06e0f098d6
The ongoing QEMU multi-queue block layer effort makes it possible for multiple threads to process I/O in parallel. The nbd block driver is not compatible with the multi-queue block layer yet because QIOChannel cannot be used easily from coroutines running in multiple threads. This series changes the QIOChannel API to make that possible. In the current API, calling qio_channel_attach_aio_context() sets the AioContext where qio_channel_yield() installs an fd handler prior to yielding: qio_channel_attach_aio_context(ioc, my_ctx); ... qio_channel_yield(ioc); // my_ctx is used here ... qio_channel_detach_aio_context(ioc); This API design has limitations: reading and writing must be done in the same AioContext and moving between AioContexts involves a cumbersome sequence of API calls that is not suitable for doing on a per-request basis. There is no fundamental reason why a QIOChannel needs to run within the same AioContext every time qio_channel_yield() is called. QIOChannel only uses the AioContext while inside qio_channel_yield(). The rest of the time, QIOChannel is independent of any AioContext. In the new API, qio_channel_yield() queries the AioContext from the current coroutine using qemu_coroutine_get_aio_context(). There is no need to explicitly attach/detach AioContexts anymore and qio_channel_attach_aio_context() and qio_channel_detach_aio_context() are gone. One coroutine can read from the QIOChannel while another coroutine writes from a different AioContext. This API change allows the nbd block driver to use QIOChannel from any thread. It's important to keep in mind that the block driver already synchronizes QIOChannel access and ensures that two coroutines never read simultaneously or write simultaneously. This patch updates all users of qio_channel_attach_aio_context() to the new API. Most conversions are simple, but vhost-user-server requires a new qemu_coroutine_yield() call to quiesce the vu_client_trip() coroutine when not attached to any AioContext. While the API is has become simpler, there is one wart: QIOChannel has a special case for the iohandler AioContext (used for handlers that must not run in nested event loops). I didn't find an elegant way preserve that behavior, so I added a new API called qio_channel_set_follow_coroutine_ctx(ioc, true|false) for opting in to the new AioContext model. By default QIOChannel uses the iohandler AioHandler. Code that formerly called qio_channel_attach_aio_context() now calls qio_channel_set_follow_coroutine_ctx(ioc, true) once after the QIOChannel is created. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Acked-by: Daniel P. Berrangé <berrange@redhat.com> Message-ID: <20230830224802.493686-5-stefanha@redhat.com> [eblake: also fix migration/rdma.c] Signed-off-by: Eric Blake <eblake@redhat.com>
76 lines
2.7 KiB
C
76 lines
2.7 KiB
C
/*
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* QEMU I/O channels utility APIs
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*
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* Copyright (c) 2016 Red Hat, Inc.
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, see <http://www.gnu.org/licenses/>.
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*
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*/
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#ifndef QIO_CHANNEL_UTIL_H
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#define QIO_CHANNEL_UTIL_H
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#include "io/channel.h"
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/*
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* This module provides helper functions that are useful when dealing
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* with QIOChannel objects
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*/
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/**
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* qio_channel_new_fd:
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* @fd: the file descriptor
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* @errp: pointer to a NULL-initialized error object
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*
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* Create a channel for performing I/O on the file
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* descriptor @fd. The particular subclass of QIOChannel
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* that is returned will depend on what underlying object
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* the file descriptor is associated with. It may be either
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* a QIOChannelSocket or a QIOChannelFile instance. Upon
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* success, the returned QIOChannel instance will own
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* the @fd file descriptor, and take responsibility for
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* closing it when no longer required. On failure, the
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* caller is responsible for closing @fd.
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*
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* Returns: the channel object, or NULL on error
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*/
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QIOChannel *qio_channel_new_fd(int fd,
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Error **errp);
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/**
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* qio_channel_util_set_aio_fd_handler:
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* @read_fd: the file descriptor for the read handler
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* @read_ctx: the AioContext for the read handler
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* @io_read: the read handler
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* @write_fd: the file descriptor for the write handler
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* @write_ctx: the AioContext for the write handler
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* @io_write: the write handler
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* @opaque: the opaque argument to the read and write handler
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*
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* Set the read and write handlers when @read_ctx and @write_ctx are non-NULL,
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* respectively. To leave a handler in its current state, pass a NULL
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* AioContext. To clear a handler, pass a non-NULL AioContext and a NULL
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* handler.
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*/
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void qio_channel_util_set_aio_fd_handler(int read_fd,
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AioContext *read_ctx,
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IOHandler *io_read,
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int write_fd,
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AioContext *write_ctx,
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IOHandler *io_write,
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void *opaque);
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#endif /* QIO_CHANNEL_UTIL_H */
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