Rusty Russell <> writes:
> Thinking about this some more: why not make WCE the only option?
>
> (1) It's simple.
> (2) We say flush SHOULD hit the disk.
OK, here's the actual patch. It applies most of Paulo's patch,
but to the legacy section, and remove VIRTIO_BLK_F_FLUSH. It also
documents what FLUSH should do (is more clarity required?)
Cheers,
Rusty.
diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 5efeba7..9b5c29a 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 +1880,22 @@ 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_WCE 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_WCE: the cache will be in writeback mode
+after reset if and only if VIRTIO_BLK_F_WCE is negotiated.
+
+Note that buggy legacy devices are common, which always operate in
+write back mode even if VIRTIO_BLK_F_WCE isn't accepted, or writeback
+is turned off.
+
2.4.2.5. Device Operation
------------------------
@@ -1907,6 +1931,9 @@ 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
