Re: [PATCH v3] virtio-blk: Respect discard granularity

[Stefano Garzarella]

+Cc stefanha@redhat.com

Please explain a bit the changes in the commit message, for example that 
you added 'report-discard-granularity', disabled it for older machines, 
that we use blk_size as default granularity, etc.

Something like this:

    Report the configured granularity for discard operation to the 
    guest. If this is not set use the block size.

    Since until now we have ignored the configured discard granularity 
    and always reported the block size, let's add 
    'report-discard-granularity' property and disable it for older 
    machine types to avoid migration issues.

And use ./scripts/get_maintainer.pl to CC all the maintainers (e.g.  
Stefan Hajnoczi was missing)

Other than that, the patch LGTM.

Thanks,
Stefano

On Tue, Feb 23, 2021 at 09:09:40PM +0900, Akihiko Odaki wrote:
> Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
> ---
> hw/block/virtio-blk.c          | 8 +++++++-
> hw/core/machine.c              | 4 +++-
> include/hw/virtio/virtio-blk.h | 1 +
> 3 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index bac2d6fa2b2..f4378e61182 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -962,10 +962,14 @@ static void virtio_blk_update_config(VirtIODevice *vdev, 
> uint8_t *config)
>     blkcfg.wce = blk_enable_write_cache(s->blk);
>     virtio_stw_p(vdev, &blkcfg.num_queues, s->conf.num_queues);
>     if (virtio_has_feature(s->host_features, VIRTIO_BLK_F_DISCARD)) {
> +        uint32_t discard_granularity = conf->discard_granularity;
> +        if (discard_granularity == -1 || !s->conf.report_discard_granularity) {
> +            discard_granularity = blk_size;
> +        }
>         virtio_stl_p(vdev, &blkcfg.max_discard_sectors,
>                      s->conf.max_discard_sectors);
>         virtio_stl_p(vdev, &blkcfg.discard_sector_alignment,
> -                     blk_size >> BDRV_SECTOR_BITS);
> +                     discard_granularity >> BDRV_SECTOR_BITS);
>         /*
>          * We support only one segment per request since multiple segments
>          * are not widely used and there are no userspace APIs that allow
> @@ -1299,6 +1303,8 @@ static Property virtio_blk_properties[] = {
>                      IOThread *),
>     DEFINE_PROP_BIT64("discard", VirtIOBlock, host_features,
>                       VIRTIO_BLK_F_DISCARD, true),
> +    DEFINE_PROP_BOOL("report-discard-granularity", VirtIOBlock,
> +                     conf.report_discard_granularity, true),
>     DEFINE_PROP_BIT64("write-zeroes", VirtIOBlock, host_features,
>                       VIRTIO_BLK_F_WRITE_ZEROES, true),
>     DEFINE_PROP_UINT32("max-discard-sectors", VirtIOBlock,
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index de3b8f1b318..e4df5797e72 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -33,7 +33,9 @@
> #include "migration/global_state.h"
> #include "migration/vmstate.h"
>
> -GlobalProperty hw_compat_5_2[] = {};
> +GlobalProperty hw_compat_5_2[] = {
> +    { "virtio-blk-device", "report-discard-granularity", "off" },
> +};
> const size_t hw_compat_5_2_len = G_N_ELEMENTS(hw_compat_5_2);
>
> GlobalProperty hw_compat_5_1[] = {
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index 214ab748229..29655a406dd 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -41,6 +41,7 @@ struct VirtIOBlkConf
>     uint16_t num_queues;
>     uint16_t queue_size;
>     bool seg_max_adjust;
> +    bool report_discard_granularity;
>     uint32_t max_discard_sectors;
>     uint32_t max_write_zeroes_sectors;
>     bool x_enable_wce_if_config_wce;
> --
> 2.24.3 (Apple Git-128)