[Xen-devel] [PATCH 3/3] xen-blkfront: Introduce BLKIF_OP_FLUSH_DISKCACHE support.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

If the backend supports the 'feature-flush-cache' mode, use that
instead of the 'feature-barrier' support.

Currently there are three backends that support the 'feature-flush-cache'
mode: NetBSD, Solaris and Linux kernel. The 'flush' option is much
light-weight version than the 'barrier' support so lets try to use as
there are no filesystems in the kernel that use full barriers anymore.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 drivers/block/xen-blkfront.c |   49 ++++++++++++++++++++++++++++++-----------
 1 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 2075981..b536a9c 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -97,6 +97,7 @@ struct blkfront_info
        struct blk_shadow shadow[BLK_RING_SIZE];
        unsigned long shadow_free;
        unsigned int feature_flush;
+       unsigned int flush_op;
        int is_ready;
 };
 
@@ -250,8 +251,7 @@ static int blkif_ioctl(struct block_device *bdev, fmode_t 
mode,
 
 /*
  * Generate a Xen blkfront IO request from a blk layer request.  Reads
- * and writes are handled as expected.  Since we lack a loose flush
- * request, we map flushes into a full ordered barrier.
+ * and writes are handled as expected.
  *
  * @req: a request struct
  */
@@ -293,14 +293,13 @@ static int blkif_queue_request(struct request *req)
 
        if (req->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
                /*
-                * Ideally we could just do an unordered
-                * flush-to-disk, but all we have is a full write
-                * barrier at the moment.  However, a barrier write is
+                * Ideally we can do an unordered flush-to-disk. In case the
+                * backend onlysupports barriers, use that. A barrier request
                 * a superset of FUA, so we can implement it the same
                 * way.  (It's also a FLUSH+FUA, since it is
                 * guaranteed ordered WRT previous writes.)
                 */
-               ring_req->operation = BLKIF_OP_WRITE_BARRIER;
+               ring_req->operation = info->flush_op;
        }
 
        ring_req->nr_segments = blk_rq_map_sg(req->q, req, info->sg);
@@ -433,8 +432,11 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 
sector_size)
 static void xlvbd_flush(struct blkfront_info *info)
 {
        blk_queue_flush(info->rq, info->feature_flush);
-       printk(KERN_INFO "blkfront: %s: barriers %s\n",
+       printk(KERN_INFO "blkfront: %s: %s: %s\n",
               info->gd->disk_name,
+              info->flush_op == BLKIF_OP_WRITE_BARRIER ?
+               "barrier" : (info->flush_op == BLKIF_OP_FLUSH_DISKCACHE ?
+               "flush diskcache" : "barrier or flush"),
               info->feature_flush ? "enabled" : "disabled");
 }
 
@@ -720,15 +722,20 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 
                error = (bret->status == BLKIF_RSP_OKAY) ? 0 : -EIO;
                switch (bret->operation) {
+               case BLKIF_OP_FLUSH_DISKCACHE:
                case BLKIF_OP_WRITE_BARRIER:
                        if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
-                               printk(KERN_WARNING "blkfront: %s: write 
barrier op failed\n",
+                               printk(KERN_WARNING "blkfront: %s: write %s op 
failed\n",
+                                      info->flush_op == BLKIF_OP_WRITE_BARRIER 
?
+                                      "barrier" :  "flush disk cache",
                                       info->gd->disk_name);
                                error = -EOPNOTSUPP;
                        }
                        if (unlikely(bret->status == BLKIF_RSP_ERROR &&
                                     info->shadow[id].req.nr_segments == 0)) {
-                               printk(KERN_WARNING "blkfront: %s: empty write 
barrier op failed\n",
+                               printk(KERN_WARNING "blkfront: %s: empty write 
%s op failed\n",
+                                      info->flush_op == BLKIF_OP_WRITE_BARRIER 
?
+                                      "barrier" :  "flush disk cache",
                                       info->gd->disk_name);
                                error = -EOPNOTSUPP;
                        }
@@ -736,6 +743,7 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
                                if (error == -EOPNOTSUPP)
                                        error = 0;
                                info->feature_flush = 0;
+                               info->flush_op = 0;
                                xlvbd_flush(info);
                        }
                        /* fall through */
@@ -1100,7 +1108,7 @@ static void blkfront_connect(struct blkfront_info *info)
        unsigned long sector_size;
        unsigned int binfo;
        int err;
-       int barrier;
+       int barrier, flush;
 
        switch (info->connected) {
        case BLKIF_STATE_CONNECTED:
@@ -1140,6 +1148,9 @@ static void blkfront_connect(struct blkfront_info *info)
                return;
        }
 
+       info->feature_flush = 0;
+       info->flush_op = 0;
+
        err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
                            "feature-barrier", "%d", &barrier,
                            NULL);
@@ -1151,11 +1162,23 @@ static void blkfront_connect(struct blkfront_info *info)
         *
         * If there are barriers, then we use flush.
         */
-       info->feature_flush = 0;
-
-       if (!err && barrier)
+       if (!err && barrier) {
                info->feature_flush = REQ_FLUSH | REQ_FUA;
+               info->flush_op = BLKIF_OP_WRITE_BARRIER;
+       }
+       /*
+        * And if there is "feature-flush-cache" use that above
+        * barriers.
+        */
+       err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                           "feature-flush-cache", "%d", &flush,
+                           NULL);
 
+       if (!err && flush) {
+               info->feature_flush = REQ_FLUSH;
+               info->flush_op = BLKIF_OP_FLUSH_DISKCACHE;
+       }
+               
        err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
        if (err) {
                xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
-- 
1.7.4.1


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