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Tsirkin" To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Kevin Wolf , Hanna Reitz , Sergio Lopez , Emanuele Giuseppe Esposito Subject: Re: [PATCH] virtio-blk: simplify virtio_blk_dma_restart_cb() Message-ID: <20221102195314-mutt-send-email-mst@kernel.org> References: <20221102182337.252202-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221102182337.252202-1-stefanha@redhat.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -37 X-Spam_score: -3.8 X-Spam_bar: --- X-Spam_report: (-3.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.048, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Qemu-devel" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Nov 02, 2022 at 02:23:37PM -0400, Stefan Hajnoczi wrote: > virtio_blk_dma_restart_cb() is tricky because the BH must deal with > virtio_blk_data_plane_start()/virtio_blk_data_plane_stop() being called. > > There are two issues with the code: > > 1. virtio_blk_realize() should use qdev_add_vm_change_state_handler() > instead of qemu_add_vm_change_state_handler(). This ensures the > ordering with virtio_init()'s vm change state handler that calls > virtio_blk_data_plane_start()/virtio_blk_data_plane_stop() is > well-defined. Then blk's AioContext is guaranteed to be up-to-date in > virtio_blk_dma_restart_cb() and it's no longer necessary to have a > special case for virtio_blk_data_plane_start(). > > 2. Only blk_drain() waits for virtio_blk_dma_restart_cb()'s > blk_inc_in_flight() to be decremented. The bdrv_drain() family of > functions do not wait for BlockBackend's in_flight counter to reach > zero. virtio_blk_data_plane_stop() relies on blk_set_aio_context()'s > implicit drain, but that's a bdrv_drain() and not a blk_drain(). > Note that virtio_blk_reset() already correctly relies on blk_drain(). > If virtio_blk_data_plane_stop() switches to blk_drain() then we can > properly wait for pending virtio_blk_dma_restart_bh() calls. > > Once these issues are taken care of the code becomes simpler. This > change is in preparation for multiple IOThreads in virtio-blk where we > need to clean up the multi-threading behavior. > > I ran the reproducer from commit 49b44549ace7 ("virtio-blk: On restart, > process queued requests in the proper context") to check that there is > no regression. > > Cc: Sergio Lopez > Cc: Kevin Wolf > Cc: Emanuele Giuseppe Esposito > Signed-off-by: Stefan Hajnoczi Acked-by: Michael S. Tsirkin > --- > include/hw/virtio/virtio-blk.h | 2 -- > hw/block/dataplane/virtio-blk.c | 17 +++++------- > hw/block/virtio-blk.c | 46 ++++++++++++++------------------- > 3 files changed, 26 insertions(+), 39 deletions(-) > > diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h > index 7f589b4146..dafec432ce 100644 > --- a/include/hw/virtio/virtio-blk.h > +++ b/include/hw/virtio/virtio-blk.h > @@ -55,7 +55,6 @@ struct VirtIOBlock { > VirtIODevice parent_obj; > BlockBackend *blk; > void *rq; > - QEMUBH *bh; > VirtIOBlkConf conf; > unsigned short sector_mask; > bool original_wce; > @@ -93,6 +92,5 @@ typedef struct MultiReqBuffer { > } MultiReqBuffer; > > void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq); > -void virtio_blk_process_queued_requests(VirtIOBlock *s, bool is_bh); > > #endif > diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c > index 26f965cabc..b28d81737e 100644 > --- a/hw/block/dataplane/virtio-blk.c > +++ b/hw/block/dataplane/virtio-blk.c > @@ -237,9 +237,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) > goto fail_aio_context; > } > > - /* Process queued requests before the ones in vring */ > - virtio_blk_process_queued_requests(vblk, false); > - > /* Kick right away to begin processing requests already in