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Date: Tue, 18 Dec 2012 14:35:42 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Michael S. Tsirkin" <mst@...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
Il 18/12/2012 14:37, Michael S. Tsirkin ha scritto:
> 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?
vq_lock.
Paolo
>> 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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