Commits on Jul 12, 2023

1. virtio-blk: fix host notifier issues during dataplane start/stop …

   The main loop thread can consume 100% CPU when using --device
   virtio-blk-pci,iothread=<iothread>. ppoll() constantly returns but
   reading virtqueue host notifiers fails with EAGAIN. The file descriptors
   are stale and remain registered with the AioContext because of bugs in
   the virtio-blk dataplane start/stop code.
   
   The problem is that the dataplane start/stop code involves drain
   operations, which call virtio_blk_drained_begin() and
   virtio_blk_drained_end() at points where the host notifier is not
   operational:
   - In virtio_blk_data_plane_start(), blk_set_aio_context() drains after
     vblk->dataplane_started has been set to true but the host notifier has
     not been attached yet.
   - In virtio_blk_data_plane_stop(), blk_drain() and blk_set_aio_context()
     drain after the host notifier has already been detached but with
     vblk->dataplane_started still set to true.
   
   I would like to simplify ->ioeventfd_start/stop() to avoid interactions
   with drain entirely, but couldn't find a way to do that. Instead, this
   patch accepts the fragile nature of the code and reorders it so that
   vblk->dataplane_started is false during drain operations. This way the
   virtio_blk_drained_begin() and virtio_blk_drained_end() calls don't
   touch the host notifier. The result is that
   virtio_blk_data_plane_start() and virtio_blk_data_plane_stop() have
   complete control over the host notifier and stale file descriptors are
   no longer left in the AioContext.
   
   This patch fixes the 100% CPU consumption in the main loop thread and
   correctly moves host notifier processing to the IOThread.
   
   Fixes: 1665d93 ("virtio-blk: implement BlockDevOps->drained_begin()")
   Reported-by: Lukáš Doktor <ldoktor@redhat.com>
   Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
   Tested-by: Lukas Doktor <ldoktor@redhat.com>
   Message-id: 20230704151527.193586-1-stefanha@redhat.com
   Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

   Stefan Hajnoczi committed Jul 12, 2023
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2. Merge tag 'block-pull-request' of https://gitlab.com/stefanha/qemu in… …

   …to staging
   
   Pull request
   
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   * tag 'block-pull-request' of https://gitlab.com/stefanha/qemu:
     virtio-blk: fix host notifier issues during dataplane start/stop
   
   Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

   

   rth7680 committed Jul 12, 2023
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