[Qemu-devel] [RFC PATCH v2 7/8] pc-bios/s390-ccw: Build a reasonable max_sectors limit

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

Now that we've read all the possible limits that have been defined for
a virtio-scsi controller and the disk we're booting from, it's possible
that we are STILL going to exceed the limits of the host device.
For example, a "-device scsi-generic" device does not support the
Block Limits VPD page.

So, let's fallback to something that seems to work for most boot
configurations if larger values were specified (including if nothing
was explicitly specified, and we took default values).

Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
---
 pc-bios/s390-ccw/virtio-scsi.c | 9 +++++++++
 pc-bios/s390-ccw/virtio-scsi.h | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/pc-bios/s390-ccw/virtio-scsi.c b/pc-bios/s390-ccw/virtio-scsi.c
index b722f25..f61ecf0 100644
--- a/pc-bios/s390-ccw/virtio-scsi.c
+++ b/pc-bios/s390-ccw/virtio-scsi.c
@@ -399,6 +399,15 @@ void virtio_scsi_setup(VDev *vdev)
         vdev->max_transfer = evpd_bl->max_transfer;
     }
 
+    /*
+     * The host sg driver will often be unhappy with particularly large
+     * I/Os that exceed the block iovec limits.  Let's enforce something
+     * reasonable, despite what the device configuration tells us.
+     */
+
+    vdev->max_transfer = MIN_NON_ZERO(VIRTIO_SCSI_MAX_SECTORS,
+                                      vdev->max_transfer);
+
     if (!scsi_read_capacity(vdev, data, data_size)) {
         virtio_scsi_verify_response(&resp, "virtio-scsi:setup:read_capacity");
     }
diff --git a/pc-bios/s390-ccw/virtio-scsi.h b/pc-bios/s390-ccw/virtio-scsi.h
index f50b38b..4c4f4bb 100644
--- a/pc-bios/s390-ccw/virtio-scsi.h
+++ b/pc-bios/s390-ccw/virtio-scsi.h
@@ -19,6 +19,8 @@
 #define VIRTIO_SCSI_CDB_SIZE   SCSI_DEFAULT_CDB_SIZE
 #define VIRTIO_SCSI_SENSE_SIZE SCSI_DEFAULT_SENSE_SIZE
 
+#define VIRTIO_SCSI_MAX_SECTORS 2048
+
 /* command-specific response values */
 #define VIRTIO_SCSI_S_OK                     0x00
 #define VIRTIO_SCSI_S_BAD_TARGET             0x03
-- 
2.10.2