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 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(-) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
