Re: [Qemu-devel] [PATCH v4 7/8] block: core copy-on-read logic

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Nov 23, 2011 at 11:47:57AM +0000, Stefan Hajnoczi wrote:
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
>  block.c      |   72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  trace-events |    1 +
>  2 files changed, 73 insertions(+), 0 deletions(-)
> 
> diff --git a/block.c b/block.c
> index c30c8f2..a598a19 100644
> --- a/block.c
> +++ b/block.c
> @@ -1469,6 +1469,61 @@ 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);
> +
> +    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
>   */
> @@ -1496,7 +1551,24 @@ 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;
> +        }

Stefan,

It is not clear where support for shared backing files would fit.
Let us consider the following block copy example:

1) Original chain:
[ BASE ] -> [ IMAGE-1 ] -> [ IMAGE-2 ] -> [ IMAGE-3 ]

2) Final chain:
[ BASE ] -> [ IMAGE-3 ]

I was talking to Kevin and we don't have code/monitor command to the
switch from 1) to 2). But that is a separate issue.

Question is, how do you plan to stream the contents of IMAGE-1 and
IMAGE-2 (but not BASE), into IMAGE-3 ? That is an important use case.

Also, do you have status on the external COW file work, for raw images?

Thanks