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[Xen-devel] [PATCH 2/3] xen-blkfront: Provide for 'feature-flush-cache' the BLKIF_OP_WRITE_FLUSH_CACHE operation.

The operation BLKIF_OP_WRITE_FLUSH_CACHE has existed in the Xen
tree header file for years but it was never present in the Linux tree
because the frontend (nor the backend) supported this interface.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 include/xen/interface/io/blkif.h |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
index 61e523a..3d5d6db 100644
--- a/include/xen/interface/io/blkif.h
+++ b/include/xen/interface/io/blkif.h
@@ -45,6 +45,19 @@ typedef uint64_t blkif_sector_t;
+ * Recognised if "feature-flush-cache" is present in backend xenbus
+ * info.  A flush will ask the underlying storage hardware to flush its
+ * non-volatile caches as appropriate.  The "feature-flush-cache" node
+ * contains a boolean indicating whether flush requests are likely to
+ * succeed or fail. Either way, a flush request may fail at any time
+ * with BLKIF_RSP_EOPNOTSUPP if it is unsupported by the underlying
+ * block-device hardware. The boolean simply indicates whether or not it
+ * is worthwhile for the frontend to attempt flushes.  If a backend does
+ * not recognise BLKIF_OP_WRITE_FLUSH_CACHE, it should *not* create the
+ * "feature-flush-cache" node!
+ */
  * Maximum scatter/gather segments per request.
  * This is carefully chosen so that sizeof(struct blkif_ring) <= PAGE_SIZE.
  * NB. This could be 12 if the ring indexes weren't stored in the same page.

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