protocol-level sector size for virtio-scsi

[Laszlo Ersek <lersek@redhat.com>]

Hi All,

I'm looking at Committee Specification 03 of virtio V1.

The specification of the virtio-block device is very explicit about the
fact that on the virtio protocol level, the sector size is 512 bytes.
See eg. "5.2.4 Device configuration layout", and "5.2.5 Device
Initialization" step 2. Also "5.2.6 Device Operation", field
virtio_blk_req.sector.

The virtio-scsi HBA has one config field for whose interpretation the
size of a sector, on the virtio protocol level, is necessary:
virtio_scsi_config.max_sectors. (See "5.6.4 Device configuration
layout".)

This size is however not spelled out. I think it is assumed by the
standard that readers will just "inherit" the 512 byte sector size (on
the virtio protocl level) for the purposes of the
"virtio_scsi_config.max_sectors" field. However, since the virtio-block
and virtio-scsi chapters are siblings, not ancestors/descendants, this
assumption of inheritance is incorrect in my option.

With regard to virtio implementations,

(1) the Linux kernel guest does this:

* In "drivers/scsi/virtio_scsi.c", function virtscsi_probe(), the
  max_sectors value is assigned to "shost->max_sectors". ("shost" being
  of type Scsi_Host, allocated with scsi_host_alloc().)

* In "drivers/scsi/scsi_lib.c", function __scsi_init_queue() calls
  blk_queue_max_hw_sectors() with "shost->max_sectors".

* In "block/blk-settings.c", the function blk_queue_max_hw_sectors() has
  the following comment:

  @max_hw_sectors: max hardware sectors in the usual 512b unit

(2) The virtio-win guest driver (called vioscsi) retrieves the
    max_sectors setting (see GetScsiConfig() in "vioscsi/helper.c"), but
    doesn't use it for anything, apparently.

(3) SeaBIOS does not use this config field.

(4) OVMF uses the field and considers the (virtio protocol level) sector
    size to be 512 bytes, similarly to Linux (1), and to the
    virtio-block chapter in the specification.

Therefore I request that the sector size, for the purposes of
"virtio_scsi_config.max_sectors", be specified as 512 bytes.

> --- content.tex.orig	2015-09-07 14:58:04.769524235 +0200
> +++ content.tex	2015-09-07 15:02:08.763888809 +0200
> @@ -5013,7 +5013,7 @@
>      segments and \field{seg_max} output segments.
>
>  \item[\field{max_sectors}] is a hint to the driver about the maximum transfer
> -    size to use.
> +    size to use, expressed in 512-byte sectors.
>
>  \item[\field{cmd_per_lun}] is tells the driver the maximum number of
>      linked commands it can send to one LUN.

(Sorry about not sending a real patch; I don't know where the git repo
lives. The old repo at git://github.com/rustyrussell/virtio-spec.git
seems to be defunct.)

Thank you,
Laszlo