summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
-rw-r--r--virtio-v1.0-wd01-part1-specification.txt54
1 files changed, 39 insertions, 15 deletions
diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 36f630c..e8fbe69 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -2224,8 +2224,6 @@ 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.
@@ -2258,9 +2256,19 @@ bits as indicated above.
u16 min_io_size;
u32 opt_io_size;
} topology;
+ u8 reserved;
};
+ 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
-----------------------------
@@ -2282,6 +2290,21 @@ bits as indicated above.
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_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
------------------------
@@ -2317,12 +2340,26 @@ 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 non-volatile storage by the device.
+
2.4.2.5.1 Legacy Interface: Device Operation
------------------------
The 'reserved' field was previously called ioprio. The ioprio field
is a hint about the relative priorities of requests to the device:
higher numbers indicate more important requests.
+ #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.
+
If the device has VIRTIO_BLK_F_SCSI feature, it can also support
scsi packet command requests, each of these requests is of form:
@@ -2382,19 +2419,6 @@ 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
====================