Re: [Qemu-devel] [RFC PATCH v1 1/5] hw/scsi: Override the max_sectors value for virtio-scsi

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/04/2017 16:46, Eric Farman wrote:
> The virtio spec states that the max_sectors field is
> "a hint to the driver for the maximum transfer size"
> that would be used for a virtio-scsi request.
> 
> It's currently hardcoded to xFFFF unless one is established
> by the max_sectors parameter on the command line, but let's
> roll through the associated devices and set it to anything
> lower if one is set for the underlying block device and
> retrieved by the BLKSECTGET ioctl.

Have you tried looking at the block limits VPD page in the firmware?
QEMU is passing BLKSECTGET down by patching that VPD page.  Your
solution would not work with hotplug.

Thanks,

Paolo

> Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
> ---
>  hw/scsi/virtio-scsi.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> index 46a3e3f..bca9461 100644
> --- a/hw/scsi/virtio-scsi.c
> +++ b/hw/scsi/virtio-scsi.c
> @@ -640,7 +640,11 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
>                                     uint8_t *config)
>  {
>      VirtIOSCSIConfig *scsiconf = (VirtIOSCSIConfig *)config;
> +    VirtIOSCSI *vs = VIRTIO_SCSI(vdev);
>      VirtIOSCSICommon *s = VIRTIO_SCSI_COMMON(vdev);
> +    SCSIDevice *d;
> +    BusChild *kid;
> +    unsigned int max_transfer;
>  
>      virtio_stl_p(vdev, &scsiconf->num_queues, s->conf.num_queues);
>      virtio_stl_p(vdev, &scsiconf->seg_max, 128 - 2);
> @@ -652,6 +656,14 @@ static void virtio_scsi_get_config(VirtIODevice *vdev,
>      virtio_stw_p(vdev, &scsiconf->max_channel, VIRTIO_SCSI_MAX_CHANNEL);
>      virtio_stw_p(vdev, &scsiconf->max_target, VIRTIO_SCSI_MAX_TARGET);
>      virtio_stl_p(vdev, &scsiconf->max_lun, VIRTIO_SCSI_MAX_LUN);
> +
> +    QTAILQ_FOREACH(kid, &vs->bus.qbus.children, sibling) {
> +        d = SCSI_DEVICE(kid->child);
> +        max_transfer = blk_get_max_transfer(d->conf.blk) / d->blocksize;
> +        virtio_stl_p(vdev,
> +                     &scsiconf->max_sectors,
> +                     MIN_NON_ZERO(max_transfer, scsiconf->max_sectors));
> +    }
>  }
>  
>  static void virtio_scsi_set_config(VirtIODevice *vdev,
>