Re: [virtio-comment] protocol-level sector size for virtio-scsi

[James Bottomley <jbottomley@odin.com>]

On Mon, 2015-09-07 at 15:15 +0200, Paolo Bonzini wrote:
> 
> On 07/09/2015 15:07, Laszlo Ersek wrote:
> > 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.
> 
> Yes, this is the correct interpretation.

So it should be stated, I think.  And the statement should include
wording that says this is for the purposes of virtio config only and
doesn't restrict the actual block size returned by the inquiry data.

Looking at the section, I think we could do with defining "segment" as
well.  It's a Linux specific term meaning element in a scatter/gather
list, but I suspect our terminology isn't industry standard (yet).

James