scsi-disk: add UNMAP limits to block limits VPD page

Linux prefers WRITE SAME to UNMAP if the limits are zero, and WRITE
SAME does not discard anything unless the device can guarantee that
the resulting block is zero.

Setting the maximum unmap block and descriptor counts to non-zero
makes Linux choose UNMAP and fixes thin provisioning on glusterfs.

While the maximum unmap block count can have some effect on performance,
the (suggested) maximum number of descriptors is not particularly
important so I didn't add a customization option.  SCSI drivers are
used to online firmware updates so I'm not yet adding versioning support
for SCSI, but we're probably getting close to the point when it's worth
thinking about it.
Reported-by: NBharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-disk.c

@@ -47,6 +47,7 @@ do { printf("scsi-disk: " fmt , ## __VA_ARGS__); } while (0)
 #define SCSI_MAX_MODE_LEN           256
 
 #define DEFAULT_DISCARD_GRANULARITY 4096
+#define DEFAULT_MAX_UNMAP_SIZE      (1 << 30)   /* 1 GB */
 
 typedef struct SCSIDiskState SCSIDiskState;
 
@@ -74,6 +75,7 @@ struct SCSIDiskState
     bool media_event;
     bool eject_request;
     uint64_t wwn;
+    uint64_t max_unmap_size;
     QEMUBH *bh;
     char *version;
     char *serial;
@@ -625,6 +627,8 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
                     s->qdev.conf.min_io_size / s->qdev.blocksize;
             unsigned int opt_io_size =
                     s->qdev.conf.opt_io_size / s->qdev.blocksize;
+            unsigned int max_unmap_sectors =
+                    s->max_unmap_size / s->qdev.blocksize;
 
             if (s->qdev.type == TYPE_ROM) {
                 DPRINTF("Inquiry (EVPD[%02X] not supported for CDROM\n",
@@ -647,6 +651,18 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
             outbuf[14] = (opt_io_size >> 8) & 0xff;
             outbuf[15] = opt_io_size & 0xff;
 
+            /* max unmap LBA count, default is 1GB */
+            outbuf[20] = (max_unmap_sectors >> 24) & 0xff;
+            outbuf[21] = (max_unmap_sectors >> 16) & 0xff;
+            outbuf[22] = (max_unmap_sectors >> 8) & 0xff;
+            outbuf[23] = max_unmap_sectors & 0xff;
+
+            /* max unmap descriptors, 255 fit in 4 kb with an 8-byte header.  */
+            outbuf[24] = 0;
+            outbuf[25] = 0;
+            outbuf[26] = 0;
+            outbuf[27] = 255;
+
             /* optimal unmap granularity */
             outbuf[28] = (unmap_sectors >> 24) & 0xff;
             outbuf[29] = (unmap_sectors >> 16) & 0xff;
@@ -2519,6 +2535,8 @@ static Property scsi_hd_properties[] = {
     DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
                     SCSI_DISK_F_DPOFUA, false),
     DEFINE_PROP_HEX64("wwn", SCSIDiskState, wwn, 0),
+    DEFINE_PROP_UINT64("max_unmap_size", SCSIDiskState, max_unmap_size,
+                       DEFAULT_MAX_UNMAP_SIZE),
     DEFINE_BLOCK_CHS_PROPERTIES(SCSIDiskState, qdev.conf),
     DEFINE_PROP_END_OF_LIST(),
 };
@@ -2628,6 +2646,8 @@ static Property scsi_disk_properties[] = {
     DEFINE_PROP_BIT("dpofua", SCSIDiskState, features,
                     SCSI_DISK_F_DPOFUA, false),
     DEFINE_PROP_HEX64("wwn", SCSIDiskState, wwn, 0),
+    DEFINE_PROP_UINT64("max_unmap_size", SCSIDiskState, max_unmap_size,
+                       DEFAULT_MAX_UNMAP_SIZE),
     DEFINE_PROP_END_OF_LIST(),
 };