Re: [Qemu-devel] Do you have a use for a tester of virtio-scsi with CD drives ?

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/03/2011 10:15 AM, Thomas Schmitt wrote:
> Which gives me something to work on. libburn does not take into
> respect the Block Descriptor of 8 bytes which sits between
> Mode Data Header and mode page.
> So it misinterpets the result and demands the wrong Allocation
> Length.
>
> This error is explainable by my reading of MMC-5 which prescribes
> in table 653:
>    "Block Descriptor Length = 0"
> MMC-1 on the other hand has in Annex B.4.1 :
>    "The Mode Parameter Block Descriptor does not apply to ATAPI
>     devices, and the Block Descriptor Length in the Mode
>     Parameter Header shall be set to 0."
>
> My thanks to qemu and the developers of its SCSI CD-ROM for showing
> me this misconception in libburn.

MMC-3 also has some text saying that for compatibility you can have 
block descriptors.

> ------------------------------------------------------------------------
>
> If i do only -drive file=/dev/sr0,if=scsi,medium=cdrom i get
> one empty drive, obviously from hw/ide/atapi.c:cmd_mode_sense():
>
>    MODE SENSE
>    5a 00 2a 00 00 00 00 00 1c 00
>    From drive: 28b
>    00 22 70 00 00 00 00 00 2a 12 3b 00 71 60 29 00 02 c0 00 02
>    02 00 02 c0 00 00 00 00
>
> So i mistook the default DVD-ROM drive, which has no source data and
> thus is empty, for the CD-ROM drive which i expected from -drive if=scsi
> which would not be empty, but does not show up at all.
>
> The question remains, why the CD-ROM drive is missing if i do -drive
> but not -cdrom.

Ok, so we have something to test now. :)

> I will repeat my experiments with
>     -drive file=/dev/sg2,if=scsi,media=cdrom -cdrom /dvdbuffer/pseudo_drive
>
> Maybe the presence of -cdrom cures the problems i had with passthrough.
> (The bug in libburn is not to blame for that. The passthrough drive did not
>   deliver a block descriptor.)

Yes, otherwise it wouldn't be passthrough. :)

> ------------------------------------------------------------------------
>
>> Yeah, looks like all the
>>
>>          case MODE_PAGE_R_W_ERROR: /* error recovery */
>>              cpu_to_ube16(&buf[0], 16 + 6);
>> should have "- 2" instead of "+ 6".
>
> I came to the same conclusion.

Good.

> MMC-1 allows MODE SENSE(6), MMC-3 implicitely assumes MODE SENSE(10) only,
> MMC-5 prescribes to use MODE SENSE(10).
>
> MMC-2 has a list of required ATAPI commands. MODE SENSE(10) is listed.
> MODE SENSE(6) is not listed.
>
>
> I compared this with the SCSI counterpart of cmd_mode_sense():
> hw/scsi-disk.c:scsi_disk_emulate_mode_sense()
>
> It distinguishes between MODE_SENSE, which gets 4 bytes of header,
> and MODE_SENSE_10, which gets 8. This looks correct.

Yep.  Seems easiest to just drop MODE SENSE(6) from ATAPI.

Paolo