OASIS Mailing List ArchivesView the OASIS mailing list archive below
or browse/search using MarkMail.


Help: OASIS Mailing Lists Help | MarkMail Help

virtio message

[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]

Subject: Re: [virtio-dev] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks

Rusty Russell <rusty@au1.ibm.com> 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?)


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
@@ -1843,6 +1841,7 @@ device except where noted.
 			u16 min_io_size;
 			u32 opt_io_size;
 		} topology;
+		u8 reserved;
 	}; 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
+ 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.
+ 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.
+ 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.
+ Legacy Interface: Device Operation
 The 'reserved' field was previously called ioprio.  The ioprio field

[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]