target/iblock: Use backend REQ_FLUSH hint for WriteCacheEnabled status

This patch allows IBLOCK to check block hints in request_queue->flush_flags
when reporting current backend device WriteCacheEnabled status to a remote
SCSI initiator port.

This is done via a se_subsystem_api->get_write_cache() call instead of a
backend se_device creation time flag, as we expect REQ_FLUSH bits to possibly
change from an underlying blk_queue_flush() by the SCSI disk driver, or
internal raw struct block_device driver usage.

Also go ahead and update iblock_execute_rw() bio I/O path code to use
REQ_FLUSH + REQ_FUA hints when determining WRITE_FUA usage, and make SPC
emulation code use a spc_check_dev_wce() helper to handle both types of
cases for virtual backend subsystem drivers.

(asias: Drop unnecessary comparsion operators)

Reported-by: majianpeng <majianpeng@gmail.com>
Cc: majianpeng <majianpeng@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/target/target_core_backend.h b/include/target/target_core_backend.h
index 819b0fc45215..f9a80169775d 100644
--- a/include/target/target_core_backend.h
+++ b/include/target/target_core_backend.h
@@ -35,6 +35,7 @@ struct se_subsystem_api {
         u32 (*get_device_type)(struct se_device *);
         sector_t (*get_blocks)(struct se_device *);
         unsigned char *(*get_sense_buffer)(struct se_cmd *);
+        bool (*get_write_cache)(struct se_device *);
 };
 
 struct sbc_ops {