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Re: [Qemu-devel] [PATCH v2 7/8] block: core copy-on-read logic


From: Zhi Yong Wu
Subject: Re: [Qemu-devel] [PATCH v2 7/8] block: core copy-on-read logic
Date: Wed, 23 Nov 2011 12:42:47 +0800

On Thu, Nov 17, 2011 at 9:40 PM, Stefan Hajnoczi
<address@hidden> wrote:
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block.c      |   72 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  trace-events |    1 +
>  2 files changed, 73 insertions(+), 0 deletions(-)
>
> diff --git a/block.c b/block.c
> index 0eef122..d5faa6c 100644
> --- a/block.c
> +++ b/block.c
> @@ -1464,6 +1464,61 @@ int bdrv_pwrite_sync(BlockDriverState *bs, int64_t 
> offset,
>     return 0;
>  }
>
> +static int coroutine_fn bdrv_co_copy_on_readv(BlockDriverState *bs,
> +        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov)
> +{
> +    /* Perform I/O through a temporary buffer so that users who scribble over
> +     * their read buffer while the operation is in progress do not end up
> +     * modifying the image file.  This is critical for zero-copy guest I/O
> +     * where anything might happen inside guest memory.
> +     */
> +    void *bounce_buffer;
> +
> +    struct iovec iov;
> +    QEMUIOVector bounce_qiov;
> +    int64_t cluster_sector_num;
> +    int cluster_nb_sectors;
> +    size_t skip_bytes;
> +    int ret;
> +
> +    /* Cover entire cluster so no additional backing file I/O is required 
> when
> +     * allocating cluster in the image file.
> +     */
> +    round_to_clusters(bs, sector_num, nb_sectors,
> +                      &cluster_sector_num, &cluster_nb_sectors);
Why need to round down/up sector_num/nb_sectors in this function? The
detection of race condition for write request has been done before
this function.

> +
> +    trace_bdrv_co_copy_on_readv(bs, sector_num, nb_sectors,
> +                                cluster_sector_num, cluster_nb_sectors);
> +
> +    iov.iov_len = cluster_nb_sectors * BDRV_SECTOR_SIZE;
> +    iov.iov_base = bounce_buffer = qemu_blockalign(bs, iov.iov_len);
> +    qemu_iovec_init_external(&bounce_qiov, &iov, 1);
> +
> +    ret = bs->drv->bdrv_co_readv(bs, cluster_sector_num, cluster_nb_sectors,
> +                                 &bounce_qiov);
> +    if (ret < 0) {
> +        goto err;
> +    }
> +
> +    ret = bs->drv->bdrv_co_writev(bs, cluster_sector_num, cluster_nb_sectors,
> +                                  &bounce_qiov);
> +    if (ret < 0) {
> +        /* It might be okay to ignore write errors for guest requests.  If 
> this
> +         * is a deliberate copy-on-read then we don't want to ignore the 
> error.
> +         * Simply report it in all cases.
> +         */
> +        goto err;
> +    }
> +
> +    skip_bytes = (sector_num - cluster_sector_num) * BDRV_SECTOR_SIZE;
> +    qemu_iovec_from_buffer(qiov, bounce_buffer + skip_bytes,
> +                           nb_sectors * BDRV_SECTOR_SIZE);
> +
> +err:
> +    qemu_vfree(bounce_buffer);
> +    return ret;
> +}
> +
>  /*
>  * Handle a read request in coroutine context
>  */
> @@ -1491,7 +1546,24 @@ static int coroutine_fn 
> bdrv_co_do_readv(BlockDriverState *bs,
>     }
>
>     tracked_request_begin(&req, bs, sector_num, nb_sectors, false);
> +
> +    if (bs->copy_on_read) {
> +        int pnum;
> +
> +        ret = bdrv_co_is_allocated(bs, sector_num, nb_sectors, &pnum);
> +        if (ret < 0) {
> +            goto out;
> +        }
> +
> +        if (!ret || pnum != nb_sectors) {
> +            ret = bdrv_co_copy_on_readv(bs, sector_num, nb_sectors, qiov);
> +            goto out;
> +        }
> +    }
> +
>     ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> +
> +out:
>     tracked_request_end(&req);
>     return ret;
>  }
> diff --git a/trace-events b/trace-events
> index 962caca..518b76b 100644
> --- a/trace-events
> +++ b/trace-events
> @@ -69,6 +69,7 @@ bdrv_lock_medium(void *bs, bool locked) "bs %p locked %d"
>  bdrv_co_readv(void *bs, int64_t sector_num, int nb_sector) "bs %p sector_num 
> %"PRId64" nb_sectors %d"
>  bdrv_co_writev(void *bs, int64_t sector_num, int nb_sector) "bs %p 
> sector_num %"PRId64" nb_sectors %d"
>  bdrv_co_io_em(void *bs, int64_t sector_num, int nb_sectors, int is_write, 
> void *acb) "bs %p sector_num %"PRId64" nb_sectors %d is_write %d acb %p"
> +bdrv_co_copy_on_readv(void *bs, int64_t sector_num, int nb_sectors, int64_t 
> cluster_sector_num, int cluster_nb_sectors) "bs %p sector_num %"PRId64" 
> nb_sectors %d cluster_sector_num %"PRId64" cluster_nb_sectors %d"
>
>  # hw/virtio-blk.c
>  virtio_blk_req_complete(void *req, int status) "req %p status %d"
> --
> 1.7.7.1
>
>
>



-- 
Regards,

Zhi Yong Wu



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