Re: [PATCH 1/2] virtio-blk: report the effective zone write granularity

[Damien Le Moal <dlemoal@kernel.org>]

On 8/20/26 04:10, Niklas Cassel wrote:
> For ZBC/ZAC devices the write granularity is the physical block size, so
> a 512e SMR disk exposed through a host_device backend has a logical block
> size of 512 and a zone write granularity of 4096. We told the driver 512
> while raw_co_zone_append() rejects anything that is not 4096 byte
> aligned, so the driver saw a plain I/O error for a request it had been
> told was valid.
> 
> Report the larger of the backend granularity and the logical block size
> instead. The driver cannot issue writes finer than the logical block
> size, so the larger of the two is the constraint that applies. This
> matches what a Linux guest derives for itself: blk_validate_zoned_limits()
> raises zone_write_granularity to the logical block size, and
> blk_stack_limits() stacks it with max().
> 
> Add it as a helper and use it for the zone append offset check in
> check_zoned_request(), which validated against bs->bl.write_granularity,
> so that the value reported to the driver and the value that requests are
> validated against cannot drift apart. The helper cannot return zero
> because blkconf_blocksizes() always leaves a logical block size behind,
> so the check no longer needs to guard against an unset granularity.
> 
> Fixes: 4f7366506a96 ("virtio-blk: add zoned storage emulation for zoned devices")
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
>  hw/block/virtio-blk.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index 6b92066aff..ef5581c1e6 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -478,6 +478,18 @@ typedef struct ZoneCmdData {
>      };
>  } ZoneCmdData;
>  
> +/*
> + * The write granularity that the device reports to the driver in
> + * virtio_blk_zoned_characteristics: the offset and size alignment constraint
> + * for writes and zone appends to sequential zones.
> + */
> +static uint32_t virtio_blk_write_granularity(VirtIOBlock *s)
> +{
> +    BlockDriverState *bs = blk_bs(s->blk);
> +
> +    return MAX(bs->bl.write_granularity, s->conf.conf.logical_block_size);
> +}
> +
>  /*
>   * check zoned_request: error checking before issuing requests. If all checks
>   * passed, return true.
> @@ -500,11 +512,11 @@ static bool check_zoned_request(VirtIOBlock *s, int64_t offset, int64_t len,
>      }
>  
>      if (append) {
> -        if (bs->bl.write_granularity) {
> -            if ((offset % bs->bl.write_granularity) != 0) {
> -                *status = VIRTIO_BLK_S_ZONE_UNALIGNED_WP;
> -                return false;
> -            }
> +        uint32_t wg = virtio_blk_write_granularity(s);
> +
> +        if ((offset % wg) != 0) {

	   if (offset & (wg - 1)) }

is more efficient. logical/physical block sizes are always power of 2s.

> +            *status = VIRTIO_BLK_S_ZONE_UNALIGNED_WP;
> +            return false;
>          }
>  
>          index = offset / bs->bl.zone_size;
> @@ -1254,7 +1266,8 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>                       bs->bl.max_active_zones);
>          virtio_stl_p(vdev, &blkcfg.zoned.max_open_zones,
>                       bs->bl.max_open_zones);
> -        virtio_stl_p(vdev, &blkcfg.zoned.write_granularity, blk_size);
> +        virtio_stl_p(vdev, &blkcfg.zoned.write_granularity,
> +                     virtio_blk_write_granularity(s));
>          virtio_stl_p(vdev, &blkcfg.zoned.max_append_sectors,
>                       bs->bl.max_append_sectors);
>      } else {


-- 
Damien Le Moal
Western Digital Research