scsi: target: tcmu: Fix size in calls to tcmu_flush_dcache_range

1) If remaining ring space before the end of the ring is smaller then the
   next cmd to write, tcmu writes a padding entry which fills the remaining
   space at the end of the ring.

   Then tcmu calls tcmu_flush_dcache_range() with the size of struct
   tcmu_cmd_entry as data length to flush.  If the space filled by the
   padding was smaller then tcmu_cmd_entry, tcmu_flush_dcache_range() is
   called for an address range reaching behind the end of the vmalloc'ed
   ring.

   tcmu_flush_dcache_range() in a loop calls
   flush_dcache_page(virt_to_page(start)); for every page being part of the
   range. On x86 the line is optimized out by the compiler, as
   flush_dcache_page() is empty on x86.

   But I assume the above can cause trouble on other architectures that
   really have a flush_dcache_page().  For paddings only the header part of
   an entry is relevant due to alignment rules the header always fits in
   the remaining space, if padding is needed.  So tcmu_flush_dcache_range()
   can safely be called with sizeof(entry->hdr) as the length here.

2) After it has written a command to cmd ring, tcmu calls
   tcmu_flush_dcache_range() using the size of a struct tcmu_cmd_entry as
   data length to flush.  But if a command needs many iovecs, the real size
   of the command may be bigger then tcmu_cmd_entry, so a part of the
   written command is not flushed then.

Link: https://lore.kernel.org/r/20200528193108.9085-1-bstroesser@ts.fujitsu.comAcked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_user.c

@@ -1007,7 +1007,7 @@ static int queue_cmd_ring(struct tcmu_cmd *tcmu_cmd, sense_reason_t *scsi_err)
 		entry->hdr.cmd_id = 0; /* not used for PAD */
 		entry->hdr.kflags = 0;
 		entry->hdr.uflags = 0;
-		tcmu_flush_dcache_range(entry, sizeof(*entry));
+		tcmu_flush_dcache_range(entry, sizeof(entry->hdr));
 
 		UPDATE_HEAD(mb->cmd_head, pad_size, udev->cmdr_size);
 		tcmu_flush_dcache_range(mb, sizeof(*mb));
@@ -1072,7 +1072,7 @@ static int queue_cmd_ring(struct tcmu_cmd *tcmu_cmd, sense_reason_t *scsi_err)
 	cdb_off = CMDR_OFF + cmd_head + base_command_size;
 	memcpy((void *) mb + cdb_off, se_cmd->t_task_cdb, scsi_command_size(se_cmd->t_task_cdb));
 	entry->req.cdb_off = cdb_off;
-	tcmu_flush_dcache_range(entry, sizeof(*entry));
+	tcmu_flush_dcache_range(entry, command_size);
 
 	UPDATE_HEAD(mb->cmd_head, command_size, udev->cmdr_size);
 	tcmu_flush_dcache_range(mb, sizeof(*mb));