Re: [RFC v2] virtio-blk: add discard and write zeroes features to specification

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Feb 28, 2018 at 02:54:28PM +0800, Changpeng Liu wrote:
> @@ -4211,6 +4227,17 @@ according to the native endian of the guest rather than
>    after reset can be either writeback or writethrough.  The actual
>    mode can be determined by reading \field{writeback} after feature
>    negotiation.
> +
> +\item If the VIRTIO_BLK_F_DISCARD feature is negotiated,
> +    \field{max_discard_sectors} and \field{max_discard_seg} can be read
> +    to determine the maximum discard sectors and maximum number of discard
> +    segments for the block driver to use. \field{discard_sector_alignment}
> +    can be used by OS when splitting a request based on alignment.
> +
> +\item if the VIRTIO_BLK_F_WRITE_ZEROES feature is negotiated,
> +    \field{max_write_zeroes_sectors} and \field{max_write_zeroes_seg} can
> +    be read to determine the maximum write zeroes sectors and maximum
> +    number of write zeroes segments for the block driver to use.

Does "sectors" use the same 512-byte units as the capacity field?  The
term "sectors" is slightly ambiguous in this spec so it would be helpful
to describe it explicitly.

What are the units of discard_sector_alignment?

>  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 scsi packet commands and for flush commands.
>  
> +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.

This paragraph has been duplicated.

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