Re: [Qemu-devel] [PATCH 02/11] block/io: add bdrv_co_write_compressed

[Kevin Wolf <kwolf@redhat.com>]

Am 31.05.2016 um 11:15 hat Denis V. Lunev geschrieben:
> From: Pavel Butsykin <pbutsykin@virtuozzo.com>
> 
> This patch just adds the interface to the bdrv_co_pwritev_compressed,
> which is currently not used but will be useful for safe implementation of the
> bdrv_co_write_compressed callback in format drivers.
> 
> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Jeff Cody <jcody@redhat.com>
> CC: Markus Armbruster <armbru@redhat.com>
> CC: Eric Blake <eblake@redhat.com>
> CC: John Snow <jsnow@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/io.c                | 78 +++++++++++++++++++++++++++++++++++++++++++----
>  include/block/block_int.h |  5 +++
>  qemu-img.c                |  2 +-
>  3 files changed, 78 insertions(+), 7 deletions(-)
> 
> diff --git a/block/io.c b/block/io.c
> index c5bb6ae..54cd9a4 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1779,8 +1779,8 @@ int bdrv_is_allocated_above(BlockDriverState *top,
>      return 0;
>  }
>  
> -int bdrv_pwrite_compressed(BlockDriverState *bs, int64_t offset,
> -                           const void *buf, int count)
> +int coroutine_fn bdrv_co_pwritev_compressed(BlockDriverState *bs,
> +    int64_t offset, unsigned int bytes, QEMUIOVector *qiov)
>  {
>      BlockDriver *drv = bs->drv;
>      int ret;
> @@ -1788,18 +1788,84 @@ int bdrv_pwrite_compressed(BlockDriverState *bs, int64_t offset,
>      if (!drv) {
>          return -ENOMEDIUM;
>      }
> -    if (!drv->bdrv_write_compressed) {
> +
> +    if (!drv->bdrv_co_write_compressed) {
>          return -ENOTSUP;
>      }
> -    ret = bdrv_check_byte_request(bs, offset, count);
> +
> +    ret = bdrv_check_byte_request(bs, offset, bytes);
>      if (ret < 0) {
>          return ret;
>      }
>  
>      assert(QLIST_EMPTY(&bs->dirty_bitmaps));
> +    assert(qemu_in_coroutine());
> +
> +    return drv->bdrv_co_write_compressed(bs, offset >> BDRV_SECTOR_BITS,
> +                                         bytes >> BDRV_SECTOR_BITS, qiov);
> +}
> +
> +typedef struct BdrvWriteCompressedCo {
> +    BlockDriverState *bs;
> +    int64_t offset;
> +    QEMUIOVector *qiov;
> +    int ret;
> +} BdrvWriteCompressedCo;

I think you could just reuse RwCo here.

And in fact, this made we wonder whether we can reuse more from the
normal I/O path. If compressed writes could be made just another REQ_*
flag and we went through bdrv_co_pwritev(), we would automatically get
the alignment code if we don't want to convert drivers to byte-based.
And also the existing coroutine wrapper code would be reused instead
implementing it once more.

I'm not completely sure how special compressed writes really are, so
reusing the normal I/O path may or may not work. It might be worth
investigating, though.

> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -207,6 +207,9 @@ struct BlockDriver {
>      int (*bdrv_write_compressed)(BlockDriverState *bs, int64_t sector_num,
>                                   const uint8_t *buf, int nb_sectors);
>  
> +    int coroutine_fn (*bdrv_co_write_compressed)(BlockDriverState *bs,
> +        int64_t sector_num, int nb_sectors, QEMUIOVector *qiov);

This should be called bdrv_co_writev_compressed because it's a vectored
interface.

Kevin