Re: [Qemu-devel] [PATCH 3/4] scsi-disk: introduce dma_readv and dma_writev

[Eric Blake <eblake@redhat.com>]

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On 05/11/2016 05:41 AM, Paolo Bonzini wrote:
> These are replacements for blk_aio_readv and blk_aio_writev that allow
> customization of the data path.  They reuse the DMA helpers' DMAIOFunc
> callback type, so that the same function can be used in either the
> QEMUSGList or the bounce-buffered case.
> 
> This customization will be needed in the next patch to do zero-copy
> SG_IO on scsi-block.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/scsi/scsi-disk.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 56 insertions(+), 8 deletions(-)
> 

> @@ -339,14 +354,16 @@ static void scsi_do_read(SCSIDiskReq *r, int ret)
>      if (r->req.sg) {
>          dma_acct_start(s->qdev.conf.blk, &r->acct, r->req.sg, BLOCK_ACCT_READ);
>          r->req.resid -= r->req.sg->size;
> -        r->req.aiocb = dma_blk_read(s->qdev.conf.blk, r->req.sg, r->sector,
> -                                    scsi_dma_complete, r);
> +        r->req.aiocb = dma_blk_io(blk_get_aio_context(s->qdev.conf.blk),
> +                                  r->req.sg, r->sector,
> +                                  sdc->dma_readv, r, scsi_dma_complete, r,
> +                                  DMA_DIRECTION_FROM_DEVICE);

Is it worth considering byte-based rather than sector-based interfaces,
as part of this series?

> +static
> +BlockAIOCB *scsi_dma_readv(int64_t sector_num,
> +                           QEMUIOVector *iov, int nb_sectors,
> +                           BlockCompletionFunc *cb, void *cb_opaque,
> +                           void *opaque)
> +{
> +    SCSIDiskReq *r = opaque;
> +    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
> +    return blk_aio_readv(s->qdev.conf.blk, sector_num, iov, nb_sectors,
> +                         cb, cb_opaque);
> +}

Especially since commit 7b1deac killed blk_aio_readv() in favor of
byte-based blk_aio_preadv().

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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