Re: [PATCH v4 1/2] virtio: make seg_max virtqueue size dependent

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Dec 16, 2019 at 01:04:50PM +0300, Denis Plotnikov wrote:
> Before the patch, seg_max parameter was immutable and hardcoded
> to 126 (128 - 2) without respect to queue size. This has two negative effects:
> 
> 1. when queue size is < 128, we have Virtio 1.1 specfication violation:
>    (2.6.5.3.1 Driver Requirements) seq_max must be <= queue_size.
>    This violation affects the old Linux guests (ver < 4.14). These guests
>    crash on these queue_size setups.
> 
> 2. when queue_size > 128, as was pointed out by Denis Lunev <den@virtuozzo.com>,
>    seg_max restrics guest's block request length which affects guests'
>    performance making them issues more block request than needed.
>    https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03721.html
> 
> To mitigate this two effects, the patch adds the property adjusting seg_max
> to queue size automaticaly. Since seg_max is a guest visible parameter,
> the property is machine type managable and allows to choose between
> old (seg_max = 126 always) and new (seg_max = queue_size - 2) behaviors.
> 
> Not to change the behavior of the older VMs, prevent setting the default
> seg_max_adjust value for older machine types.
> 
> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>

Stefan I think you acked this already? Could you ack?

Denis could you rebase on latest master pls so I can apply?

Thanks!

