Re: [PATCH v4 2/4] block/nbd: define new max_write_zero_fast limit

[Eric Blake]

On 6/11/20 11:26 AM, Vladimir Sementsov-Ogievskiy wrote:
> The NBD spec was recently updated to clarify that max_block doesn't
> relate to NBD_CMD_WRITE_ZEROES with NBD_CMD_FLAG_FAST_ZERO (which
> mirrors Qemu flag BDRV_REQ_NO_FALLBACK).
>
> bs->bl.max_write_zero_fast is zero by default which means using
> max_pwrite_zeroes. Update nbd driver to allow larger requests with
> BDRV_REQ_NO_FALLBACK.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/nbd.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/block/nbd.c b/block/nbd.c
> index 4ac23c8f62..b0584cf68d 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -1956,6 +1956,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>   
>       bs->bl.request_alignment = min;
>       bs->bl.max_pdiscard = QEMU_ALIGN_DOWN(INT_MAX, min);
> +    bs->bl.max_pwrite_zeroes_fast = bs->bl.max_pdiscard;
>       bs->bl.max_pwrite_zeroes = max;

Do we even need max_pwrite_zeroes_fast?  Doesn't qemu behave correctly 
if we just blindly assign max_pdiscard and max_pwrite_zeroes to the same 
value near 2G?

>       bs->bl.max_transfer = max;
>   

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org