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Date: Thu, 14 Feb 2013 10:23:37 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Rusty Russell <rusty@...tcorp.com.au>
CC: linux-kernel@...r.kernel.org,
Wanlong Gao <gaowanlong@...fujitsu.com>, asias@...hat.com,
mst@...hat.com, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 0/9] virtio: new API for addition of buffers, scatterlist
changes
Il 14/02/2013 07:00, Rusty Russell ha scritto:
> Paolo Bonzini <pbonzini@...hat.com> writes:
>> 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 related locking.
>
> They are ugly though. It's convoluted because we do actually know all
> the buffers at once, we don't need a piecemeal API.
>
> As a result, you now have arbitrary changes to the indirect heuristic,
> because the API is now piecemeal.
Note that I have sent v2 of patch 1/9, keeping the original indirect
heuristic. It was indeed a bad idea to conflate it in this series (it
was born there because originally virtqueue_add_buf was not sharing any
code, but now it's a different story)
> How about this as a first step?
>
> virtio_ring: virtqueue_add_sgs, to add multiple sgs.
>
> virtio_scsi and virtio_blk can really use these, to avoid their current
> hack of copying the whole sg array.
>
> Signed-off-by: Ruty Russell <rusty@...tcorp.com.au>
It's much better than the other prototype you had posted, but I still
dislike this... You pay for additional counting of scatterlists when
the caller knows the number of buffers; and the nested loops aren't
free, either.
My piecemeal API tried hard to keep things as fast as virtqueue_add_buf
when possible; I'm worried that this approach requires a lot more
benchmarking. Probably you would also need a fast-path
virtqueue_add_buf_single, and (unlike my version) that one couldn't
share much code if any with virtqueue_add_sgs.
So I can resend based on this patch, but I'm not sure it's really better...
Also, see below for a comment.
> @@ -197,8 +213,47 @@ int virtqueue_add_buf(struct virtqueue *_vq,
> void *data,
> gfp_t gfp)
> {
> + struct scatterlist *sgs[2];
> + unsigned int i;
> +
> + sgs[0] = sg;
> + sgs[1] = sg + out;
> +
> + /* Workaround until callers pass well-formed sgs. */
> + for (i = 0; i < out + in; i++)
> + sg_unmark_end(sg + i);
> +
> + sg_unmark_end(sg + out + in);
> + if (out && in)
> + sg_unmark_end(sg + out);
What's this second sg_unmark_end block for? Doesn't it access after the
end of sg? If you wanted it to be sg_mark_end, that must be:
if (out)
sg_mark_end(sg + out - 1);
if (in)
sg_mark_end(sg + out + in - 1);
with a corresponding unmark afterwards.
Paolo
> + return virtqueue_add_sgs(_vq, sgs, out ? 1 : 0, in ? 1 : 0, data, gfp);
> +}
> +
> +/**
> + * virtqueue_add_sgs - expose buffers to other end
> + * @vq: the struct virtqueue we're talking about.
> + * @sgs: array of terminated scatterlists.
> + * @out_num: the number of scatterlists readable by other side
> + * @in_num: the number of scatterlists which are writable (after readable ones)
> + * @data: the token identifying the buffer.
> + * @gfp: how to do memory allocations (if necessary).
> + *
> + * Caller must ensure we don't call this with other virtqueue operations
> + * at the same time (except where noted).
> + *
> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM).
> + */
> +int virtqueue_add_sgs(struct virtqueue *_vq,
> + struct scatterlist *sgs[],
> + unsigned int out_sgs,
> + unsigned int in_sgs,
> + void *data,
> + gfp_t gfp)
> +{
> struct vring_virtqueue *vq = to_vvq(_vq);
> - unsigned int i, avail, uninitialized_var(prev);
> + struct scatterlist *sg;
> + unsigned int i, n, avail, uninitialized_var(prev), total_sg;
> int head;
>
> START_USE(vq);
> @@ -218,46 +273,59 @@ int virtqueue_add_buf(struct virtqueue *_vq,
> }
> #endif
>
> + /* Count them first. */
> + for (i = total_sg = 0; i < out_sgs + in_sgs; i++) {
> + struct scatterlist *sg;
> + for (sg = sgs[i]; sg; sg = sg_next(sg))
> + total_sg++;
> + }
> +
> +
> /* If the host supports indirect descriptor tables, and we have multiple
> * buffers, then go indirect. FIXME: tune this threshold */
> - if (vq->indirect && (out + in) > 1 && vq->vq.num_free) {
> - head = vring_add_indirect(vq, sg, out, in, gfp);
> + if (vq->indirect && total_sg > 1 && vq->vq.num_free) {
> + head = vring_add_indirect(vq, sgs, total_sg, out_sgs, in_sgs,
> + gfp);
> if (likely(head >= 0))
> goto add_head;
> }
>
> - BUG_ON(out + in > vq->vring.num);
> - BUG_ON(out + in == 0);
> + BUG_ON(total_sg > vq->vring.num);
> + BUG_ON(total_sg == 0);
>
> - if (vq->vq.num_free < out + in) {
> + if (vq->vq.num_free < total_sg) {
> pr_debug("Can't add buf len %i - avail = %i\n",
> - out + in, vq->vq.num_free);
> + total_sg, vq->vq.num_free);
> /* FIXME: for historical reasons, we force a notify here if
> * there are outgoing parts to the buffer. Presumably the
> * host should service the ring ASAP. */
> - if (out)
> + if (out_sgs)
> vq->notify(&vq->vq);
> END_USE(vq);
> return -ENOSPC;
> }
>
> /* We're about to use some buffers from the free list. */
> - vq->vq.num_free -= out + in;
> -
> - head = vq->free_head;
> - for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
> - vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
> - vq->vring.desc[i].addr = sg_phys(sg);
> - vq->vring.desc[i].len = sg->length;
> - prev = i;
> - sg++;
> + vq->vq.num_free -= total_sg;
> +
> + head = i = vq->free_head;
> + for (n = 0; n < out_sgs; n++) {
> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> + vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
> + vq->vring.desc[i].addr = sg_phys(sg);
> + vq->vring.desc[i].len = sg->length;
> + prev = i;
> + i = vq->vring.desc[i].next;
> + }
> }
> - for (; in; i = vq->vring.desc[i].next, in--) {
> - vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> - vq->vring.desc[i].addr = sg_phys(sg);
> - vq->vring.desc[i].len = sg->length;
> - prev = i;
> - sg++;
> + for (; n < (out_sgs + in_sgs); n++) {
> + for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> + vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
> + vq->vring.desc[i].addr = sg_phys(sg);
> + vq->vring.desc[i].len = sg->length;
> + prev = i;
> + i = vq->vring.desc[i].next;
> + }
> }
> /* Last one doesn't continue. */
> vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index ff6714e..6eff15b 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -40,6 +40,13 @@ int virtqueue_add_buf(struct virtqueue *vq,
> void *data,
> gfp_t gfp);
>
> +int virtqueue_add_sgs(struct virtqueue *vq,
> + struct scatterlist *sgs[],
> + unsigned int out_sgs,
> + unsigned int in_sgs,
> + void *data,
> + gfp_t gfp);
> +
> void virtqueue_kick(struct virtqueue *vq);
>
> bool virtqueue_kick_prepare(struct virtqueue *vq);
>
--
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