Re: [RFC v6 2/4] virtio-blk: add zoned storage emulation for zoned devices

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sun, Jan 29, 2023 at 06:39:49PM +0800, Sam Li wrote:
> This patch extends virtio-blk emulation to handle zoned device commands
> by calling the new block layer APIs to perform zoned device I/O on
> behalf of the guest. It supports Report Zone, four zone oparations (open,
> close, finish, reset), and Append Zone.
> 
> The VIRTIO_BLK_F_ZONED feature bit will only be set if the host does
> support zoned block devices. Regular block devices(conventional zones)
> will not be set.
> 
> The guest os can use blktests, fio to test those commands on zoned devices.
> Furthermore, using zonefs to test zone append write is also supported.
> 
> Signed-off-by: Sam Li <faithilikerun@gmail.com>
> ---
>  hw/block/virtio-blk-common.c |   2 +
>  hw/block/virtio-blk.c        | 394 +++++++++++++++++++++++++++++++++++
>  2 files changed, 396 insertions(+)
> 

> @@ -949,6 +1311,30 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>          blkcfg.write_zeroes_may_unmap = 1;
>          virtio_stl_p(vdev, &blkcfg.max_write_zeroes_seg, 1);
>      }
> +    if (bs->bl.zoned != BLK_Z_NONE) {
> +        switch (bs->bl.zoned) {
> +        case BLK_Z_HM:
> +            blkcfg.zoned.model = VIRTIO_BLK_Z_HM;
> +            break;
> +        case BLK_Z_HA:
> +            blkcfg.zoned.model = VIRTIO_BLK_Z_HA;
> +            break;
> +        default:
> +            g_assert_not_reached();
> +        }
> +
> +        virtio_stl_p(vdev, &blkcfg.zoned.zone_sectors,
> +                     bs->bl.zone_size / 512);
> +        virtio_stl_p(vdev, &blkcfg.zoned.max_active_zones,
> +                     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.max_append_sectors,
> +                     bs->bl.max_append_sectors);

So these are all ABI sensitive frontend device settings, but they are
not exposed as tunables on the virtio-blk device, instead they are
implicitly set from the backend.

We have done this kind of thing before in QEMU, but several times it
has bitten QEMU maintainers/users, as having a backend affect the
frontend ABI is not to typical. It wouldn't be immediately obvious
when starting QEMU on a target host that the live migration would
be breaking ABI if the target host wasn't using a zoned device with
exact same settings.

This also limits mgmt flexibility across live migration, if the
mgmt app wants/needs to change the storage backend. eg maybe they
need to evacuate the host for an emergency, but don't have spare
hosts with same kind of storage. It might be desirable to migrate
and switch to a plain block device or raw/qcow2 file, rather than
let the VM die.

Can we make these virtio setting be explicitly controlled on the
virtio-blk device.  If not specified explicitly they could be
auto-populated from the backend for ease of use, but if specified
then simply validate the backend is a match. libvirt would then
make sure these are always explicitly set on the frontend.


With regards,
Daniel
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