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Message-ID: <20121218133719.GD26110@redhat.com>
Date: Tue, 18 Dec 2012 15:37:19 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
gaowanlong@...fujitsu.com, hutao@...fujitsu.com,
linux-scsi@...r.kernel.org,
virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
asias@...hat.com, stefanha@...hat.com, nab@...ux-iscsi.org
Subject: Re: [PATCH v2 2/5] virtio-scsi: use functions for piecewise
composition of buffers
On Tue, Dec 18, 2012 at 01:32:49PM +0100, Paolo Bonzini wrote:
> Using the new virtio_scsi_add_sg function lets us simplify the queueing
> path. In particular, all data protected by the tgt_lock is just gone
> (multiqueue will find a new use for the lock).
vq access still needs some protection: virtio is not reentrant
by itself. with tgt_lock gone what protects vq against
concurrent add_buf calls?
> The speedup is relatively small (2-4%) but it is worthwhile because of
> the code simplification---both in this patches and in the next ones.
>
> Signed-off-by: Wanlong Gao <gaowanlong@...fujitsu.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
> v1->v2: new
>
> drivers/scsi/virtio_scsi.c | 94 +++++++++++++++++++------------------------
> 1 files changed, 42 insertions(+), 52 deletions(-)
>
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 74ab67a..2b93b6e 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -59,11 +59,8 @@ struct virtio_scsi_vq {
>
> /* Per-target queue state */
> struct virtio_scsi_target_state {
> - /* Protects sg. Lock hierarchy is tgt_lock -> vq_lock. */
> + /* Never held at the same time as vq_lock. */
> spinlock_t tgt_lock;
> -
> - /* For sglist construction when adding commands to the virtqueue. */
> - struct scatterlist sg[];
> };
>
> /* Driver instance state */
> @@ -351,57 +348,58 @@ static void virtscsi_event_done(struct virtqueue *vq)
> spin_unlock_irqrestore(&vscsi->event_vq.vq_lock, flags);
> };
>
> -static void virtscsi_map_sgl(struct scatterlist *sg, unsigned int *p_idx,
> - struct scsi_data_buffer *sdb)
> -{
> - struct sg_table *table = &sdb->table;
> - struct scatterlist *sg_elem;
> - unsigned int idx = *p_idx;
> - int i;
> -
> - for_each_sg(table->sgl, sg_elem, table->nents, i)
> - sg[idx++] = *sg_elem;
> -
> - *p_idx = idx;
> -}
> -
> /**
> - * virtscsi_map_cmd - map a scsi_cmd to a virtqueue scatterlist
> + * virtscsi_add_cmd - add a virtio_scsi_cmd to a virtqueue
> * @vscsi : virtio_scsi state
> * @cmd : command structure
> - * @out_num : number of read-only elements
> - * @in_num : number of write-only elements
> * @req_size : size of the request buffer
> * @resp_size : size of the response buffer
> - *
> - * Called with tgt_lock held.
> + * @gfp : flags to use for memory allocations
> */
> -static void virtscsi_map_cmd(struct virtio_scsi_target_state *tgt,
> - struct virtio_scsi_cmd *cmd,
> - unsigned *out_num, unsigned *in_num,
> - size_t req_size, size_t resp_size)
> +static int virtscsi_add_cmd(struct virtqueue *vq,
> + struct virtio_scsi_cmd *cmd,
> + size_t req_size, size_t resp_size, gfp_t gfp)
> {
> struct scsi_cmnd *sc = cmd->sc;
> - struct scatterlist *sg = tgt->sg;
> - unsigned int idx = 0;
> + struct scatterlist sg;
> + unsigned int count, count_sg;
> + struct sg_table *out, *in;
> + struct virtqueue_buf buf;
> + int ret;
> +
> + out = in = NULL;
> +
> + if (sc && sc->sc_data_direction != DMA_NONE) {
> + if (sc->sc_data_direction != DMA_FROM_DEVICE)
> + out = &scsi_out(sc)->table;
> + if (sc->sc_data_direction != DMA_TO_DEVICE)
> + in = &scsi_in(sc)->table;
> + }
> +
> + count_sg = 2 + (out ? 1 : 0) + (in ? 1 : 0);
> + count = 2 + (out ? out->nents : 0) + (in ? in->nents : 0);
> + ret = virtqueue_start_buf(vq, &buf, cmd, count, count_sg, gfp);
> + if (ret < 0)
> + return ret;
>
> /* Request header. */
> - sg_set_buf(&sg[idx++], &cmd->req, req_size);
> + sg_init_one(&sg, &cmd->req, req_size);
> + virtqueue_add_sg(&buf, &sg, 1, DMA_TO_DEVICE);
>
> /* Data-out buffer. */
> - if (sc && sc->sc_data_direction != DMA_FROM_DEVICE)
> - virtscsi_map_sgl(sg, &idx, scsi_out(sc));
> -
> - *out_num = idx;
> + if (out)
> + virtqueue_add_sg(&buf, out->sgl, out->nents, DMA_TO_DEVICE);
>
> /* Response header. */
> - sg_set_buf(&sg[idx++], &cmd->resp, resp_size);
> + sg_init_one(&sg, &cmd->resp, resp_size);
> + virtqueue_add_sg(&buf, &sg, 1, DMA_FROM_DEVICE);
>
> /* Data-in buffer */
> - if (sc && sc->sc_data_direction != DMA_TO_DEVICE)
> - virtscsi_map_sgl(sg, &idx, scsi_in(sc));
> + if (in)
> + virtqueue_add_sg(&buf, in->sgl, in->nents, DMA_FROM_DEVICE);
>
> - *in_num = idx - *out_num;
> + virtqueue_end_buf(&buf);
> + return 0;
> }
>
> static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
> @@ -409,25 +407,20 @@ static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
> struct virtio_scsi_cmd *cmd,
> size_t req_size, size_t resp_size, gfp_t gfp)
> {
> - unsigned int out_num, in_num;
> unsigned long flags;
> - int err;
> + int ret;
> bool needs_kick = false;
>
> - spin_lock_irqsave(&tgt->tgt_lock, flags);
> - virtscsi_map_cmd(tgt, cmd, &out_num, &in_num, req_size, resp_size);
> -
> - spin_lock(&vq->vq_lock);
> - err = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
> - spin_unlock(&tgt->tgt_lock);
> - if (!err)
> + spin_lock_irqsave(&vq->vq_lock, flags);
> + ret = virtscsi_add_cmd(vq->vq, cmd, req_size, resp_size, gfp);
> + if (!ret)
> needs_kick = virtqueue_kick_prepare(vq->vq);
>
> spin_unlock_irqrestore(&vq->vq_lock, flags);
>
> if (needs_kick)
> virtqueue_notify(vq->vq);
> - return err;
> + return ret;
> }
>
> static int virtscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
> @@ -592,14 +585,11 @@ static struct virtio_scsi_target_state *virtscsi_alloc_tgt(
> gfp_t gfp_mask = GFP_KERNEL;
>
> /* We need extra sg elements at head and tail. */
> - tgt = kmalloc(sizeof(*tgt) + sizeof(tgt->sg[0]) * (sg_elems + 2),
> - gfp_mask);
> -
> + tgt = kmalloc(sizeof(*tgt), gfp_mask);
> if (!tgt)
> return NULL;
>
> spin_lock_init(&tgt->tgt_lock);
> - sg_init_table(tgt->sg, sg_elems + 2);
> return tgt;
> }
>
> --
> 1.7.1
>
--
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