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>
73 lines
2.2 KiB
C
73 lines
2.2 KiB
C
/*
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* Sharing QEMU devices via vhost-user protocol
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*
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* Copyright (c) Coiby Xu <coiby.xu@gmail.com>.
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* Copyright (c) 2020 Red Hat, Inc.
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or
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* later. See the COPYING file in the top-level directory.
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*/
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#ifndef VHOST_USER_SERVER_H
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#define VHOST_USER_SERVER_H
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#include "subprojects/libvhost-user/libvhost-user.h" /* only for the type definitions */
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#include "io/channel-socket.h"
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#include "io/channel-file.h"
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#include "io/net-listener.h"
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#include "qapi/error.h"
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#include "standard-headers/linux/virtio_blk.h"
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/* A kick fd that we monitor on behalf of libvhost-user */
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typedef struct VuFdWatch {
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VuDev *vu_dev;
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int fd; /*kick fd*/
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void *pvt;
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vu_watch_cb cb;
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QTAILQ_ENTRY(VuFdWatch) next;
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} VuFdWatch;
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/**
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* VuServer:
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* A vhost-user server instance with user-defined VuDevIface callbacks.
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* Vhost-user device backends can be implemented using VuServer. VuDevIface
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* callbacks and virtqueue kicks run in the given AioContext.
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*/
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typedef struct {
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QIONetListener *listener;
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QEMUBH *restart_listener_bh;
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AioContext *ctx;
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int max_queues;
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const VuDevIface *vu_iface;
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unsigned int in_flight; /* atomic */
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/* Protected by ctx lock */
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bool in_qio_channel_yield;
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bool wait_idle;
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VuDev vu_dev;
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QIOChannel *ioc; /* The I/O channel with the client */
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QIOChannelSocket *sioc; /* The underlying data channel with the client */
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QTAILQ_HEAD(, VuFdWatch) vu_fd_watches;
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Coroutine *co_trip; /* coroutine for processing VhostUserMsg */
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} VuServer;
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bool vhost_user_server_start(VuServer *server,
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SocketAddress *unix_socket,
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AioContext *ctx,
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uint16_t max_queues,
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const VuDevIface *vu_iface,
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Error **errp);
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void vhost_user_server_stop(VuServer *server);
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void vhost_user_server_inc_in_flight(VuServer *server);
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void vhost_user_server_dec_in_flight(VuServer *server);
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bool vhost_user_server_has_in_flight(VuServer *server);
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void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx);
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void vhost_user_server_detach_aio_context(VuServer *server);
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#endif /* VHOST_USER_SERVER_H */
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