7448 nvme: performance regression when volatile write cache not present
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Hans Rosenfeld <hans.rosenfeld@nexenta.com>

*** 2505,2514 ****
--- 2505,2515 ----
          int instance;
          int nregs;
          off_t regsize;
          int i;
          char name[32];
+         bd_ops_t l_bd_ops = nvme_bd_ops;
  
          if (cmd != DDI_ATTACH)
                  return (DDI_FAILURE);
  
          instance = ddi_get_instance(dip);
*** 2641,2652 ****
           */
          for (i = 0; i != nvme->n_namespace_count; i++) {
                  if (nvme->n_ns[i].ns_ignore)
                          continue;
  
                  nvme->n_ns[i].ns_bd_hdl = bd_alloc_handle(&nvme->n_ns[i],
!                     &nvme_bd_ops, &nvme->n_prp_dma_attr, KM_SLEEP);
  
                  if (nvme->n_ns[i].ns_bd_hdl == NULL) {
                          dev_err(dip, CE_WARN,
                              "!failed to get blkdev handle for namespace %d", i);
                          goto fail;
--- 2642,2657 ----
           */
          for (i = 0; i != nvme->n_namespace_count; i++) {
                  if (nvme->n_ns[i].ns_ignore)
                          continue;
  
+                 if (!nvme->n_write_cache_present) {
+                         l_bd_ops.o_sync_cache = NULL;
+                 }
+ 
                  nvme->n_ns[i].ns_bd_hdl = bd_alloc_handle(&nvme->n_ns[i],
!                     &l_bd_ops, &nvme->n_prp_dma_attr, KM_SLEEP);
  
                  if (nvme->n_ns[i].ns_bd_hdl == NULL) {
                          dev_err(dip, CE_WARN,
                              "!failed to get blkdev handle for namespace %d", i);
                          goto fail;