linux-kernel - Re: [RFC PATCH 0/8] virtio: new API for addition of buffers, scatterlist changes

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Message-ID: <20130207132349.GA21707@redhat.com>
Date:	Thu, 7 Feb 2013 15:23:49 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Wanlong Gao <gaowanlong@...fujitsu.com>, asias@...hat.com,
	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 0/8] virtio: new API for addition of buffers,
 scatterlist changes

On Thu, Feb 07, 2013 at 02:14:24PM +0100, Paolo Bonzini wrote:
> Il 07/02/2013 14:09, Michael S. Tsirkin ha scritto:
> >> 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.
> > 
> > What are the changes as compared to the previous version?
> > How about some comments made on the previous version?
> > See e.g.
> > https://patchwork.kernel.org/patch/1891541/
> 
> Two changes: 1) added virtqueue_add_sg_single; 2) reimplemented
> virtqueue_add_buf in terms of the new API, which requires virtio-blk and
> virtio-net changes.
> 
> The virtio-blk and virtio-net changes are based on some ideas in the
> patch Rusty posted, but virtio-net is a bit simpler and virtio-blk was
> redone from scratch.
> 
> > Generally we have code for direct and indirect which is already
> > painful. We do not want 4 more variants of this code.
> 
> Yes, indeed, the other main difference is that I'm now reimplementing
> virtqueue_add_buf using the new functions.  So:
> 
> - we previously had 2 variants (direct/indirect)
> 
> - v1 had 4 variants (direct/indirect x add_buf/add_sg)
> 
> - v2 has 4 variants (direct/indirect x add_sg/add_sg_single)

single is never indirect so should have a single variant.

> >> 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*.
> > 
> > Hide the whole structure as part of vring struct, the problem will go
> > away.
> 
> Yes, that's the other possibility.  Will do for the next submission.
> 
> Paolo
> 
> >> 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(-)
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