[PATCH v2 1/1] virtio-blk: Add description for blk_size field

[Max Gurtovoy <mgurtovoy@nvidia.com>]

This field is only valid when the VIRTIO_BLK_F_BLK_SIZE feature bit is
offered by the device.

The blk_size field actually represents the logical block size of the
device. It is always a power of two and typically ranges from 512 bytes
to larger values such as 4 KB.

Add description for this field to provide clarity on its constraints.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
---

changes from V1:
 - Addressed Stefan's and Daniel's comments:
       1. use SHOULD instead of MUST
       2. Add a note that devices may return IOERR upon misaligned IO
 - Add a note that devices may return IOERR if IO size is not following
   the block size granularity.
---
 device-types/blk/description.tex | 34 ++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/device-types/blk/description.tex b/device-types/blk/description.tex
index 2712ada..77eed80 100644
--- a/device-types/blk/description.tex
+++ b/device-types/blk/description.tex
@@ -135,6 +135,9 @@ \subsection{Device configuration layout}\label{sec:Device Types / Block Device /
 present. The availability of the others all depend on various feature
 bits as indicated above.
 
+The field \field{blk_size} exists only if VIRTIO_BLK_F_BLK_SIZE is offered by the device.
+This field reports the block size of the device, expressed in bytes.
+
 The field \field{num_queues} only exists if VIRTIO_BLK_F_MQ is set. This field specifies
 the number of queues.
 
@@ -282,6 +285,13 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
 
 \drivernormative{\subsubsection}{Device Initialization}{Device Types / Block Device / Device Initialization}
 
+Drivers SHOULD negotiate VIRTIO_BLK_F_BLK_SIZE if the feature is offered by the
+device. When negotiated, drivers SHOULD interpret the \field{blk_size} as the
+logical block size.
+
+If the VIRTIO_BLK_F_BLK_SIZE feature is not offered by the device, then drivers
+MAY assume that the logical block size is 512 bytes.
+
 Drivers SHOULD NOT negotiate VIRTIO_BLK_F_FLUSH if they are incapable of
 sending VIRTIO_BLK_T_FLUSH commands.
 
@@ -319,6 +329,10 @@ \subsection{Device Initialization}\label{sec:Device Types / Block Device / Devic
 
 \devicenormative{\subsubsection}{Device Initialization}{Device Types / Block Device / Device Initialization}
 
+Devices SHOULD always offer VIRTIO_BLK_F_BLK_SIZE feature. When this feature is
+offered, devices MUST initialize \field{blk_size} to a power of two greater
+than or equal to 512.
+
 Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it
 if they offer VIRTIO_BLK_F_CONFIG_WCE.
 
@@ -879,6 +893,14 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
 The length of \field{data} MUST be a multiple of 512 bytes for VIRTIO_BLK_T_IN
 and VIRTIO_BLK_T_OUT requests.
 
+The length of \field{data} SHOULD be a multiple of \field{blk_size} for VIRTIO_BLK_T_IN
+and VIRTIO_BLK_T_OUT requests, when the VIRTIO_BLK_F_BLK_SIZE feature has been
+offered by the device.
+
+The value of \field{sector} (multiplied by 512) SHOULD be aligned with
+\field{blk_size} for VIRTIO_BLK_T_IN and VIRTIO_BLK_T_OUT requests, when the
+VIRTIO_BLK_F_BLK_SIZE feature has been offered by the device.
+
 The length of \field{data} MUST be a multiple of the size of struct
 virtio_blk_discard_write_zeroes for VIRTIO_BLK_T_DISCARD,
 VIRTIO_BLK_T_SECURE_ERASE and VIRTIO_BLK_T_WRITE_ZEROES requests.
@@ -966,6 +988,18 @@ \subsection{Device Operation}\label{sec:Device Types / Block Device / Device Ope
 for a write request if the VIRTIO_BLK_F_RO feature if offered, and MUST NOT
 write any data.
 
+A device MAY set the \field{status} to VIRTIO_BLK_S_IOERR for
+VIRTIO_BLK_T_IN or VIRTIO_BLK_T_OUT requests if the requested
+\field{sector} (multiplied by 512) is not aligned with the device's
+\field{blk_size}, when the VIRTIO_BLK_F_BLK_SIZE feature has been offered by
+the device.
+
+A device MAY set the \field{status} to VIRTIO_BLK_S_IOERR for
+VIRTIO_BLK_T_IN or VIRTIO_BLK_T_OUT requests if the length of the
+requested \field{data} is not an integer multiple of the device's
+\field{blk_size}, when the VIRTIO_BLK_F_BLK_SIZE feature has been offered by
+the device.
+
 The device MUST set the \field{status} byte to VIRTIO_BLK_S_UNSUPP for
 discard, secure erase and write zeroes commands if any unknown flag is set.
 Furthermore, the device MUST set the \field{status} byte to
-- 
2.18.1