Virtualization - Feb 2010 - [PATCH] virtio-spec: document block CMD and FLUSH

I took a stub at documenting CMD and FLUSH request types in virtio
block.  Christoph, could you look over this please?

I note that the interface seems full of warts to me,
this might be a first step to cleaning them.

One issue I struggled with especially is how type
field mixes bits and non-bit values. I ended up
simply defining all legal values, so that we have
CMD = 2, CMD_OUT = 3 and so on.

I also avoided instroducing inhdr/outhdr structures
that virtio blk driver in linux uses, I was concerned
that nesting tables will confuse the reader.

Comments welcome.

Signed-off-by: Michael S. Tsirkin <mst at redhat.com>

--

diff --git a/virtio-spec.lyx b/virtio-spec.lyx
index d16104a..ed35893 100644
--- a/virtio-spec.lyx
+++ b/virtio-spec.lyx
@@ -67,7 +67,11 @@ IBM Corporation
 \end_layout
 
 \begin_layout Standard
+
+\change_deleted 0 1266531118
 FIXME: virtio block scsi passthrough section
+\change_unchanged
+
 \end_layout
 
 \begin_layout Standard
@@ -4376,7 +4380,7 @@ struct virtio_net_ctrl_mac {
 The device can filter incoming packets by any number of destination MAC
  addresses.
 \begin_inset Foot
-status open
+status collapsed
 
 \begin_layout Plain Layout
 Since there are no guarentees, it can use a hash filter orsilently switch
@@ -4549,6 +4553,22 @@ blk_size
 \end_inset
 
 .
+\change_inserted 0 1266444580
+
+\end_layout
+
+\begin_layout Description
+
+\change_inserted 0 1266471229
+VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
+\end_layout
+
+\begin_layout Description
+
+\change_inserted 0 1266444605
+VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
+\change_unchanged
+
 \end_layout
 
 \begin_layout Description
@@ -4700,17 +4720,25 @@ struct virtio_blk_req {
 
 \begin_layout Plain Layout
 
+\change_deleted 0 1266472188
+
 #define VIRTIO_BLK_T_IN          0
 \end_layout
 
 \begin_layout Plain Layout
 
+\change_deleted 0 1266472188
+
 #define VIRTIO_BLK_T_OUT         1
 \end_layout
 
 \begin_layout Plain Layout
 
+\change_deleted 0 1266472188
+
 #define VIRTIO_BLK_T_BARRIER	 0x80000000
+\change_unchanged
+
 \end_layout
 
 \begin_layout Plain Layout
@@ -4735,11 +4763,15 @@ struct virtio_blk_req {
 
 \begin_layout Plain Layout
 
+\change_deleted 0 1266472204
+
 #define VIRTIO_BLK_S_OK        0
 \end_layout
 
 \begin_layout Plain Layout
 
+\change_deleted 0 1266472204
+
 #define VIRTIO_BLK_S_IOERR     1
 \end_layout
 
@@ -4759,32 +4791,481 @@ struct virtio_blk_req {
 \end_layout
 
 \begin_layout Standard
-The type of the request is either a read (VIRTIO_BLK_T_IN) or a write
(VIRTIO_BL
-K_T_OUT); the high bit 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.
+
+\change_inserted 0 1266472490
+If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi packet
+ command requests, each of these requests is of form:
+\begin_inset listings
+inline false
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472395
+
+struct virtio_scsi_pc_req {
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472375
+
+	u32 type;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472375
+
+	u32 ioprio;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266474298
+
+	u64 sector;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266474308
+
+    char cmd[];
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266505809
+
+	char data[][512];
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266505825
+
+#define SCSI_SENSE_BUFFERSIZE   96
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266505848
+
+    u8 sense[SCSI_SENSE_BUFFERSIZE];
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472969
+
+    u32 errors;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472979
+
+    u32 data_len;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472984
+
+    u32 sense_len;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472987
+
+    u32 residual;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472375
+
+	u8 status;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472375
+
+};
+\end_layout
+
+\end_inset
+
+
+\change_unchanged
+
 \end_layout
 
