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

[Eric Farman <farman@linux.vnet.ibm.com>]

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.

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,
-- 
2.10.2