Re: [PATCH 1/2] virtio-scsi: fix parsing of hotplug/hot-unplug LUN number

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2012-07-26 at 10:17 +0000, Bart Van Assche wrote:
> On 07/26/12 09:21, James Bottomley wrote:
> > On Thu, 2012-07-26 at 11:04 +0200, Paolo Bonzini wrote:
> >> Il 26/07/2012 10:52, James Bottomley ha scritto:
> >>>>> +static unsigned int virtscsi_get_lun(u8 *lun_bytes)
> >>>>> +{
> >>>>> +	unsigned int lun = (lun_bytes[2] << 8) | lun_bytes[3];
> >>>>> +	return lun & 16383;
> >>>>> +}
> >>>>> +
> >>> Why are you rolling your own incomplete version of scsilun_to_int here?
> >>
> >> Because scsilun_to_int does not do the AND, so it would have exactly the
> >> same bug I'm fixing.
> > 
> > It's not a bug ... it's the encoding.  All the other devices use this
> > too.  Ideally we should have switched to 64 bit lun numbers for the
> > encoding to be exact, but nothing so far has gone over 32 bits.  If we
> > don't encode the Address method as part of the lun number, we don't get
> > the reverse transform right and the addressing often fails.
> > 
> > That does mean that arrays that use address method=1 in REPORT LUNS have
> > their lun numbers start at 16384.
> 
> Has it already been considered to modify scsilun_to_int() such that LUN
> numbers start at zero even for addressing method 1 ? This is what e.g.
> the function scst_unpack_lun() already does. See also
> http://scst.svn.sourceforge.net/viewvc/scst/trunk/scst/src/scst_lib.c?revision=HEAD&view=markup.

Yes, as I said before, the problem is that the actual numbers are

1. not 1:1: there are several possible encodings of luns 0-255
2. hierarchical, so once you go beyond a single level you can't properly
use a numeric representation either.

the mid layer just uses the lun number as an encoding of the actual SAM
lun.  The key for us is that int_to_scsilun has to go back the other
way.

James