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From: Paolo Bonzini <pbonzini <at> redhat.com>
Subject: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes
Newsgroups: gmane.comp.emulators.kvm.devel
Date: Thursday 7th February 2013 12:22:24 UTC (over 5 years ago)
The virtqueue_add_buf function has two limitations:

1) it requires the caller to provide all the buffers in a single call;

2) it does not support chained scatterlists: the buffers must be
provided as an array of struct scatterlist.

Because of these limitations, virtio-scsi has to copy each request into
a scatterlist internal to the driver.  It cannot just use the one that
was prepared by the upper SCSI layers.

This series adds a different set of APIs for adding a buffer to a
virtqueue.  The new API lets you pass the buffers piecewise, wrapping
multiple calls to virtqueue_add_sg between virtqueue_start_buf and
virtqueue_end_buf.  Letting drivers call virtqueue_add_sg multiple times
if they already have a scatterlist provided by someone else simplifies the
code and, for virtio-scsi, it saves the copying and the related locking.

One major difference between virtqueue_add_buf and virtqueue_add_sg
is that the latter uses scatterlist iterators, which follow chained
scatterlist structs and stop at ending markers.  In order to avoid code
duplication, and use the new API from virtqueue_add_buf (patch 8), we need
to change all existing callers of virtqueue_add_buf to provide well-formed
scatterlists.  This is what patches 2-7 do.  For virtio-blk it is easiest
to just switch to the new API, just like for virtio-scsi.  For virtio-net
the ending marker must be reset after calling virtqueue_add_buf, in
preparation for the next usage of the scatterlist.  Other drivers are
safe already.

This is an RFC for two reasons.  First, because I haven't done enough
testing yet (especially with all the variations on receiving that
virtio-net has).  Second, because I still have two struct vring_desc *
fields in virtqueue API, which is a layering violation.  I'm not really
sure how important that is and how to fix that---except by making the
fields void*.


Paolo Bonzini (8):
  virtio: add functions for piecewise addition of buffers
  virtio-blk: reorganize virtblk_add_req
  virtio-blk: use virtqueue_start_buf on bio path
  virtio-blk: use virtqueue_start_buf on req path
  scatterlist: introduce sg_unmark_end
  virtio-net: unmark scatterlist ending after virtqueue_add_buf
  virtio-scsi: use virtqueue_start_buf
  virtio: reimplement virtqueue_add_buf using new functions

 block/blk-integrity.c        |    2 +-
 block/blk-merge.c            |    2 +-
 drivers/block/virtio_blk.c   |  165 +++++++++--------
 drivers/net/virtio_net.c     |   21 ++-
 drivers/scsi/virtio_scsi.c   |  103 +++++------
 drivers/virtio/virtio_ring.c |  417
 include/linux/scatterlist.h  |   16 ++
 include/linux/virtio.h       |   25 +++
 8 files changed, 460 insertions(+), 291 deletions(-)
CD: 3ms