On Tue, Feb 21, 2017 at 6:45 AM Eric Blake <ebl...@redhat.com> wrote:
> The NBD protocol would like to advertise the optimal I/O
> size to the client; but it would be a layering violation to
> peek into blk_bs(blk)->bl, when we only have a BB.
>
> This copies the existing blk_get_max_transfer() in reading
> a value from the top BDS; where that value was picked via
> bdrv_refresh_limits() to reflect the overall constraints of
> the entire BDS chain.
>
> Signed-off-by: Eric Blake <ebl...@redhat.com>
>
Reviewed-by: Marc-André Lureau <marcandre.lur...@redhat.com>
>
> ---
> v4: retitle, as part of rebasing to byte limits
> v3: new patch
> ---
> include/sysemu/block-backend.h | 1 +
> block/block-backend.c | 12 ++++++++++++
> 2 files changed, 13 insertions(+)
>
> diff --git a/include/sysemu/block-backend.h
> b/include/sysemu/block-backend.h
> index 6444e41..882480a 100644
> --- a/include/sysemu/block-backend.h
> +++ b/include/sysemu/block-backend.h
> @@ -174,6 +174,7 @@ void blk_lock_medium(BlockBackend *blk, bool locked);
> void blk_eject(BlockBackend *blk, bool eject_flag);
> int blk_get_flags(BlockBackend *blk);
> uint32_t blk_get_max_transfer(BlockBackend *blk);
> +uint32_t blk_get_opt_transfer(BlockBackend *blk);
> int blk_get_max_iov(BlockBackend *blk);
> void blk_set_guest_block_size(BlockBackend *blk, int align);
> void *blk_try_blockalign(BlockBackend *blk, size_t size);
> diff --git a/block/block-backend.c b/block/block-backend.c
> index efbf398..4d91ff8 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -1426,6 +1426,18 @@ uint32_t blk_get_max_transfer(BlockBackend *blk)
> return MIN_NON_ZERO(max, INT_MAX);
> }
>
> +/* Returns the optimum transfer length, in bytes; may be 0 if no optimum
> */
> +uint32_t blk_get_opt_transfer(BlockBackend *blk)
> +{
> + BlockDriverState *bs = blk_bs(blk);
> +
> + if (bs) {
> + return bs->bl.opt_transfer;
> + } else {
> + return 0;
> + }
> +}
> +
> int blk_get_max_iov(BlockBackend *blk)
> {
> return blk->root->bs->bl.max_iov;
> --
> 2.9.3
>
>
> --
Marc-André Lureau
