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virtio-blk: simplify virtio_blk_dma_restart_cb()

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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 49b44549ac ("virtio-blk: On restart,
process queued requests in the proper context") to check that there is
no regression.

Cc: Sergio Lopez <slp@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-id: 20221102182337.252202-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2022-11-02 14:23:37 -04:00
parent 816a430c51
commit a937f8e857
3 changed files with 26 additions and 39 deletions
Download patch file Download diff file Expand all files Collapse all files

2
include/hw/virtio/virtio-blk.h
View file

@ -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