 \begin_layout Standard
-The ioprio field is a hint about the relative priorities of requests to
- the device: higher numbers indicate more important requests.
+The 
+\emph on
+type
+\emph default
+ of the request is either a read (VIRTIO_BLK_T_IN)
+\change_inserted 0 1266495815
+,
+\change_unchanged
+ 
+\change_deleted 0 1266495817
+or
+\change_unchanged
+ a write (VIRTIO_BLK_T_OUT)
+\change_inserted 0 1266497316
+, a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT
+\begin_inset Foot
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266497390
+the SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device does not
+ distinguish between them
+\change_unchanged
+
+\end_layout
+
+\end_inset
+
+) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT
+\begin_inset Foot
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266497402
+the FLUSH and FLUSH_OUT types are equivalent, the device does not distinguish
+ between them
+\change_unchanged
+
+\end_layout
+
+\end_inset
+
+)
+\change_deleted 0 1266503753
+;
+\change_inserted 0 1266503758
+.
+
+\change_unchanged
+ 
+\change_inserted 0 1266497301
+If the device has VIRTIO_BLK_F_BARRIER feature
+\begin_inset space ~
+\end_inset
+
+
+\change_unchanged
+the high bit
+\change_inserted 0 1266497301
+ (VIRTIO_BLK_T_BARRIER)
+\change_unchanged
+ 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.
+
+\change_inserted 0 1266504385
+ 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.
+\change_unchanged
+
 \end_layout
 
 \begin_layout Standard
-The sector number indicates the offset (multiplied by 512) where the read
- or write is to occur.
+
+\change_inserted 0 1266472135
+\begin_inset listings
+inline false
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266495783
+
+#define VIRTIO_BLK_T_IN           0
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266495782
+
+#define VIRTIO_BLK_T_OUT          1
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266495781
+
+#define VIRTIO_BLK_T_SCSI_CMD     2
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266495799
+
+#define VIRTIO_BLK_T_SCSI_CMD_OUT 3
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266497225
+
+#define VIRTIO_BLK_T_FLUSH        4
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266497237
+
+#define VIRTIO_BLK_T_FLUSH_OUT    5
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266472135
+
+#define VIRTIO_BLK_T_BARRIER	 0x80000000
+\end_layout
+
+\end_inset
+
+
 \end_layout
 
 \begin_layout Standard
-Note that these first three fields are always read-only: the data field
- is either read-only or write-only, depending on the type of the request.
+The 
+\emph on
+ioprio
+\emph default
+ field is a hint about the relative priorities of requests to the device:
+ higher numbers indicate more important requests.
+\end_layout
+
+\begin_layout Standard
+The 
+\emph on
+sector
+\emph default
+ number indicates the offset (multiplied by 512) where the read or write
+ is to occur.
+
+\change_inserted 0 1266504683
+ This field is unused and set to 0 for scsi packet commands and for flush
+ commands.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1266527996
+The 
+\emph on
+cmd
+\emph default
+ field is only present for scsi packet command requests, and indicates the
+ command to perform.
+ This field must reside in a single, separate read-only buffer; command
+ length can be derived from the length of this buffer.
+ 
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+Note that these first three
+\change_inserted 0 1266504407
+ (four for scsi packet commands)
+\change_unchanged
+ fields are always read-only: the 
+\emph on
+data
+\emph default
+ field is either read-only or write-only, depending on 
+\change_deleted 0 1266505122
+the type of
+\change_unchanged
+ the request.
  The size of the read or write can be derived from the total size of the
- request buffer.
+ request buffer
+\change_inserted 0 1266504916
+s
+\change_unchanged
+.
+\change_inserted 0 1266506030
+
 \end_layout
 
