CVS commit: src/sys/dev/pci

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Fri Aug 12 10:49:58 UTC 2022

Modified Files:
        src/sys/dev/pci: virtio.c

Log Message:
virtio(4): Membar and bus_dmamap_sync audit.

- Don't use membar_* for DMA.

- Sync only the header and payload of the rings separately as needed.
  If we bounce, this avoids large memcpy when we only care about the
  header.

- Sync uring with PREREAD before triggering anything that will return
  data in it.

  => Move uring PREREAD in virtio_enqueue_commit to _before_ updating
     vq->vq_avail->idx, which is the pointat which the DMA read is
     triggered in the `device' (host).

  => Omit needless membar_consumer in virtio_enqueue_commit -- may not
     work with DMA memory, and even if it does, redundant with
     bus_dmamap_sync uring PREREAD here.

  => XXX Does the device/host ever return unsolicited entries in the
     queue, or only solicited ones?  If only solicited ones, the
     PREREAD in virtio_init_vq is redundant.

- Sync uring with POSTREAD before we read from it.  This way the DMA
  read into our buffer has finished before we read from the buffer.

  => Add missing uring POSTREAD in virtio_vq_is_enqueued, between read of
     vq->vq_used_idx and return to caller, so that the caller can
     safely use virtio_dequeue.

  => Add missing uring POSTREADs in virtio_start_vq_intr:
     . between entry from caller and the read of vq->vq_used_idx
     . between the read of vq->vq_used_idx and return to caller,
       so that the caller can safely use virtio_dequeue, just like
       virtio_vq_is_enqueued

  => Move uring POSTREADs in virtio_enqueue_commit to _before_ reading
     vq->vq_used->flags or *vq->vq_avail_event, not after.

- After we write to aring, sync it with PREWRITE.  This way we finish
  writing to our buffer before the DMA write from it.

  => Omit needless PREWRITE in virtio_init_vq -- we do the appropriate
     PREWRITE in virtio_enqueue_commit now.

  => Convert membar_producer to bus_dmamap_sync PREWRITE in
     virtio_enqueue_commit.

  => Omit incorrect aring POSTWRITE in virtio_enqueue_commit -- no need
     because the DMA write may not have completed yet at this point,
     and we already do a POSTWRITE in virtio_vq_is_enqueued.

  => Omit needless membar_producer in virtio_postpone_intr -- may not
     work with DMA memory, and even if it does, redundant with
     bus_dmamap_sync PREWRITE here.

- After xfers to aring have completed, sync it with POSTWRITE.

  => Add missing aring POSTWRITE in virtio_free_vq, in case there are
     paths from virtio_enqueue_commit to here that don't go through
     virtio_is_enqueued.  (If there are no such paths, then maybe we
     should KASSERT(vq->vq_queued == 0) in virtio_free_vq.)


To generate a diff of this commit:
cvs rdiff -u -r1.56 -r1.57 src/sys/dev/pci/virtio.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.