scsi: Fix transfer length for READ POSITION commands.

The transfer length depends on the specific service action
code, as defined in the SCSI stream commands spec section 7.7.
Up to now only the extended form was supported.
Signed-off-by: NChristian Hoff <christian.hoff@de.ibm.com>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi-bus.c

@@ -901,7 +901,21 @@ static int scsi_req_stream_length(SCSICommand *cmd, SCSIDevice *dev, uint8_t *bu
         cmd->xfer = buf[13] | (buf[12] << 8);
         break;
     case READ_POSITION:
-        cmd->xfer = buf[8] | (buf[7] << 8);
+        switch (buf[1] & 0x1f) /* operation code */ {
+        case SHORT_FORM_BLOCK_ID:
+        case SHORT_FORM_VENDOR_SPECIFIC:
+            cmd->xfer = 20;
+            break;
+        case LONG_FORM:
+            cmd->xfer = 32;
+            break;
+        case EXTENDED_FORM:
+            cmd->xfer = buf[8] | (buf[7] << 8);
+            break;
+        default:
+            return -1;
+        }
+
         break;
     case FORMAT_UNIT:
         cmd->xfer = buf[4] | (buf[3] << 8);

hw/scsi-defs.h

@@ -146,6 +146,14 @@
  */
 #define SAI_READ_CAPACITY_16  0x10
 
+/*
+ * READ POSITION service action codes
+ */
+#define SHORT_FORM_BLOCK_ID  0x00
+#define SHORT_FORM_VENDOR_SPECIFIC 0x01
+#define LONG_FORM            0x06
+#define EXTENDED_FORM        0x08
+
 /*
  *  SAM Status codes
  */