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Date: Thu, 03 Nov 2011 08:49:36 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Do you have a use for a tester of virtio-scsi with
 CD drives ?
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To: Thomas Schmitt <scdbackup@gmx.net>
Cc: kwolf@redhat.com, stefanha@gmail.com, qemu-devel@nongnu.org

On 11/02/2011 10:22 PM, Thomas Schmitt wrote:
> Hi,
>
> i wrote:
>>> So how is this altered to 0x12 in the further course of processing ?
>
> Paolo Bonzini wrote:
>> Because you're using an *IDE* (ATAPI) CD-ROM, not SCSI.  See hw/ide/atapi.c.
>
> You convinced me.
> But this does not explain yet the difference in behavior of
> both groups of IDE DVD-ROMs. Why are those of -drive if=scsi empty ?

They are not "of -drive if=scsi".  They are "of -cdrom", and unlike the 
SCSI ones they are always present, even if no -cdrom is given (you can 
use -nodefaults to remove all default devices, including the IDE 
CD-ROM).  So if you specified "-drive if=scsi" *and* "-cdrom", you'd get 
two non-empty drives.

>> Ok, in your counting I should have written 20 for IDE (0x12 + page number +
>> page size) and 22 for SCSI (0x14 + page number + page size).
>
> Why the distinction by transport bus ? Mode page 2Ah is a matter
> of the drive alone. MMC is independent of the bus.

It's just an artefact of having two separate implementations for ATAPI 
and SCSI.

> I did not yet find out, why the IDE drive emulation emits a Mode Data
> Length of 0x22. With Page Length 0x12 it should have been 0x1a.
> 8 too many.

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".

Paolo