 \begin_layout Standard
-The final byte is written by the device: either VIRTIO_BLK_S_OK or
VIRTIO_BLK_S_
-IOERR.
+
+\change_inserted 0 1266528308
+The
+\emph on
+ sense
+\emph default
+ field is only present for scsi packet command requests, and indicates the
+ buffer for scsi sense data.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1266528658
+The 
+\emph on
+data_len
+\emph default
+ field is only present for scsi packet command requests, this field is
deprecate
+d, and should be ignored by the driver.
+ Historically, devices copied data length there.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1266528675
+The 
+\emph on
+sense_len
+\emph default
+ field is only present for scsi packet command requests and indicates the
+ number of bytes actually written to the 
+\emph on
+sense
+\emph default
+ buffer.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1266528717
+The 
+\emph on
+residual
+\emph default
+ field is only present for scsi packet command requests and indicates the
+ residual size, calculated as data length - number of bytes actually
transferred.
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+The final 
+\change_inserted 0 1266471813
+
+\emph on
+status
+\emph default
+ 
+\change_unchanged
+byte is written by the device: either VIRTIO_BLK_S_OK
+\change_inserted 0 1266528888
+ for success,
+\change_deleted 0 1266528889
+ or
+\change_unchanged
+ VIRTIO_BLK_S_IOERR
+\change_inserted 0 1266529171
+ for host or guest error or VIRTIO_BLK_S_UNSUPP for a request unsupported
+ by host:
+\change_deleted 0 1266471769
+.
+\change_inserted 0 1266471782
+
+\begin_inset listings
+inline false
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266471782
+
+#define VIRTIO_BLK_S_OK        0
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266528846
+
+#define VIRTIO_BLK_S_IOERR     1
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 0 1266528863
+
+#define VIRTIO_BLK_S_UNSUPP    2
+\change_unchanged
+
+\end_layout
+
+\end_inset
+
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 0 1266528247
+Historically, devices assumed that the fields 
+\emph on
+type
+\emph default
+, 
+\emph on
+ioprio
+\emph default
+ and 
+\emph on
+sector
+\emph default
+ reside in a single, separate read-only buffer; the fields 
+\emph on
+errors
+\emph default
+, 
+\emph on
+data_len
+\emph default
+, 
+\emph on
+sense_len
+\emph default
+ and 
+\emph on
+residual
+\emph default
+ reside in a single, separate write-only buffer; the 
+\emph on
+sense
+\emph default
+ field in a separate write-only buffer of size 96 bytes, by itself; the
+ fields 
+\emph on
+errors
+\emph default
+, 
+\emph on
+data_len
+\emph default
+, 
+\emph on
+sense_len
+\emph default
+ and 
+\emph on
+residual
+\emph default
+ in a single write-only buffer; and the 
+\emph on
+status
+\emph default
+ field is a separate read-only buffer of size 1 byte, by itself.
 \end_layout
 
 \begin_layout Chapter*

On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin
wrote:
> I took a stub at documenting CMD and FLUSH request types in virtio
> block.
Any comments?

