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 <s...@redhat.com>
> Cc: Kevin Wolf <kw...@redhat.com>
> Cc: Emanuele Giuseppe Esposito <eespo...@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
Acked-by: Michael S. Tsirkin <m...@redhat.com>
block tree I presume?
> ---
> 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