> ---
>  hw/block/virtio-blk.c           |  9 ++++++++-
>  hw/core/machine.c               |  3 +++
>  hw/scsi/vhost-scsi.c            |  2 ++
>  hw/scsi/virtio-scsi.c           | 10 +++++++++-
>  include/hw/virtio/virtio-blk.h  |  1 +
>  include/hw/virtio/virtio-scsi.h |  1 +
>  6 files changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index d62e6377c2..0f6f8113b7 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -908,7 +908,8 @@ static void virtio_blk_update_config(VirtIODevice *vdev, uint8_t *config)
>      blk_get_geometry(s->blk, &capacity);
>      memset(&blkcfg, 0, sizeof(blkcfg));
>      virtio_stq_p(vdev, &blkcfg.capacity, capacity);
> -    virtio_stl_p(vdev, &blkcfg.seg_max, 128 - 2);
> +    virtio_stl_p(vdev, &blkcfg.seg_max,
> +                 s->conf.seg_max_adjust ? s->conf.queue_size - 2 : 128 - 2);
>      virtio_stw_p(vdev, &blkcfg.geometry.cylinders, conf->cyls);
>      virtio_stl_p(vdev, &blkcfg.blk_size, blk_size);
>      virtio_stw_p(vdev, &blkcfg.min_io_size, conf->min_io_size / blk_size);
> @@ -1133,6 +1134,11 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
>          error_setg(errp, "num-queues property must be larger than 0");
>          return;
>      }
> +    if (conf->queue_size <= 2) {
> +        error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
> +                   "must be > 2", conf->queue_size);
> +        return;
> +    }
>      if (!is_power_of_2(conf->queue_size) ||
>          conf->queue_size > VIRTQUEUE_MAX_SIZE) {
>          error_setg(errp, "invalid queue-size property (%" PRIu16 "), "
> @@ -1262,6 +1268,7 @@ static Property virtio_blk_properties[] = {
>                      true),
>      DEFINE_PROP_UINT16("num-queues", VirtIOBlock, conf.num_queues, 1),
>      DEFINE_PROP_UINT16("queue-size", VirtIOBlock, conf.queue_size, 128),
> +    DEFINE_PROP_BOOL("seg-max-adjust", VirtIOBlock, conf.seg_max_adjust, true),
>      DEFINE_PROP_LINK("iothread", VirtIOBlock, conf.iothread, TYPE_IOTHREAD,
>                       IOThread *),
>      DEFINE_PROP_BIT64("discard", VirtIOBlock, host_features,
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 023548b4f3..bfa320387e 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -29,6 +29,9 @@
>  
>  GlobalProperty hw_compat_4_2[] = {
>      { "virtio-blk-device", "x-enable-wce-if-config-wce", "off" },
> +    { "virtio-blk-device", "seg-max-adjust", "off"},
> +    { "virtio-scsi-device", "seg_max_adjust", "off"},
> +    { "vhost-blk-device", "seg_max_adjust", "off"},
>  };
>  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
>  
> diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
> index c693fc748a..26f710d3ec 100644
> --- a/hw/scsi/vhost-scsi.c
> +++ b/hw/scsi/vhost-scsi.c
> @@ -275,6 +275,8 @@ static Property vhost_scsi_properties[] = {
>      DEFINE_PROP_UINT32("num_queues", VirtIOSCSICommon, conf.num_queues, 1),
>      DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSICommon, conf.virtqueue_size,
>                         128),
> +    DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSICommon, conf.seg_max_adjust,
> +                      true),
>      DEFINE_PROP_UINT32("max_sectors", VirtIOSCSICommon, conf.max_sectors,
>                         0xFFFF),
>      DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSICommon, conf.cmd_per_lun, 128),
> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> index e8b2b64d09..405cb6c953 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -654,7 +654,8 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
>      VirtIOSCSICommon *s = VIRTIO_SCSI_COMMON(vdev);
>  
>      virtio_stl_p(vdev, &scsiconf->num_queues, s->conf.num_queues);
> -    virtio_stl_p(vdev, &scsiconf->seg_max, 128 - 2);
> +    virtio_stl_p(vdev, &scsiconf->seg_max,
> +                 s->conf.seg_max_adjust ? s->conf.virtqueue_size - 2 : 128 - 2);
>      virtio_stl_p(vdev, &scsiconf->max_sectors, s->conf.max_sectors);
>      virtio_stl_p(vdev, &scsiconf->cmd_per_lun, s->conf.cmd_per_lun);
>      virtio_stl_p(vdev, &scsiconf->event_info_size, sizeof(VirtIOSCSIEvent));
> @@ -893,6 +894,11 @@ void virtio_scsi_common_realize(DeviceState *dev,
>          virtio_cleanup(vdev);
>          return;
>      }
> +    if (s->conf.virtqueue_size <= 2) {
> +        error_setg(errp, "invalid virtqueue_size property (= %" PRIu16 "), "
> +                   "must be > 2", s->conf.virtqueue_size);
> +        return;
> +    }
>      s->cmd_vqs = g_new0(VirtQueue *, s->conf.num_queues);
>      s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
>      s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
> @@ -949,6 +955,8 @@ static Property virtio_scsi_properties[] = {
>      DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
>      DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI,
>                                           parent_obj.conf.virtqueue_size, 128),
> +    DEFINE_PROP_BOOL("seg_max_adjust", VirtIOSCSI,
> +                      parent_obj.conf.seg_max_adjust, true),
>      DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
>                                                    0xFFFF),
>      DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSI, parent_obj.conf.cmd_per_lun,
> diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
> index 9c19f5b634..1e62f869b2 100644
> --- a/include/hw/virtio/virtio-blk.h
> +++ b/include/hw/virtio/virtio-blk.h
> @@ -38,6 +38,7 @@ struct VirtIOBlkConf
>      uint32_t request_merging;
>      uint16_t num_queues;
>      uint16_t queue_size;
> +    bool seg_max_adjust;
>      uint32_t max_discard_sectors;
>      uint32_t max_write_zeroes_sectors;
>      bool x_enable_wce_if_config_wce;
> diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
> index 122f7c4b6f..24e768909d 100644
> --- a/include/hw/virtio/virtio-scsi.h
> +++ b/include/hw/virtio/virtio-scsi.h
> @@ -48,6 +48,7 @@ typedef struct virtio_scsi_config VirtIOSCSIConfig;
>  struct VirtIOSCSIConf {
>      uint32_t num_queues;
>      uint32_t virtqueue_size;
> +    bool seg_max_adjust;
>      uint32_t max_sectors;
>      uint32_t cmd_per_lun;
>  #ifdef CONFIG_VHOST_SCSI
> -- 
> 2.17.0