Re: [virtio] [PATCH 2/4] virtio-blk: clarify what seg_max and size_max are for

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

On Fri, Aug 17, 2018 at 07:04:42PM +0200, Paolo Bonzini wrote:
> Define them in terms of descriptors, not segments, and explain what the
> driver and device are to use them for.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  content.tex | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index 4cdfeef..fd3a3c2 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3515,11 +3515,11 @@ device except where noted.
>  \subsection{Feature bits}\label{sec:Device Types / Block Device / Feature bits}
>  
>  \begin{description}
> -\item[VIRTIO_BLK_F_SIZE_MAX (1)] Maximum size of any single segment is
> -    in \field{size_max}.
> +\item[VIRTIO_BLK_F_SIZE_MAX (1)] Maximum size of any single descriptor in
> +    the data portion of a request ("segment") is in \field{size_max}.
>  
> -\item[VIRTIO_BLK_F_SEG_MAX (2)] Maximum number of segments in a
> -    request is in \field{seg_max}.
> +\item[VIRTIO_BLK_F_SEG_MAX (2)] Maximum number of descriptors in the
> +    data portion of a request ("segments") is in \field{seg_max}.
>  
>  \item[VIRTIO_BLK_F_GEOMETRY (4)] Disk-style geometry specified in
>      \field{geometry}.

So it's a bit of a mess. For example when you count descriptors with an
indirect descriptor you don't count the pointer. Split and packed ring
terminology also differ.  We need to have a definition in the common
part. Up to cleaning this up?


> @@ -3762,6 +3762,10 @@ error or VIRTIO_BLK_S_UNSUPP for a request unsupported by device:
>  A driver SHOULD NOT submit a request which would cause a read or write
>  beyond \field{capacity}.
>  
> +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.
> +
>  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.
> @@ -3782,6 +3786,11 @@ a range of sectors has been discarded.
>  A device MUST set the \field{status} byte to VIRTIO_BLK_S_IOERR for a
>  request which would cause a read or write beyond \field{capacity}.
>  
> +A device MAY set the \field{status} byte to VIRTIO_BLK_S_IOERR for 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.
> +
>  A device MUST set the \field{status} byte to VIRTIO_BLK_S_IOERR
>  for a write request if the VIRTIO_BLK_F_RO feature if offered, and MUST NOT
>  write any data.
> -- 
> 2.17.1
> 
> 
> 
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