[PATCH 3/4] virtio-blk: clarify units for sector and data length

[Paolo Bonzini <pbonzini@redhat.com>]

For reads and writes, the sector is the offset divided by 512, not
multiplied.  For everything else, it is zero.

Mention VIRTIO_BLK_F_BLK_SIZE in the normative section.

The data length need not be a multiple of 512, so do not express that
requirement in the virtio_blk_req struct.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 content.tex | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/content.tex b/content.tex
index fd3a3c2..b7d0bea 100644
--- a/content.tex
+++ b/content.tex
@@ -3711,7 +3711,7 @@ struct virtio_blk_req {
         le32 type;
         le32 reserved;
         le64 sector;
-        u8 data[][512];
+        u8 data[];
         u8 status;
 };
 
@@ -3737,7 +3737,7 @@ The type of the request is either a read (VIRTIO_BLK_T_IN), a write
 #define VIRTIO_BLK_T_WRITE_ZEROES 13
 \end{lstlisting}
 
-The \field{sector} number indicates the offset (multiplied by 512) where
+The \field{sector} number indicates the offset (in 512-byte units) where
 the read or write is to occur. This field is unused and set to 0 for
 commands other than read or write.
 
@@ -3766,9 +3766,15 @@ A driver SHOULD NOT submit a request which has more than \field{seg_max}
 descriptors in the data portion, or which has more than \field{size_max}
 bytes in a single descriptor in the data portion.
 
+The byte offset of each operation (sector number multiplied by
+512) and data length SHOULD be aligned to the block size if the
+VIRTIO_BLK_F_BLK_SIZE feature is negotiated.
+
 A driver SHOULD accept the VIRTIO_BLK_F_RO feature if offered.
 
-A driver MUST set \field{sector} to 0 for a VIRTIO_BLK_T_FLUSH request.
+A driver MUST set \field{sector} to 0 for a VIRTIO_BLK_T_FLUSH,
+VIRTIO_BLK_T_DISCARD or VIRTIO_BLK_T_WRITE_ZEROES request.
+
 A driver SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.
 
 If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY
-- 
2.17.1