Michael S. Tsirkin wrote:
> I took a stub at documenting CMD and FLUSH request types in virtio
> block.  Christoph, could you look over this please?
> 
> I note that the interface seems full of warts to me,
> this might be a first step to cleaning them.
> 
> One issue I struggled with especially is how type
> field mixes bits and non-bit values. I ended up
> simply defining all legal values, so that we have
> CMD = 2, CMD_OUT = 3 and so on.
> 
> I also avoided instroducing inhdr/outhdr structures
> that virtio blk driver in linux uses, I was concerned
> that nesting tables will confuse the reader.
> 
> Comments welcome.
> 
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> 
> --
> 
> diff --git a/virtio-spec.lyx b/virtio-spec.lyx
> index d16104a..ed35893 100644
> --- a/virtio-spec.lyx
> +++ b/virtio-spec.lyx
> @@ -67,7 +67,11 @@ IBM Corporation
>  \end_layout
>  
>  \begin_layout Standard
> +
> +\change_deleted 0 1266531118
>  FIXME: virtio block scsi passthrough section
> +\change_unchanged
> +
>  \end_layout
>  
>  \begin_layout Standard
> @@ -4376,7 +4380,7 @@ struct virtio_net_ctrl_mac {
>  The device can filter incoming packets by any number of destination MAC
>   addresses.
>  \begin_inset Foot
> -status open
> +status collapsed
>  
>  \begin_layout Plain Layout
>  Since there are no guarentees, it can use a hash filter orsilently switch
> @@ -4549,6 +4553,22 @@ blk_size
>  \end_inset
>  
>  .
> +\change_inserted 0 1266444580
> +
> +\end_layout
> +
> +\begin_layout Description
> +
> +\change_inserted 0 1266471229
> +VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
> +\end_layout
> +
> +\begin_layout Description
> +
> +\change_inserted 0 1266444605
> +VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
> +\change_unchanged
> +
>  \end_layout
>  
>  \begin_layout Description
> @@ -4700,17 +4720,25 @@ struct virtio_blk_req {
>  
>  \begin_layout Plain Layout
>  
> +\change_deleted 0 1266472188
> +
>  #define VIRTIO_BLK_T_IN          0
>  \end_layout
>  
>  \begin_layout Plain Layout
>  
> +\change_deleted 0 1266472188
> +
>  #define VIRTIO_BLK_T_OUT         1
>  \end_layout
>  
>  \begin_layout Plain Layout
>  
> +\change_deleted 0 1266472188
> +
>  #define VIRTIO_BLK_T_BARRIER	 0x80000000
> +\change_unchanged
> +
>  \end_layout
>  
>  \begin_layout Plain Layout
> @@ -4735,11 +4763,15 @@ struct virtio_blk_req {
>  
>  \begin_layout Plain Layout
>  
> +\change_deleted 0 1266472204
> +
>  #define VIRTIO_BLK_S_OK        0
>  \end_layout
>  
>  \begin_layout Plain Layout
>  
> +\change_deleted 0 1266472204
> +
>  #define VIRTIO_BLK_S_IOERR     1
>  \end_layout
>  
> @@ -4759,32 +4791,481 @@ struct virtio_blk_req {
>  \end_layout
>  
>  \begin_layout Standard
> -The type of the request is either a read (VIRTIO_BLK_T_IN) or a write
(VIRTIO_BL
> -K_T_OUT); the high bit 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.
> +
> +\change_inserted 0 1266472490
> +If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi
packet
> + command requests, each of these requests is of form:
> +\begin_inset listings
> +inline false
> +status open
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472395
> +
> +struct virtio_scsi_pc_req {
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472375
> +
> +	u32 type;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472375
> +
> +	u32 ioprio;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266474298
> +
> +	u64 sector;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266474308
> +
> +    char cmd[];
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266505809
> +
> +	char data[][512];
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266505825
> +
> +#define SCSI_SENSE_BUFFERSIZE   96
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266505848
> +
> +    u8 sense[SCSI_SENSE_BUFFERSIZE];
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472969
> +
> +    u32 errors;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472979
> +
> +    u32 data_len;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472984
> +
> +    u32 sense_len;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472987
> +
> +    u32 residual;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472375
> +
> +	u8 status;
> +\end_layout
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266472375
> +
> +};
> +\end_layout
> +
> +\end_inset
> +
> +
> +\change_unchanged
> +
>  \end_layout
>  
>  \begin_layout Standard
> -The ioprio field is a hint about the relative priorities of requests to
> - the device: higher numbers indicate more important requests.
> +The 
> +\emph on
> +type
> +\emph default
> + of the request is either a read (VIRTIO_BLK_T_IN)
> +\change_inserted 0 1266495815
> +,
> +\change_unchanged
> + 
> +\change_deleted 0 1266495817
> +or
> +\change_unchanged
> + a write (VIRTIO_BLK_T_OUT)
> +\change_inserted 0 1266497316
> +, a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
VIRTIO_BLK_T_SCSI_CMD_OUT
> +\begin_inset Foot
> +status open
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266497390
> +the SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device does not
> + distinguish between them
> +\change_unchanged
> +
> +\end_layout
> +
> +\end_inset
> +
> +) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT
> +\begin_inset Foot
> +status open
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 0 1266497402
> +the FLUSH and FLUSH_OUT types are equivalent, the device does not
distinguish
> + between them
> +\change_unchanged
> +
> +\end_layout
> +
> +\end_inset
> +
> +)
> +\change_deleted 0 1266503753
> +;
> +\change_inserted 0 1266503758
> +.
> +
> +\change_unchanged
> + 
> +\change_inserted 0 1266497301
> +If the device has VIRTIO_BLK_F_BARRIER feature
> +\begin_inset space ~
> +\end_inset
> +
> +
> +\change_unchanged
> +the high bit
> +\change_inserted 0 1266497301
> + (VIRTIO_BLK_T_BARRIER)
> +\change_unchanged
> + 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.
> +
> +\change_inserted 0 1266504385
> + 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.
> +\change_unchanged
> +
Does this mean that virtio-blk supports all three combinations?

