Re: [PATCH v2 1/4] virtio-blk: document data[] size constraints

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 31, 2019 at 10:36:14AM +0800, Stefan Hajnoczi wrote:
> The struct virtio_blk_req->data[] field is a multiple of 512 bytes long
> for read and write requests.  Flush requests don't use data[] at all.
> 
> The new discard and write zeroes requests being introduced in VIRTIO 1.1
> put struct virtio_blk_discard_write_zeroes elements into data[], so it
> must be a multiple of the struct size.
> 
> The uint8_t data[][512] pseudo-code makes it look like discard and write
> zeroes requests must pad to 512 bytes.  This wastes memory since struct
> virtio_blk_discard_write_data is only 16 bytes long.
> 
> Furthermore, all known implementations wishing to take advantage of this
> upcoming VIRTIO 1.1 feature do not use 512-byte padding (Linux
> virtio_blk.ko, QEMU virtio-blk device emulation, the SPDK virtio-blk
> driver, and the SPDK vhost-user-blk device backend).
> 
> This patch documents the data[] size constraints clearly in the driver
> normative section.  This is clearer than the current pseudo-code.
> 
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Changpeng Liu <changpeng.liu@intel.com>
> Cc: Stefano Garzarella <sgarzare@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  content.tex | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/content.tex b/content.tex
> index 836ee52..b185bb0 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3941,7 +3941,7 @@ struct virtio_blk_req {
>          le32 type;
>          le32 reserved;
>          le64 sector;
> -        u8 data[][512];
> +        u8 data[];
>          u8 status;
>  };
>  
> @@ -3971,6 +3971,11 @@ The \field{sector} number indicates the offset (multiplied by 512) where
>  the read or write is to occur. This field is unused and set to 0 for
>  commands other than read or write.
>  
> +VIRTIO_BLK_T_IN requests populate \field{data} with the contents of sectors
> +read from the block device (in multiples of 512 bytes).  VIRTIO_BLK_T_OUT
> +requests write the contents of \field{data} to the block device (in multiples
> +of 512 bytes).
> +
>  The \field{data} used for discard or write zeroes command is described
>  by one or more virtio_blk_discard_write_zeroes structs. \field{sector}
>  indicates the starting offset (in 512-byte units) of the segment, while
> @@ -3997,6 +4002,13 @@ 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 SHOULD NOT include any data in a VIRTIO_BLK_T_FLUSH request.
>  
> +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} MUST be a multiple of the size of struct
> +virtio_blk_discard_write_zeroes for VIRTIO_BLK_T_DISCARD and
> +VIRTIO_BLK_T_WRITE_ZEROES requests.
> +

So a single request can discard/write multiple ranges?
It might be a good idea to make this explicit.
Also is this capability useful/used? And what's the value of status
in case some of the requests fail?


>  If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY
>  switch to writethrough or writeback mode by writing respectively 0 and
>  1 to the \field{writeback} field.  After writing a 0 to \field{writeback},
> -- 
> 2.20.1