Paolo Bonzini <> writes:
> Il 02/10/2013 02:50, Rusty Russell ha scritto:
>>> > I would make this even stronger, like "It is undefined whether a disk
>>> > operates in writeback or writethrough mode after a guest announces lack
>>> > of support for VIRTIO_BLK_F_FLUSH".
>> At this point it's documenting historical behavior, rather than
>> proscribing it. How about we simply note:
>>
>> Some older legacy devices did not operate in writethrough mode even
>> after a guest announced lack of support for VIRTIO_BLK_F_FLUSH.
>
> That's fine.
>
> Paolo
Here's the final patch then:
commit ff7486a8699c06743473c0078e9b511275d65306
Author: Rusty Russell <>
Date: Tue Oct 1 14:53:17 2013 +0930
virtio_blk: deprecate flush.
This is partially a port of Paolo Bonzini <>'s
WCE spec changes, but puts them in the legacy section. It reserves
the writeback field for compatibility.
Flush is now required.
See: https://lists.oasis-open.org/archives/virtio-comment/201308/msg00078.html
Signed-off-by: Rusty Russell <>
diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 5efeba7..a9c0cce 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -1817,8 +1817,6 @@ device except where noted.
VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.
- VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
-
VIRTIO_BLK_F_TOPOLOGY (10) Device exports information on optimal I/O
alignment.
@@ -1843,6 +1841,7 @@ device except where noted.
u16 min_io_size;
u32 opt_io_size;
} topology;
+ u8 reserved;
};
2.4.2.3.1 Legacy Interface: Feature bits
@@ -1851,6 +1850,15 @@ device except where noted.
VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
+ VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
+
+ VIRTIO_BLK_F_CONFIG_WCE (11) Device can toggle its cache between writeback
+ and writethrough modes.
+
+VIRTIO_BLK_F_FLUSH was also called VIRTIO_BLK_F_WCE: Legacy drivers
+should only negotiate this feature if they are capable of sending
+VIRTIO_BLK_T_FLUSH commands.
+
2.4.2.4. Device Initialization
-----------------------------
@@ -1872,6 +1880,21 @@ device except where noted.
I/O lengths for the driver to use. This also does not affect the units
in the protocol, only performance.
+2.4.2.4.1. Legacy Interface: Device Initialization
+-----------------------------
+
+The reserved field used to be called writeback. If the
+VIRTIO_BLK_F_CONFIG_FLUSH feature is offered, the cache mode should be
+read from the writeback field of the configuration if available; the
+driver can also write to the field in order to toggle the cache
+between writethrough (0) and writeback (1) mode. If the feature is
+not available, the driver can instead look at the result of
+negotiating VIRTIO_BLK_F_FLUSH: the cache will be in writeback mode
+after reset if and only if VIRTIO_BLK_F_FLUSH is negotiated.
+
+Some older legacy devices did not operate in writethrough mode even
+after a guest announced lack of support for VIRTIO_BLK_F_FLUSH.
+
2.4.2.5. Device Operation
------------------------
@@ -1907,6 +1930,9 @@ error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:
#define VIRTIO_BLK_S_IOERR 1
#define VIRTIO_BLK_S_UNSUPP 2
+Any writes completed before the submission of the flush command should
+be committed to permanent storage by the device.
+
2.4.2.5.1 Legacy Interface: Device Operation
------------------------
The 'reserved' field was previously called ioprio. The ioprio field
