Re: [virtio] [OASIS Issue Tracker] Created: (VIRTIO-19) block: Revisit flush/barrier semantics

[Rusty Russell <rusty@au1.ibm.com>]

OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
> block: Revisit flush/barrier semantics
> --------------------------------------
>
>                  Key: VIRTIO-19
>                  URL: http://tools.oasis-open.org/issues/browse/VIRTIO-19
>              Project: OASIS Virtual I/O Device (VIRTIO) TC
>           Issue Type: Improvement
>             Reporter: Rusty Russell
>
>
> Flush and barrier semantics seem to be insufficient or inefficient for Linux, at least.

It would look like this:

commit c0886e1c0e94eddad44d675f2ccae66835d3afda
Author: Rusty Russell <rusty@au1.ibm.com>
Date:   Tue Sep 17 14:26:55 2013 +0930

    2.4.2.3: Remove VIRTIO_BLK_F_BARRIER.
    
    As per VIRTIO-19.
    
    Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 743467a..8f95592 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1584,8 +1584,6 @@ device except where noted.
 2.4.2.3. Feature bits
 --------------------
 
-  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.
-
   VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
     in “size_max”.
 
@@ -1620,6 +1618,10 @@ device except where noted.
 		u32 blk_size;
 	};
 
+2.4.2.3.1. Legacy Ingterface: Feature bits
+--------------------
+  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.
+
 2.4.2.4. Device Initialization
 -----------------------------
 
@@ -1671,14 +1673,7 @@ scsi packet command requests, each of these requests is of form:
 The type of the request is either a read (VIRTIO_BLK_T_IN), a write
 (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
 VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or
-VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER
-feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
-request acts as a barrier and that all preceeding requests must be
-complete before this one, and all following requests must not be
-started until this is complete. Note that a barrier does not flush
-caches in the underlying backend device in host, and thus does not
-serve as data consistency guarantee. Driver must use FLUSH request to
-flush the host cache.
+VIRTIO_BLK_T_FLUSH_OUT[23]).
 
 	#define VIRTIO_BLK_T_IN           0
 	#define VIRTIO_BLK_T_OUT          1
@@ -1686,7 +1681,6 @@ flush the host cache.
 	#define VIRTIO_BLK_T_SCSI_CMD_OUT 3
 	#define VIRTIO_BLK_T_FLUSH        4
 	#define VIRTIO_BLK_T_FLUSH_OUT    5
-	#define VIRTIO_BLK_T_BARRIER	 0x80000000
 
 The ioprio field is a hint about the relative priorities of
 requests to the device: higher numbers indicate more important
@@ -1738,6 +1732,19 @@ data_len, sense_len and residual in a single write-only buffer;
 and the status field is a separate read-only buffer of size 1
 byte, by itself.
 
+2.4.2.5.1. Legacy Interface: Device Operation
+------------------------
+	#define VIRTIO_BLK_T_BARRIER	 0x80000000
+
+If the device has VIRTIO_BLK_F_BARRIER
+feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
+request acts as a barrier and that all preceeding requests must be
+complete before this one, and all following requests must not be
+started until this is complete. Note that a barrier does not flush
+caches in the underlying backend device in host, and thus does not
+serve as data consistency guarantee. Driver must use FLUSH request to
+flush the host cache.
+
 
 2.4.3. Console Device
 ====================