemu-api/src/futures.rs
> @@ -0,0 +1,77 @@
> +use crate::bindings;
> +use std::ffi::c_void;
> +use std::future::Future;
> +use std::mem::MaybeUninit;
> +use std::sync::Arc;
> +use std::task::{Context, Poll, Wake, Waker};
> +
> +struct RunFutureWaker {
> + co: *mut bindings::Coroutine,
> +}
> +unsafe impl Send for RunFutureWaker {}
> +unsafe impl Sync for RunFutureWaker {}
> +
> +impl Wake for RunFutureWaker {
> + fn wake(self: Arc<Self>) {
> + unsafe {
> + bindings::aio_co_wake(self.co);
> + }
> + }
> +}
> +
> +/// Use QEMU's event loops to run a Rust [`Future`] to completion and return
> its result.
> +///
> +/// This function must be called in coroutine context. If the future isn't
> ready yet, it yields.
> +pub fn qemu_co_run_future<F: Future>(future: F) -> F::Output {
> + let waker = Waker::from(Arc::new(RunFutureWaker {
> + co: unsafe { bindings::qemu_coroutine_self() },
> + }));
> + let mut cx = Context::from_waker(&waker);
> +
> + let mut pinned_future = std::pin::pin!(future);
> + loop {
> + match pinned_future.as_mut().poll(&mut cx) {
> + Poll::Ready(res) => return res,
> + Poll::Pending => unsafe {
> + bindings::qemu_coroutine_yield();
> + },
> + }
> + }
> +}
> +
> +/// Wrapper around [`qemu_co_run_future`] that can be called from C.
> +///
> +/// # Safety
> +///
> +/// `future` must be a valid pointer to an owned `F` (it will be freed in
> this function). `output`
> +/// must be a valid pointer representing a mutable reference to an
> `F::Output` where the result can
> +/// be stored.
> +unsafe extern "C" fn rust_co_run_future<F: Future>(
> + future: *mut bindings::RustBoxedFuture,
> + output: *mut c_void,
> +) {
> + let future = unsafe { Box::from_raw(future.cast::<F>()) };
> + let output = output.cast::<F::Output>();
> + let ret = qemu_co_run_future(*future);
> + unsafe {
> + output.write(ret);
> + }
> +}
> +
> +/// Use QEMU's event loops to run a Rust [`Future`] to completion and return
> its result.
> +///
> +/// This function must be called outside of coroutine context to avoid
> deadlocks. It blocks and
rust_run_future() has GLOBAL_STATE_CODE() so qemu_run_future() needs to
run not just outside coroutine context, but also under the BQL. Should
this be mentioned?
> +/// runs a nested even loop until the future is ready and returns a result.
> +pub fn qemu_run_future<F: Future>(future: F) -> F::Output {
> + let future_ptr = Box::into_raw(Box::new(future));
> + let mut output = MaybeUninit::<F::Output>::uninit();
> + unsafe {
> + bindings::rust_run_future(
> + future_ptr.cast::<bindings::RustBoxedFuture>(),
> + #[allow(clippy::as_underscore)]
> + Some(rust_co_run_future::<F> as _),
This line is hard to follow. I think it's casting to the C equivalent
type:
void coroutine_fn (*)(RustBoxedFuture *future, void *opaque)
I wonder if there's a clearer way of writing this. Maybe being explicit
rather than implicit here would be helpful.
If not, it's not a big deal, but I spent some time trying to figure out
what this does and others might too.
> + output.as_mut_ptr().cast::<c_void>(),
> + );
> + output.assume_init()
> + }
> +}
> diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
> index 825443abde..84928905f1 100644
> --- a/rust/qemu-api/src/lib.rs
> +++ b/rust/qemu-api/src/lib.rs
> @@ -20,6 +20,7 @@
> pub mod cell;
> pub mod chardev;
> pub mod errno;
> +pub mod futures;
> #[cfg(feature = "system")]
> pub mod irq;
> #[cfg(feature = "system")]
> diff --git a/util/meson.build b/util/meson.build
> index 780b5977a8..14a2ae17fd 100644
> --- a/util/meson.build
> +++ b/util/meson.build
> @@ -101,6 +101,9 @@ if have_block
> util_ss.add(files('qemu-coroutine-sleep.c'))
> util_ss.add(files('qemu-co-shared-resource.c'))
> util_ss.add(files('qemu-co-timeout.c'))
> + if have_rust
> + util_ss.add(files('qemu-co-rust-async.c'))
> + endif
> util_ss.add(files('readline.c'))
> util_ss.add(files('throttle.c'))
> util_ss.add(files('timed-average.c'))
> --
> 2.48.1
>
>
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