Re: [Qemu-devel] [PATCH v1 1/2] scsi-disk: support reporting of rotation rate

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

On Wed, Oct 04, 2017 at 05:49:44PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > The Linux kernel will query the SCSI "Block device characteristics"
> > VPD to determine the rotations per minute of the disk. If this has
> > the value 1, it is taken to be an SSD and so Linux sets the
> > 'rotational' flag to 0 for the I/O queue and will stop using that
> > disk as a source of random entropy. Other operating systems may
> > also take into account rotation rate when setting up default
> > behaviour.
> > 
> > Mgmt apps should be able to set the rotation rate for virtualized
> > block devices, based on characteristics of the host storage in use,
> > so that the guest OS gets sensible behaviour out of the box. This
> > patch thus adds a 'rotation-rate' parameter for 'scsi-hd' and
> > 'scsi-block' device types. For the latter, this parameter will be
> > ignored unless the host device has TYPE_DISK.
> > 
> > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> > ---
> >  hw/scsi/scsi-disk.c | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> > index 6e841fb5ff..a518080e7d 100644
> > --- a/hw/scsi/scsi-disk.c
> > +++ b/hw/scsi/scsi-disk.c
> > @@ -104,6 +104,14 @@ typedef struct SCSIDiskState
> >      char *product;
> >      bool tray_open;
> >      bool tray_locked;
> > +    /*
> > +     * 0x0000        - rotation rate not reported
> > +     * 0x0001        - non-rotating medium (SSD)
> > +     * 0x0002-0x0400 - reserved
> > +     * 0x0401-0xffe  - rotations per minute
> > +     * 0xffff        - reserved
> > +     */
> > +    uint16_t rotation_rate;
> >  } SCSIDiskState;
> >  
> >  static bool scsi_handle_rw_error(SCSIDiskReq *r, int error, bool acct_failed);
> > @@ -605,6 +613,7 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
> >              outbuf[buflen++] = 0x83; // device identification
> >              if (s->qdev.type == TYPE_DISK) {
> >                  outbuf[buflen++] = 0xb0; // block limits
> > +                outbuf[buflen++] = 0xb1; /* block device characteristics */
> >                  outbuf[buflen++] = 0xb2; // thin provisioning
> >              }
> >              break;
> > @@ -747,6 +756,15 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
> >              outbuf[43] = max_io_sectors & 0xff;
> >              break;
> >          }
> > +        case 0xb1: /* block device characteristics */
> > +        {
> > +            buflen = 8;
> > +            outbuf[4] = (s->rotation_rate >> 8) & 0xff;
> > +            outbuf[5] = s->rotation_rate & 0xff;
> > +            outbuf[6] = 0;
> > +            outbuf[7] = 0;
> > +            break;
> > +        }
> >          case 0xb2: /* thin provisioning */
> >          {
> >              buflen = 8;
> > @@ -2911,6 +2929,7 @@ static Property scsi_hd_properties[] = {
> >                         DEFAULT_MAX_UNMAP_SIZE),
> >      DEFINE_PROP_UINT64("max_io_size", SCSIDiskState, max_io_size,
> >                         DEFAULT_MAX_IO_SIZE),
> > +    DEFINE_PROP_UINT16("rotation_rate", SCSIDiskState, rotation_rate, 0),
> >      DEFINE_BLOCK_CHS_PROPERTIES(SCSIDiskState, qdev.conf),
> >      DEFINE_PROP_END_OF_LIST(),
> >  };
> > @@ -2982,6 +3001,7 @@ static const TypeInfo scsi_cd_info = {
> >  static Property scsi_block_properties[] = {
> >      DEFINE_BLOCK_ERROR_PROPERTIES(SCSIDiskState, qdev.conf),         \
> >      DEFINE_PROP_DRIVE("drive", SCSIDiskState, qdev.conf.blk),
> > +    DEFINE_PROP_UINT16("rotation_rate", SCSIDiskState, rotation_rate, 0),
> >      DEFINE_PROP_END_OF_LIST(),
> >  };
> 
> Are you sure you want this as a property of SCSIDiskState etc rather
> than a property on the underlying drive?
> Then if virtio devices etc wanted to pick this up later they could
> without needing extra parameters.

That would make 'rotation_rate' available for everything, regardless
of whether its used/supported by the frontend device. I was trying to
restrict this to only the places where its appropriate, so only disks,
not cdroms for example.

Regards,
Daniel
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