Re: [Qemu-devel] [PATCH v2 05/17] nbd: Advertise realistic limits to block layer

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/06/2016 23:30, Eric Blake wrote:
> We were basing the advertisement of maximum discard and transfer
> length off of UINT32_MAX, but since the rest of the block layer
> has signed int limits on a transaction, nothing could ever reach
> that maximum, and we risk overflowing an int once things are
> converted to byte-based rather than sector-based limits.  What's
> more, we DO have a much smaller limit: both the current kernel
> and qemu-nbd have a hard limit of 32M on a read or write
> transaction, and while they may also permit up to a full 32 bits
> on a discard transaction, the upstream NBD protocol is proposing
> wording that without any explicit advertisement otherwise,
> clients should limit ALL requests to the same limits as read and
> write, even though the other requests do not actually require as
> many bytes across the wire.  So the better limit to tell the
> block layer is 32M for both values.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> v2: new patch
> ---
>  block/nbd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index 6015e8b..bf67c8a 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -362,8 +362,8 @@ static int nbd_co_flush(BlockDriverState *bs)
> 
>  static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
>  {
> -    bs->bl.max_discard = UINT32_MAX >> BDRV_SECTOR_BITS;
> -    bs->bl.max_transfer_length = UINT32_MAX >> BDRV_SECTOR_BITS;
> +    bs->bl.max_discard = NBD_MAX_SECTORS;
> +    bs->bl.max_transfer_length = NBD_MAX_SECTORS;
>  }
> 
>  static int nbd_co_discard(BlockDriverState *bs, int64_t sector_num,
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org