vring */ > for (i = 0; i < nvqs; i++) { > VirtQueue *vq = virtio_get_queue(s->vdev, i); > @@ -272,11 +269,6 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) > fail_host_notifiers: > k->set_guest_notifiers(qbus->parent, nvqs, false); > fail_guest_notifiers: > - /* > - * If we failed to set up the guest notifiers queued requests will be > - * processed on the main context. > - */ > - virtio_blk_process_queued_requests(vblk, false); > vblk->dataplane_disabled = true; > s->starting = false; > vblk->dataplane_started = true; > @@ -325,8 +317,13 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev) > aio_context_acquire(s->ctx); > aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s); > > - /* Drain and try to switch bs back to the QEMU main loop. If other users > - * keep the BlockBackend in the iothread, that's ok */ > + /* Wait for virtio_blk_dma_restart_bh() and in flight I/O to complete */ > + blk_drain(s->conf->conf.blk); > + > + /* > + * Try to switch bs back to the QEMU main loop. If other users keep the > + * BlockBackend in the iothread, that's ok > + */ > blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context(), NULL); > > aio_context_release(s->ctx); > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c > index f717550fdc..1762517878 100644 > --- a/hw/block/virtio-blk.c > +++ b/hw/block/virtio-blk.c > @@ -806,8 +806,10 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq) > virtio_blk_handle_vq(s, vq); > } > > -void virtio_blk_process_queued_requests(VirtIOBlock *s, bool is_bh) > +static void virtio_blk_dma_restart_bh(void *opaque) > { > + VirtIOBlock *s = opaque; > + > VirtIOBlockReq *req = s->rq; > MultiReqBuffer mrb = {}; > > @@ -834,43 +836,27 @@ void virtio_blk_process_queued_requests(VirtIOBlock *s, bool is_bh) > if (mrb.num_reqs) { > virtio_blk_submit_multireq(s, &mrb); > } > - if (is_bh) { > - blk_dec_in_flight(s->conf.conf.blk); > - } > + > + /* Paired with inc in virtio_blk_dma_restart_cb() */ > + blk_dec_in_flight(s->conf.conf.blk); > + > aio_context_release(blk_get_aio_context(s->conf.conf.blk)); > } > > -static void virtio_blk_dma_restart_bh(void *opaque) > -{ > - VirtIOBlock *s = opaque; > - > - qemu_bh_delete(s->bh); > - s->bh = NULL; > - > - virtio_blk_process_queued_requests(s, true); > -} > - > static void virtio_blk_dma_restart_cb(void *opaque, bool running, > RunState state) > { > VirtIOBlock *s = opaque; > - BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s))); > - VirtioBusState *bus = VIRTIO_BUS(qbus); > > if (!running) { > return; > } > > - /* > - * If ioeventfd is enabled, don't schedule the BH here as queued > - * requests will be processed while starting the data plane. > - */ > - if (!s->bh && !virtio_bus_ioeventfd_enabled(bus)) { > - s->bh = aio_bh_new(blk_get_aio_context(s->conf.conf.blk), > - virtio_blk_dma_restart_bh, s); > - blk_inc_in_flight(s->conf.conf.blk); > - qemu_bh_schedule(s->bh); > - } > + /* Paired with dec in virtio_blk_dma_restart_bh() */ > + blk_inc_in_flight(s->conf.conf.blk); > + > + aio_bh_schedule_oneshot(blk_get_aio_context(s->conf.conf.blk), > + virtio_blk_dma_restart_bh, s); > } > > static void virtio_blk_reset(VirtIODevice *vdev) > @@ -1213,7 +1199,13 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp) > return; > } > > - s->change = qemu_add_vm_change_state_handler(virtio_blk_dma_restart_cb, s); > + /* > + * This must be after virtio_init() so virtio_blk_dma_restart_cb() gets > + * called after ->start_ioeventfd() has already set blk's AioContext. > + */ > + s->change = > + qdev_add_vm_change_state_handler(dev, virtio_blk_dma_restart_cb, s); > + > blk_ram_registrar_init(&s->blk_ram_registrar, s->blk); > blk_set_dev_ops(s->blk, &virtio_block_ops, s); > > -- > 2.38.1