   1. FLUSH that isn't a barrier
   2. FLUSH that is also a barrier
   3. Barrier that is not a flush

1 is good for fsync-like operations;
2 is good for journalling-like ordered operations.
3 sounds like it doesn't mean a lot as the host cache provides no
guarantees and has no ordering facility that can be used.

-- Jamie

On Tue, Apr 20, 2010 at 12:26:27AM +0300, Michael S. Tsirkin
wrote:
> On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin wrote:
> > I took a stub at documenting CMD and FLUSH request types in virtio
> > block.
> 
> Any comments?
Rusty?

On Fri, 19 Feb 2010 08:52:20 am Michael S. Tsirkin
wrote:
> I took a stub at documenting CMD and FLUSH request types in virtio
> block.  Christoph, could you look over this please?
> 
> I note that the interface seems full of warts to me,
> this might be a first step to cleaning them.
ISTR Christoph had withdrawn some patches in this area, and was waiting
for him to resubmit?

I've given up on figuring out the block device.  What seem to me to be sane
semantics along the lines of memory barriers are foreign to disk people: they
want (and depend on) flushing everywhere.

For example, tdb transactions do not require a flush, they only require what
I would call a barrier: that prior data be written out before any future data.
Surely that would be more efficient in general than a flush!  In fact, TDB
wants only writes to *that file* (and metadata) written out first; it has no
ordering issues with other I/O on the same device.

A generic I/O interface would allow you to specify "this request depends on
these
outstanding requests" and leave it at that.  It might have some sync flush
command for dumb applications and OSes.  The userspace API might be not be as
precise and only allow such a barrier against all prior writes on this fd.

ISTR someone mentioning a desire for such an API years ago, so CC'ing the
usual I/O suspects...

Cheers,
Rusty.

On Fri, Feb 19, 2010 at 12:22:20AM +0200, Michael S. Tsirkin
wrote:
> I took a stub at documenting CMD and FLUSH request types in virtio
> block.  Christoph, could you look over this please?
> 
> I note that the interface seems full of warts to me,
> this might be a first step to cleaning them.
The whole virtio-blk interface is full of warts.  It has been
extended rather ad-hoc, so that is rather expected.
> One issue I struggled with especially is how type
> field mixes bits and non-bit values. I ended up
> simply defining all legal values, so that we have
> CMD = 2, CMD_OUT = 3 and so on.
It's basically a complete mess without much logic behind it.
> +\change_unchanged
> +the high bit
> +\change_inserted 0 1266497301
> + (VIRTIO_BLK_T_BARRIER)
> +\change_unchanged
> + 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.
> +
> +\change_inserted 0 1266504385
> + 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.
> +\change_unchanged
I'm not sure it's even worth documenting it.  I can't see any way to
actually implement safe behaviour with the VIRTIO_BLK_T_BARRIER-style
barriers.


Btw, did I mention that .lyx is a a really horrible format to review
diffs for?  Plain latex would be a lot better..