virtio_blk: add support for cache flush

Recent qemu has added a VIRTIO_BLK_F_FLUSH flag to advertise that the
virtual disk has a volatile write cache that needs to be flushed.  In case
we see this feature implement tell the Linux block layer about the fact
and use the new VIRTIO_BLK_T_FLUSH to flush the cache when required.  This
allows for an correct and simple implementation of write barriers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 40 insertions, 5 deletions

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 73de7532fcf5..3d5fe97569c7 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -92,15 +92,26 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
                 return false;
 
         vbr->req = req;
-        if (blk_fs_request(vbr->req)) {
+        switch (req->cmd_type) {
+        case REQ_TYPE_FS:
                 vbr->out_hdr.type = 0;
                 vbr->out_hdr.sector = blk_rq_pos(vbr->req);
                 vbr->out_hdr.ioprio = req_get_ioprio(vbr->req);
-        } else if (blk_pc_request(vbr->req)) {
+                break;
+        case REQ_TYPE_BLOCK_PC:
                 vbr->out_hdr.type = VIRTIO_BLK_T_SCSI_CMD;
                 vbr->out_hdr.sector = 0;
                 vbr->out_hdr.ioprio = req_get_ioprio(vbr->req);
-        } else {
+                break;
+        case REQ_TYPE_LINUX_BLOCK:
+                if (req->cmd[0] == REQ_LB_OP_FLUSH) {
+                        vbr->out_hdr.type = VIRTIO_BLK_T_FLUSH;
+                        vbr->out_hdr.sector = 0;
+                        vbr->out_hdr.ioprio = req_get_ioprio(vbr->req);
+                        break;
+                }
+                /*FALLTHRU*/
+        default:
                 /* We don't put anything else in the queue. */
                 BUG();
         }
@@ -200,6 +211,12 @@ out:
         return err;
 }
 
+static void virtblk_prepare_flush(struct request_queue *q, struct request *req)
+{
+        req->cmd_type = REQ_TYPE_LINUX_BLOCK;
+        req->cmd[0] = REQ_LB_OP_FLUSH;
+}
+
 static int virtblk_ioctl(struct block_device *bdev, fmode_t mode,
                          unsigned cmd, unsigned long data)
 {
@@ -338,7 +355,10 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
         index++;
 
         /* If barriers are supported, tell block layer that queue is ordered */
-        if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
+        if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
+                blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN_FLUSH,
+                                  virtblk_prepare_flush);
+        else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
                 blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL);
 
         /* If disk is read-only in the host, the guest should obey */
@@ -425,7 +445,7 @@ static struct virtio_device_id id_table[] = {
 static unsigned int features[] = {
         VIRTIO_BLK_F_BARRIER, VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX,
         VIRTIO_BLK_F_GEOMETRY, VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE,
-        VIRTIO_BLK_F_SCSI, VIRTIO_BLK_F_IDENTIFY
+        VIRTIO_BLK_F_SCSI, VIRTIO_BLK_F_IDENTIFY, VIRTIO_BLK_F_FLUSH
 };
 
 /*
diff --git a/include/linux/virtio_blk.h b/include/linux/virtio_blk.h
index 25fbabfa90d4..15cb666581d7 100644
--- a/include/linux/virtio_blk.h
+++ b/include/linux/virtio_blk.h
@@ -14,6 +14,7 @@
 #define VIRTIO_BLK_F_BLK_SIZE   6       /* Block size of disk is available*/
 #define VIRTIO_BLK_F_SCSI       7       /* Supports scsi command passthru */
 #define VIRTIO_BLK_F_IDENTIFY   8       /* ATA IDENTIFY supported */
+#define VIRTIO_BLK_F_FLUSH      9       /* Cache flush command support */
 
 #define VIRTIO_BLK_ID_BYTES     (sizeof(__u16[256]))    /* IDENTIFY DATA */
 
@@ -35,6 +36,17 @@ struct virtio_blk_config {
         __u8 identify[VIRTIO_BLK_ID_BYTES];
 } __attribute__((packed));
 
+/*
+ * Command types
+ *
+ * Usage is a bit tricky as some bits are used as flags and some are not.
+ *
+ * Rules:
+ *   VIRTIO_BLK_T_OUT may be combined with VIRTIO_BLK_T_SCSI_CMD or
+ *   VIRTIO_BLK_T_BARRIER.  VIRTIO_BLK_T_FLUSH is a command of its own
+ *   and may not be combined with any of the other flags.
+ */
+
 /* These two define direction. */
 #define VIRTIO_BLK_T_IN         0
 #define VIRTIO_BLK_T_OUT        1
@@ -42,6 +54,9 @@ struct virtio_blk_config {
 /* This bit says it's a scsi command, not an actual read or write. */
 #define VIRTIO_BLK_T_SCSI_CMD   2
 
+/* Cache flush command */
+#define VIRTIO_BLK_T_FLUSH      4
+
 /* Barrier before this op. */
 #define VIRTIO_BLK_T_BARRIER    0x80000000